Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint)
Here are a few illustrations and tables from Sam Brown's classic "All About Telescopes" that are useful tools for making sure the object you're imaging will be the size you want it in the frame. When the book was published, only film astrophotography was possible, but it's all directly applicable to digital imaging. Your imaging chip just takes the place of the film. The math is exactly the same. The first shows the 4 primary methods of telescope imaging. Note that a refractor is used in the illustrations, but the type of telescope is irrellevant. You can imagine the light cone coming from the objective at left to be from any type of telescope desired, the camera doesn't care. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/Imagingsystemsill... The next table will tell you the actual size of the object you want to image on your chip or film, knowing your effective focal length. It's from the pre-metric days, so you'll have to convert inches to millimeters. This will tell you if you have sufficient image scale to show the object at a satisfactory size. You need to know the actual dimensions of your chip to know if the object will be too big or too small. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/imagesizetable_zp... This final table gives you the formulae to determine spacing with various systems. This will help you decide which commercial extenstion tubes, Barlows, adapters, etc., to order to suit your imaging system. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/projectionarithme... Hope this is useful.
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 5:06 PM Subject: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Here are a few illustrations and tables from Sam Brown's classic "All About Telescopes" that are useful tools for making sure the object you're imaging will be the size you want it in the frame. When the book was published, only film astrophotography was possible, but it's all directly applicable to digital imaging. Your imaging chip just takes the place of the film. The math is exactly the same. The first shows the 4 primary methods of telescope imaging. Note that a refractor is used in the illustrations, but the type of telescope is irrellevant. You can imagine the light cone coming from the objective at left to be from any type of telescope desired, the camera doesn't care. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/Imagingsystemsill... The next table will tell you the actual size of the object you want to image on your chip or film, knowing your effective focal length. It's from the pre-metric days, so you'll have to convert inches to millimeters. This will tell you if you have sufficient image scale to show the object at a satisfactory size. You need to know the actual dimensions of your chip to know if the object will be too big or too small. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/imagesizetable_zp... This final table gives you the formulae to determine spacing with various systems. This will help you decide which commercial extenstion tubes, Barlows, adapters, etc., to order to suit your imaging system. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/projectionarithme... Hope this is useful. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
Actually Joe, no, you can't change magnification in the direct objective configuration. The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image. You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration. Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration. The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system. The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram. Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position. On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe
Ah, you're right -- I just realized when I use tubes it's with my focal reducer/field flatteners (I have both an f/6.3 and an f/3.3). So they act as a lens, or in effect, an eyepiece. Sorry for the misunderstanding. -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 7:08 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Actually Joe, no, you can't change magnification in the direct objective configuration. The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image. You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration. Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration. The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system. The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram. Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position. On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe
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No problem, Joe! I'm just trying to help. On Aug 10, 2013 7:14 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
Ah, you're right -- I just realized when I use tubes it's with my focal reducer/field flatteners (I have both an f/6.3 and an f/3.3). So they act as a lens, or in effect, an eyepiece. Sorry for the misunderstanding. -- Joe
________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 7:08 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint)
Actually Joe, no, you can't change magnification in the direct objective configuration.
The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image.
You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration.
Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration.
The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system.
The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram.
Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position.
On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe
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are not the function of telecompressors and teleextenders to change the scale of photographs? I am no photographer but I thought that was what Joe was originally referring to.
Actually Joe, no, you can't change magnification in the direct objective
configuration.
The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image.
You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration.
Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration.
The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system.
The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram.
Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position.
On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe
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Correct. The terms tele-extender and telecompressor are carryovers from the camera industry. They are ussed to change the EFL of any given lens, thus changing the image scale. They serve the same purpose on telescopes. Essentially they serve the same purpose as Barlows and eyepieces, but are mounted in housings more friendly to imaging setups. They modify the converging angle of the light cone, which modifies the EFL of the system. Extension tubes by themselves in direct-objective imaging serve no purpose if focus is achieved without them. Image scale (magnification) cannot be changed using direct-objective imaging without any other additional lenses ahead of the imaging chip or film. Joe made it clear that he had forgotten about his own use of "reducer-corrector" lenses when using extension tubes with his own SCT imaging system. On Sun, Aug 11, 2013 at 8:51 AM, Erik Hansen <erikhansen@thebluezone.net>wrote:
are not the function of telecompressors and teleextenders to change the scale of photographs? I am no photographer but I thought that was what Joe was originally referring to.
