Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles. Debbie "As Pete alluded to earlier, the smaller apertures can offer more pleasing views of colorful doubles--I saw this first hand the other night with the 6 inch mask in place--Izar, Cor Caroli, Rasalgethi and many others came alive! I can see why folks love their refractors. Interestingly, on this same night of quite poor seeing, Antares B showed better as a disk separated from the primary when using the 6 inch off-axis mask versus unmasked to the full 15 inches. So, there exists practical utility with its use as well."
Debbie, any central obstruction will increase diffration. Energy is robbed from the central portion of the Airy Disk and redistributed into the rings. Most of it in the first ring, less in succeeding rings. The larger the central obstruction, the more diffraction. This translates into "bloated" star images in the eyepiece. Refractors have no obstruction. Achromatic refractors send some uncorrected color into the rings, but this is generally less than what a central obstruction does. APOs are the best, with their nearly perfect color correction. Spiders will also add diffraction, but double-star observers can sometimes use this to advantage by rotating the tube or secondary cage such that companion stars are between diffraction spikes. The first ring in the Airy Disk is still bloated from the central obstruction, however. Smaller apertures are generally less affected by poor seeing, but this is somewhat of an optical illusion. The larger the aperture, the smaller the Airy Disk. Using an off-aperture mask effectively turns your large Newtonian into an unobstructed off-axis reflector. This eliminates diffraction effects from the central obstruction and spider. Be sure to locate the mask between the spider vanes. Sometimes, it does help, on some objects, but then you are losing the resolution of the larger aperture. So you see, it's a trade-off. Go ahead and try it if you are curious. You have a TeleVue refractor also, don't you? You might want to compare the reflector stopped-down to the same aperture as the refractor, and compare the view in both scopes. Adjust the magnification on the refractor with a good Barlow so it's effective focal length is about the same as the reflector. On Wed, Jul 10, 2013 at 9:16 AM, Debbie <astrodeb@beyondbb.com> wrote:
Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles.
Debbie
I have such a mask for my 16" f/6.25 dob -- but have yet to try it out. In part because I've mostly neglected double stars (and the moon) in my observing sessions. I really need to change that... ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, July 10, 2013 11:44 AM Subject: Re: [Utah-astronomy] Off-axis Aperture mask Debbie, any central obstruction will increase diffration. Energy is robbed from the central portion of the Airy Disk and redistributed into the rings. Most of it in the first ring, less in succeeding rings. The larger the central obstruction, the more diffraction. This translates into "bloated" star images in the eyepiece. Refractors have no obstruction. Achromatic refractors send some uncorrected color into the rings, but this is generally less than what a central obstruction does. APOs are the best, with their nearly perfect color correction. Spiders will also add diffraction, but double-star observers can sometimes use this to advantage by rotating the tube or secondary cage such that companion stars are between diffraction spikes. The first ring in the Airy Disk is still bloated from the central obstruction, however. Smaller apertures are generally less affected by poor seeing, but this is somewhat of an optical illusion. The larger the aperture, the smaller the Airy Disk. Using an off-aperture mask effectively turns your large Newtonian into an unobstructed off-axis reflector. This eliminates diffraction effects from the central obstruction and spider. Be sure to locate the mask between the spider vanes. Sometimes, it does help, on some objects, but then you are losing the resolution of the larger aperture. So you see, it's a trade-off. Go ahead and try it if you are curious. You have a TeleVue refractor also, don't you? You might want to compare the reflector stopped-down to the same aperture as the refractor, and compare the view in both scopes. Adjust the magnification on the refractor with a good Barlow so it's effective focal length is about the same as the reflector. On Wed, Jul 10, 2013 at 9:16 AM, Debbie <astrodeb@beyondbb.com> wrote:
Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles.
Debbie
_______________________________________________ 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".
Rich, on nights that won't support a larger aperture, I always go for the small refractor instead of off-axis mask on a larger reflector. I don't see the point in using a huge scope of only 4 to 6 inches aperture. Either that, or observe deep-sky objects that benefit mostly from aperture, and save the close doubles and planets for another night. I'm really liking my Orion ED100mm for backyard observing. On Jul 10, 2013 1:44 PM, "Richard Tenney" <retenney@yahoo.com> wrote:
I have such a mask for my 16" f/6.25 dob -- but have yet to try it out.