Chuck beat me to the answer. His explanation was also better. Thanks, Chuck! ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 7:08 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Actually Joe, no, you can't change magnification in the direct objective configuration. The prime focus of any telescope focused for infinity is fixed. This is the focal length of the objective, or system in the case of a compound telescope such as a Cassegrain. Moving the camera further from the prime focus by means of extension tubes just puts the image out of focus. In other words, the camera chip and prime focus of the objective must coincide. Moving a focuser does not move the image. You CAN increase the image size by moving the camera further from an eyepiece when used in a projection system, number 3 in the illustration. Note that the primary image is in the same location relative to the objective in all three configurations. The camera moves to accommodate the shifted focus, dependant on configuration. The secondary mirror in your SCT is actually a reflective Barlow. It does the exact same thing in your telescope that a Barlow does in a Newtonian or refractor. It changes the focal ratio of the objective, lengthening the EFL of the sytem. While the light cone of your primary mirror is something like f/2, the light cone from your secondary mirror is that of an f/10 system. The small tube in the illustration is supposed to depict a focuser drawtube or camera adapter. It is not important to the diagram. Your SCT focuses by changing the spacing between the primary and secondary. Your prime focus moves according to the distance of the object under view. The closer the object, the further from the back of the telescope the image will be formed. But for objects at infinity, the focus is unchanged. The same thing happens with other telescopes; the focal plane moves further out, the closer the object under view is. But again, it's always in the same place for sky objects which are optically at the infinity position. On Aug 10, 2013 5:24 PM, "Joe Bauman" <josephmbauman@yahoo.com> wrote:
In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe
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Thank YOU for weighing in, Brent! Your answer was succinct. People sometimes don't realize that focusing a system is just a matter of making focal planes coincide. Visually, the eyepiece focal plane must occupy the same location as the objective focal plane. In imaging, the chip or film surface must coincide with the final focal plane of the entire system, be it direct-objective or with additional lenses. A little bit of math goes a long way towards saving time and frustration under the stars. I'm sure there must be websites and apps available these days to do it, without having to take pencil to paper. But where's the fun in that? There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware. On Sun, Aug 11, 2013 at 9:41 AM, Brent Watson <brentjwatson@yahoo.com>wrote:
Chuck beat me to the answer. His explanation was also better. Thanks, Chuck!
I've actually been going down a similar road lately. I've been working on adapting my hi-def camcorder to the telescope. It has a non-removable lens, so eyepiece projection is my best bet. I finally found an adapter ring that allows me to directly couple the camera to my eyepiece-projection housing with T-threads. Preliminary daytime results have been good with the 80mm refractor, but I'm still lacking magnification without a lot of vignetting. I'm goint to try it with the C6 next. It has a much longer focal length. On Sun, Aug 11, 2013 at 9:53 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware.
To me the biggest advantage to building your own tube assembly is that you can match the prime focus point to the eyepieces or other equipment you plan on using. Erik
I've actually been going down a similar road lately. I've been working on adapting my hi-def camcorder to the telescope. It has a non-removable lens, so eyepiece projection is my best bet. I finally found an adapter ring that allows me to directly couple the camera to my eyepiece-projection housing with T-threads. Preliminary daytime results have been good with the 80mm refractor, but I'm still lacking magnification without a lot of vignetting. I'm goint to try it with the C6 next. It has a much longer focal length.
On Sun, Aug 11, 2013 at 9:53 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware.
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Good point, plus, it's an interesting activity in itself. On Sun, Aug 11, 2013 at 10:05 AM, Erik Hansen <erikhansen@thebluezone.net>wrote:
To me the biggest advantage to building your own tube assembly is that you can match the prime focus point to the eyepieces or other equipment you plan on using.
Oversize focusers are a good thing to have also, so you can add extensions without vignetting. Did run into a problem with coma correctors, once.
Good point, plus, it's an interesting activity in itself.
On Sun, Aug 11, 2013 at 10:05 AM, Erik Hansen <erikhansen@thebluezone.net>wrote:
To me the biggest advantage to building your own tube assembly is that you can match the prime focus point to the eyepieces or other equipment you plan on using.