In part because I've mostly neglected double stars (and the moon) in my observing sessions. I really need to change that...
I think we now know why so many Dob owners have a nice refractor mounted right on it. Be prepared.
I also have an off-axis mask for my 16" f5.5 dob. Chuck and Brent have given good technical explanations on the pluses and minuses of an aperture stop. My practical experience is that I always bring my aperture stop along, very rarely use it and even less often think the view with it in place is better than the unmasked telescope. I have tried it on the moon, planets and doubles. When observing a more than quarter moon with that scope, the mask is very nice as it cuts down the brightness. The view isn't any "better", just not so incredibly bright. All large dob owners should have an aperture mask and give it a try. Your observing may or may not benefit, but you never know until you try! Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Wednesday, July 10, 2013 7:36 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Off-axis Aperture mask Rich, on nights that won't support a larger aperture, I always go for the small refractor instead of off-axis mask on a larger reflector. I don't see the point in using a huge scope of only 4 to 6 inches aperture. Either that, or observe deep-sky objects that benefit mostly from aperture, and save the close doubles and planets for another night. I'm really liking my Orion ED100mm for backyard observing. On Jul 10, 2013 1:44 PM, "Richard Tenney" <retenney@yahoo.com> wrote:
I have such a mask for my 16" f/6.25 dob -- but have yet to try it out.
In part because I've mostly neglected double stars (and the moon) in my observing sessions. I really need to change that... _______________________________________________ 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". This message and any attachments are solely for the use of intended recipients. The information contained herein may include trade secrets, protected health or personal information, privileged or otherwise confidential information. Unauthorized review, forwarding, printing, copying, distributing, or using such information is strictly prohibited and may be unlawful. If you are not an intended recipient, you are hereby notified that you received this email in error, and that any review, dissemination, distribution or copying of this email and any attachment is strictly prohibited. If you have received this email in error, please contact the sender and delete the message and any attachment from your system. Thank you for your cooperation
Mat: One of the editors at Sky and Telescope a few years ago made a mask for his midsize dob and carried it with him for a year. He compared views with and without on many different objects over that time and could not document a single instance where the view was better with the mask. He ended up concluding that if the mask improved your view through a dob, then there was something seriously wrong with the dob. The mask has lived on for many decades as part of the popular culture because it's easy to build one and many people think that maybe it will help poor seeing. But the negative results never make it back to the public. My own take is that the mask is a relic of the age when "telescope" meant "refractor". Folks just don't trust those newfangled mirror things. But that's just my opinion. I know that operators of the Grimm and the Ealing are big on using the mask. But when they do, there is a steady stream of people at my dob telling me that my "little" telescope has a better view than the "big" telescopes in the observatory. DT ________________________________ From: "Hutchings, Mat" <mat.hutchings@siemens.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Thursday, July 11, 2013 9:18 AM Subject: Re: [Utah-astronomy] Off-axis Aperture mask I also have an off-axis mask for my 16" f5.5 dob. Chuck and Brent have given good technical explanations on the pluses and minuses of an aperture stop. My practical experience is that I always bring my aperture stop along, very rarely use it and even less often think the view with it in place is better than the unmasked telescope. I have tried it on the moon, planets and doubles. When observing a more than quarter moon with that scope, the mask is very nice as it cuts down the brightness. The view isn't any "better", just not so incredibly bright. All large dob owners should have an aperture mask and give it a try. Your observing may or may not benefit, but you never know until you try! Mat -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Wednesday, July 10, 2013 7:36 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Off-axis Aperture mask Rich, on nights that won't support a larger aperture, I always go for the small refractor instead of off-axis mask on a larger reflector. I don't see the point in using a huge scope of only 4 to 6 inches aperture. Either that, or observe deep-sky objects that benefit mostly from aperture, and save the close doubles and planets for another night. I'm really liking my Orion ED100mm for backyard observing. On Jul 10, 2013 1:44 PM, "Richard Tenney" <retenney@yahoo.com> wrote:
I have such a mask for my 16" f/6.25 dob -- but have yet to try it out.