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Plus, you get the optical system you want rather than the one the manufacturer forces upon you. I have always preferred long focal lengths. The image is better. That is why my scopes are such monsters in length. My 22 inch was f6.3. My 10 inch is f10, and my 12.5 inch is f8. ________________________________ From: Erik Hansen <erikhansen@thebluezone.net> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 10:05 AM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) To me the biggest advantage to building your own tube assembly is that you can match the prime focus point to the eyepieces or other equipment you plan on using. Erik
I've actually been going down a similar road lately. I've been working on adapting my hi-def camcorder to the telescope. It has a non-removable lens, so eyepiece projection is my best bet. I finally found an adapter ring that allows me to directly couple the camera to my eyepiece-projection housing with T-threads. Preliminary daytime results have been good with the 80mm refractor, but I'm still lacking magnification without a lot of vignetting. I'm goint to try it with the C6 next. It has a much longer focal length.
On Sun, Aug 11, 2013 at 9:53 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware.
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I believe that Joe is correct and that Chuck and Brent are wrong. Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to. He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio. Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly. DT ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 9:53 AM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Thank YOU for weighing in, Brent! Your answer was succinct. People sometimes don't realize that focusing a system is just a matter of making focal planes coincide. Visually, the eyepiece focal plane must occupy the same location as the objective focal plane. In imaging, the chip or film surface must coincide with the final focal plane of the entire system, be it direct-objective or with additional lenses. A little bit of math goes a long way towards saving time and frustration under the stars. I'm sure there must be websites and apps available these days to do it, without having to take pencil to paper. But where's the fun in that? There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware. On Sun, Aug 11, 2013 at 9:41 AM, Brent Watson <brentjwatson@yahoo.com>wrote:
Chuck beat me to the answer. His explanation was also better. Thanks, Chuck!
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Daniel, this was mentioned earlier in the thread. The fact is that Joe, or anyone shooting with a SCT, never shoots using direct objective because the secondary mirror multiplies the objective's f-ratio. You are correct, the secondary in his telescope is effectively a reflective Barlow, and I did mention that. For that reason, Joe always shoots with the SCT equivalent of configuration number 4 in the chart I posted. Projection using a negative lens, or, in his case, a convex mirror. It may be splitting hairs to you, but any time you modify the f-ratio of the objective before the prime focus, it is no longer a true direct objective system optically. Claiming that the secondary in SCT doesn't count is optically the equivalent of saying that a Barlow in a Newtonian doesn't count. Optically speaking, these configurations are not simply a matter of what extension tube or adapter you have in place between the camera and telescope. On Sun, Aug 11, 2013 at 12:45 PM, daniel turner <outwest112@yahoo.com>wrote:
I believe that Joe is correct and that Chuck and Brent are wrong.
Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to.
He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio.
Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly.
That's why I said it works with an SCT but might not with a refractor -- changing the focus is done internally, so a change in f/ratio or magnification does not require an eyepiece. -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 2:09 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Daniel, this was mentioned earlier in the thread. The fact is that Joe, or anyone shooting with a SCT, never shoots using direct objective because the secondary mirror multiplies the objective's f-ratio. You are correct, the secondary in his telescope is effectively a reflective Barlow, and I did mention that. For that reason, Joe always shoots with the SCT equivalent of configuration number 4 in the chart I posted. Projection using a negative lens, or, in his case, a convex mirror. It may be splitting hairs to you, but any time you modify the f-ratio of the objective before the prime focus, it is no longer a true direct objective system optically. Claiming that the secondary in SCT doesn't count is optically the equivalent of saying that a Barlow in a Newtonian doesn't count. Optically speaking, these configurations are not simply a matter of what extension tube or adapter you have in place between the camera and telescope. On Sun, Aug 11, 2013 at 12:45 PM, daniel turner <outwest112@yahoo.com>wrote:
I believe that Joe is correct and that Chuck and Brent are wrong.
Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to.
He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio.
Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly.