In part because I've mostly neglected double stars (and the moon) in my observing sessions. I really need to change that... _______________________________________________ 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". This message and any attachments are solely for the use of intended recipients. The information contained herein may include trade secrets, protected health or personal information, privileged or otherwise confidential information. Unauthorized review, forwarding, printing, copying, distributing, or using such information is strictly prohibited and may be unlawful. If you are not an intended recipient, you are hereby notified that you received this email in error, and that any review, dissemination, distribution or copying of this email and any attachment is strictly prohibited. If you have received this email in error, please contact the sender and delete the message and any attachment from your system. Thank you for your cooperation _______________________________________________ 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".
Hi Chuck, What I was thinking is making a 6 inch off-axis mask out of cardboard. I don't think it would be too much trouble to make. I've used most of my cardboard for fishline so I need to hunt up more. I wanted to try to split Zeta Herculis this fall. With a separation of 1.3", it should split on a good night. Also, Delta Cygni and Izar would look good in the 15". You are right I do still have the TeleVue 102 refractor. I need to buy another dovetail plate for it because 2 1/2 years ago I went to a G11 saddle plate. I was going to buy one this fall when the nights cool down some. That way I could compare both scopes on my driveway. In fact, some of the neighbors want to see Saturn soon. Debbie On Wed, Jul 10, 2013 at 11:44 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
Debbie, any central obstruction will increase diffration. Energy is robbed from the central portion of the Airy Disk and redistributed into the rings. Most of it in the first ring, less in succeeding rings. The larger the central obstruction, the more diffraction. This translates into "bloated" star images in the eyepiece. Refractors have no obstruction. Achromatic refractors send some uncorrected color into the rings, but this is generally less than what a central obstruction does. APOs are the best, with their nearly perfect color correction.
Spiders will also add diffraction, but double-star observers can sometimes use this to advantage by rotating the tube or secondary cage such that companion stars are between diffraction spikes. The first ring in the Airy Disk is still bloated from the central obstruction, however.
Smaller apertures are generally less affected by poor seeing, but this is somewhat of an optical illusion. The larger the aperture, the smaller the Airy Disk.
Using an off-aperture mask effectively turns your large Newtonian into an unobstructed off-axis reflector. This eliminates diffraction effects from the central obstruction and spider. Be sure to locate the mask between the spider vanes. Sometimes, it does help, on some objects, but then you are losing the resolution of the larger aperture. So you see, it's a trade-off.
Go ahead and try it if you are curious. You have a TeleVue refractor also, don't you? You might want to compare the reflector stopped-down to the same aperture as the refractor, and compare the view in both scopes. Adjust the magnification on the refractor with a good Barlow so it's effective focal length is about the same as the reflector.
On Wed, Jul 10, 2013 at 9:16 AM, Debbie <astrodeb@beyondbb.com> wrote:
Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles.
Debbie
_______________________________________________ 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".
Debbie, As Chuck stated previously, the secondary obstruction causes light to be moved from the Airy disk to the difraction rings. What is not stated is that as this light is removed from the Airy disk, it actually makes the Airy disk appear smaller than it would otherwise be. This means that the Airy disk from an obstructed telescope is smaller that that of an unobstructed telescope. This can actually help in splitting double stars, particularly when the second star does not coincide with the diffraction rings of the first star, as is the case on many tight doubles. How big is that Airy disk? Dawe's limit can give you an approximation. If you take the aperture of the telescope and divide by 4.56, you will have a good idea of the size in arc seconds of the Airy disk radius for an unobstructed telescope. The obstruction will shrink this size a bit. The real equation states that the resolution of a telescope is proportional the the inverse cube of the aperture. The above relationship (Dawe's limit) just happens to approximate it pretty well for our size optical systems. As you can see, the larger aperture obstructed telescope has two things working in its favor. The first is the smaller Airy disk from the increased aperture, and the second is the reduction in size of the Airy disk because of difration from the obstruction. So, why use an off axis aperture stop? The reason is that it is much more difficult to get the light path through the atmosphere to be steady for a larger aperture instrument. I am sure you have noticed that the number of nights you have difraction limited seeing with your dob is far less than the number of nights you have difraction limited seeing with your refractor. I can not authoritatively give you a number, but it seems like the number of nights I had good seeing with my 22 inch was about equal to the inverse cube of the ratio of the diameters betwen my dob and my refractor. (1/((22/8)^3) , or about a factor of 1/22. That is a rough estimate, and is only a SWAG. The off axis aperture stop will make it so your dob's aperture only six inches, and so you will have good seeing through the atmosphere more often. As you can see, it is really a situation where you are managing the light path through the atmosphere. Your dob should produce good double star images on those nights when you have equivalent seeing, but those nights will be much more rare. Just my opinions and observations, FWIW. On Wed, Jul 10, 2013 at 11:44 AM, Chuck Hards <chuck.hards@gmail.com> wrote:
Debbie, any central obstruction will increase diffration. Energy is robbed from the central portion of the Airy Disk and redistributed into the rings. Most of it in the first ring, less in succeeding rings. The larger the central obstruction, the more diffraction. This translates into "bloated" star images in the eyepiece. Refractors have no obstruction. Achromatic refractors send some uncorrected color into the rings, but this is generally less than what a central obstruction does. APOs are the best, with their nearly perfect color correction.