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Correct Joe. You are using your "internal" Barlow (secondary mirror) to modify the telescope's EFL. Negative projection. The refractor would use a Barlow to achieve the same thing as you, optically. Know that SCT's, by varying the distance between primary and secondary, almost never have optimal correction. The more you depart from the optimal mirror spacing, the worse the imagery gets- and that's where the Barlow analogy ends. That's why only the mass-produced SCT's use moving-primary focusing. The "true" astrographs such as RC Cassegrains almost all use a standard focuser at the rear of the telescope and keep the mirror spacing constant. So just adding extension tubes won't work for them anymore than it would on a refractor or Newtonian, or a standard Cassegrain with fixed mirror spacing. SCT's were designed as "everyman's" telescope. They are very versatile, but the optical design is a compromise made in the name of compactness- even the corrector plate is in a compromised position. It performs better near the ROC, not near the primary's focus, but near focus makes a convenient place to mount the secondary. Brent knows this and that's another reason he prefers long f-ratio Newtonians. I do, too, along with long refractors. While the imagery is superior, though, they are cumbersome for imaging. The SCT wins hands-down for convenience. I'm not a SCT hater, I own one and like it for what it can do. But it's not the sharpest knife in the drawer. On Sun, Aug 11, 2013 at 2:25 PM, Joe Bauman <josephmbauman@yahoo.com> wrote:
That's why I said it works with an SCT but might not with a refractor -- changing the focus is done internally, so a change in f/ratio or magnification does not require an eyepiece. -- Joe
Chuck, everything about amateur astronomy is a trade-off. I would dearly love an optically superior instrument, but try to buy one on a retiree's fixed income. Still if you're interested in astrophotography, SCTs aren't bad -- you definitely get far more bang for the aperture buck. Some things even out: bigger aperture means both faster gathering of protons and more resolution. Use the design's strengths and you might get a better photo than with a small-diameter refractor. As an "everyman" I don't mind having an "everyman's telescope." -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 2:47 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Correct Joe. You are using your "internal" Barlow (secondary mirror) to modify the telescope's EFL. Negative projection. The refractor would use a Barlow to achieve the same thing as you, optically. Know that SCT's, by varying the distance between primary and secondary, almost never have optimal correction. The more you depart from the optimal mirror spacing, the worse the imagery gets- and that's where the Barlow analogy ends. That's why only the mass-produced SCT's use moving-primary focusing. The "true" astrographs such as RC Cassegrains almost all use a standard focuser at the rear of the telescope and keep the mirror spacing constant. So just adding extension tubes won't work for them anymore than it would on a refractor or Newtonian, or a standard Cassegrain with fixed mirror spacing. SCT's were designed as "everyman's" telescope. They are very versatile, but the optical design is a compromise made in the name of compactness- even the corrector plate is in a compromised position. It performs better near the ROC, not near the primary's focus, but near focus makes a convenient place to mount the secondary. Brent knows this and that's another reason he prefers long f-ratio Newtonians. I do, too, along with long refractors. While the imagery is superior, though, they are cumbersome for imaging. The SCT wins hands-down for convenience. I'm not a SCT hater, I own one and like it for what it can do. But it's not the sharpest knife in the drawer. On Sun, Aug 11, 2013 at 2:25 PM, Joe Bauman <josephmbauman@yahoo.com> wrote:
That's why I said it works with an SCT but might not with a refractor -- changing the focus is done internally, so a change in f/ratio or magnification does not require an eyepiece. -- Joe
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Joe, they are terrific telescopes, and have probably helped popularize the hobby more than any other. In fact I mis-spoke. I own TWO SCTs. I keep forgetting about the C5. Earlier this year I collimated it with a laser, and the imagery completely deteriorated. I haven't had the time or inclination since to dive into the problem and figure it out. And I have the C6 to use when I need the advantages of a SCT. BTW, many SCT imagers lock down their primary in the optimal position, and use an aftermarket Crayford focuser threaded on the back. There are many to choose from, and not expensive at all. I may try one, myself.
Oops, photons! ________________________________ From: Joe Bauman <josephmbauman@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 3:33 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Chuck, everything about amateur astronomy is a trade-off. I would dearly love an optically superior instrument, but try to buy one on a retiree's fixed income. Still if you're interested in astrophotography, SCTs aren't bad -- you definitely get far more bang for the aperture buck. Some things even out: bigger aperture means both faster gathering of protons and more resolution. Use the design's strengths and you might get a better photo than with a small-diameter refractor. As an "everyman" I don't mind having an "everyman's telescope." -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 2:47 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Correct Joe. You are using your "internal" Barlow (secondary mirror) to modify the telescope's EFL. Negative projection. The refractor would use a Barlow to achieve the same thing as you, optically. Know that SCT's, by varying the distance between primary and secondary, almost never have optimal correction. The more you depart from the optimal mirror spacing, the worse the imagery gets- and that's where the Barlow analogy ends. That's why only the mass-produced SCT's use moving-primary focusing. The "true" astrographs such as RC Cassegrains almost all use a standard focuser at the rear of the telescope and keep the mirror spacing constant. So just adding extension tubes won't work for them anymore than it would on a refractor or Newtonian, or a standard Cassegrain with fixed mirror spacing. SCT's were designed as "everyman's" telescope. They are very versatile, but the optical design is a compromise made in the name of compactness- even the corrector plate is in a compromised position. It performs better near the ROC, not near the primary's focus, but near focus makes a convenient place to mount the secondary. Brent knows this and that's another reason he prefers long f-ratio Newtonians. I do, too, along with long refractors. While the imagery is superior, though, they are cumbersome for imaging. The SCT wins hands-down for convenience. I'm not a SCT hater, I own one and like it for what it can do. But it's not the sharpest knife in the drawer. On Sun, Aug 11, 2013 at 2:25 PM, Joe Bauman <josephmbauman@yahoo.com> wrote:
That's why I said it works with an SCT but might not with a refractor -- changing the focus is done internally, so a change in f/ratio or magnification does not require an eyepiece. -- Joe
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Sounds like a problem of nomenclature, an adapter can mean a lot of different things to different people, personally I enjoyed the large scale of the photo and seemed sharp enough that an enlargement would suffice if you wanted to change the scale. For sure different optical assemblies have their pro's and con's, try before you buy and get as much info as you can. Erik
Daniel, this was mentioned earlier in the thread.
The fact is that Joe, or anyone shooting with a SCT, never shoots using direct objective because the secondary mirror multiplies the objective's f-ratio. You are correct, the secondary in his telescope is effectively a reflective Barlow, and I did mention that. For that reason, Joe always shoots with the SCT equivalent of configuration number 4 in the chart I posted. Projection using a negative lens, or, in his case, a convex mirror.
It may be splitting hairs to you, but any time you modify the f-ratio of the objective before the prime focus, it is no longer a true direct objective system optically. Claiming that the secondary in SCT doesn't count is optically the equivalent of saying that a Barlow in a Newtonian doesn't count.
Optically speaking, these configurations are not simply a matter of what extension tube or adapter you have in place between the camera and telescope.
On Sun, Aug 11, 2013 at 12:45 PM, daniel turner <outwest112@yahoo.com>wrote:
I believe that Joe is correct and that Chuck and Brent are wrong.
Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to.
He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio.
Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly.
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Joe, That would violate the laws of optics. Any telescope has a fixed focal length. If you move the film plane out beyond this distance then you are focusing closer than infinity. You can find that distance using the lens maker's formula. You cannot just place an extension tube in the path and get a larger image scale. Brent ________________________________ From: Joe Bauman <josephmbauman@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 5:22 PM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) In the first illustration, no eyepiece is used. Note the small tube between the camera and the telescope. You can use a longer one, an extender, to get a larger image. I think you should be able to focus the telescope the telescope to accommodate the longer tube. -- Joe ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Saturday, August 10, 2013 5:06 PM Subject: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Here are a few illustrations and tables from Sam Brown's classic "All About Telescopes" that are useful tools for making sure the object you're imaging will be the size you want it in the frame. When the book was published, only film astrophotography was possible, but it's all directly applicable to digital imaging. Your imaging chip just takes the place of the film. The math is exactly the same. The first shows the 4 primary methods of telescope imaging. Note that a refractor is used in the illustrations, but the type of telescope is irrellevant. You can imagine the light cone coming from the objective at left to be from any type of telescope desired, the camera doesn't care. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/Imagingsystemsill... The next table will tell you the actual size of the object you want to image on your chip or film, knowing your effective focal length. It's from the pre-metric days, so you'll have to convert inches to millimeters. This will tell you if you have sufficient image scale to show the object at a satisfactory size. You need to know the actual dimensions of your chip to know if the object will be too big or too small. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/imagesizetable_zp... This final table gives you the formulae to determine spacing with various systems. This will help you decide which commercial extenstion tubes, Barlows, adapters, etc., to order to suit your imaging system. http://i260.photobucket.com/albums/ii24/JethroTull1958/ATM/projectionarithme... Hope this is useful. _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options". _______________________________________________ Utah-Astronomy mailing list http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Send messages to the list to Utah-Astronomy@mailman.xmission.com The Utah-Astronomy mailing list is not affiliated with any astronomy club. To unsubscribe go to: http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Then enter your email address in the space provided and click on "Unsubscribe or edit options".
participants (5)
-
Brent Watson -
Chuck Hards -
daniel turner -
Erik Hansen -
Joe Bauman