Spiders will also add diffraction, but double-star observers can sometimes use this to advantage by rotating the tube or secondary cage such that companion stars are between diffraction spikes. The first ring in the Airy Disk is still bloated from the central obstruction, however.
Smaller apertures are generally less affected by poor seeing, but this is somewhat of an optical illusion. The larger the aperture, the smaller the Airy Disk.
Using an off-aperture mask effectively turns your large Newtonian into an unobstructed off-axis reflector. This eliminates diffraction effects from the central obstruction and spider. Be sure to locate the mask between the spider vanes. Sometimes, it does help, on some objects, but then you are losing the resolution of the larger aperture. So you see, it's a trade-off.
Go ahead and try it if you are curious. You have a TeleVue refractor also, don't you? You might want to compare the reflector stopped-down to the same aperture as the refractor, and compare the view in both scopes. Adjust the magnification on the refractor with a good Barlow so it's effective focal length is about the same as the reflector.
On Wed, Jul 10, 2013 at 9:16 AM, Debbie <astrodeb@beyondbb.com> wrote:
Anyone used an off-axis aperture mask for double stars? Here is a quote from CN forums. I'm thinking a 6 inch off-axis mask would be perfect to try some good doubles.
Debbie
_______________________________________________ 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".
Sorry, I mis-spoke. Resolution is proportional to the inverse of the aperture, not the inverse of the aperture cubed. From: Brent Watson <brentjwatson@yahoo.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Wednesday, July 10, 2013 3:39 PM Subject: Re: [Utah-astronomy] Off-axis Aperture mask Debbie, As Chuck stated previously, the secondary obstruction causes light to be moved from the Airy disk to the difraction rings. What is not stated is that as this light is removed from the Airy disk, it actually makes the Airy disk appear smaller than it would otherwise be. This means that the Airy disk from an obstructed telescope is smaller that that of an unobstructed telescope. This can actually help in splitting double stars, particularly when the second star does not coincide with the diffraction rings of the first star, as is the case on many tight doubles. How big is that Airy disk? Dawe's limit can give you an approximation. If you take the aperture of the telescope and divide by 4.56, you will have a good idea of the size in arc seconds of the Airy disk radius for an unobstructed telescope. The obstruction will shrink this size a bit. The real equation states that the resolution of a telescope is proportional the the inverse cube of the aperture. The above relationship (Dawe's limit) just happens to approximate it pretty well for our size optical systems. As you can see, the larger aperture obstructed telescope has two things working in its favor. The first is the smaller Airy disk from the increased aperture, and the second is the reduction in size of the Airy disk because of difration from the obstruction. So, why use an off axis aperture stop? The reason is that it is much more difficult to get the light path through the atmosphere to be steady for a larger aperture instrument. I am sure you have noticed that the number of nights you have difraction limited seeing with your dob is far less than the number of nights you have difraction limited seeing with your refractor. I can not authoritatively give you a number, but it seems like the number of nights I had good seeing with my 22 inch was about equal to the inverse cube of the ratio of the diameters betwen my dob and my refractor. (1/((22/8)^3) , or about a factor of 1/22. That is a rough estimate, and is only a SWAG. The off axis aperture stop will make it so your dob's aperture only six inches, and so you will have good seeing through the atmosphere more often. As you can see, it is really a situation where you are managing the light path through the atmosphere. Your dob should produce good double star images on those nights when you have equivalent seeing, but those nights will be much more rare. Just my opinions and observations, FWIW.
participants (6)
-
Brent Watson -
Chuck Hards -
daniel turner -
Debbie -
Hutchings, Mat -
Richard Tenney