Brent I was able to get my mirror down to .10175 and Scott's down to .1015 inches before I ran out of 60 grit. I was shooting for .102 for both of ours so I guess I am close enough. My sagita depth is .103 for an f7 and am hoping to reach that for both of our mirrors using the 120 grit. So I want to be clear on what I am doing. I was so worried about our mirrors being so far off I may not have the next step right. I believe what I am supposed to do is about 10 wets with the 120 grit with a short stroke with about 2" exposed at the top and bottom of the stroke and extend the last 2 wets. If this isn't right please let me know. Thanks Jim --------------------------------- Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, and more

What? Does Brent like longer f ratios? ;-) LOL Dave On Friday, February 21, 2003, at 09:42 AM, Chuck Hards wrote:

I'm guessing that Brent influenced your decision?

C.

That is the main reason why you should build your own scope. Nobody buyilds long focal lengths, and they really do perform better than the fast, commercial scopes. Brent --- Chuck Hards <chuckhards@yahoo.com> wrote:

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department! :)

C.

--- David L Bennett <dlbennett@mac.com> wrote:

...

What? Does Brent like longer f ratios? ;-)

LOL

Dave

On Friday, February 21, 2003, at 09:42 AM, Chuck Hards wrote:

...

I'm guessing that Brent influenced your decision?

C.

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Imagine a 30" f/7... "Pass me that pressure suit willya?" "Sure thing; oh, and watch out for those satellites, they come a little close. By the way, did you remember your parachute?" "Naw, I'd just burn up if I fell anyhow..." Cave made some sweet mirrors, so I've been told. You can probably get some nice high power views with it too. Agreeing with what Chuck said, my 10 f/5.6 does a better job than it would if it were f/4.5, being mass produced and all. On a rare night of perfect seeing last summer (very far from here), I was able to push it to 580x without image degradation. I saw detail on Uranus! (Boy that dob driver comes in handy at those high magnifications) --- David L Bennett <dlbennett@mac.com> wrote:

Don't forget considerations of field of view! My 10" f/5 Cave with a 40mm Tele Vue Wide Field gives some very impressive Milky Way shots ;)

On Friday, February 21, 2003, at 09:58 AM, Chuck Hards wrote:

...

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department! :)

C.

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My 22 inch is f6.3. Only a couple of folks have required oxygen. Even though it is long, I believe the improvement in the image is worthwhile. I can still set it up myself, although two is nice. Brent --- Greg Taylor <astronomus_maximus@yahoo.com> wrote:

Imagine a 30" f/7...

"Pass me that pressure suit willya?"

"Sure thing; oh, and watch out for those satellites, they come a little close. By the way, did you remember your parachute?"

"Naw, I'd just burn up if I fell anyhow..."

Cave made some sweet mirrors, so I've been told. You can probably get some nice high power views with it too.

Agreeing with what Chuck said, my 10 f/5.6 does a better job than it would if it were f/4.5, being mass produced and all. On a rare night of perfect seeing last summer (very far from here), I was able to push it to 580x without image degradation. I saw detail on Uranus! (Boy that dob driver comes in handy at those high magnifications)

--- David L Bennett <dlbennett@mac.com> wrote:

...

Don't forget considerations of field of view! My 10" f/5 Cave with a 40mm Tele Vue Wide Field gives some very impressive Milky Way shots ;)

On Friday, February 21, 2003, at 09:58 AM, Chuck Hards wrote:

...

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department! :)

C.

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Chuck, Can you explain the physics behind why short focal length scopes can achieve much faster photographic imaging times than longer f-ratios? Are we back to the camera anaolgy again here? (as you stop down the lens, depth of field increases, but so does the incoming amount of light vs. opening the stops up for more light, but the depth of field gets narrow). The missing piece for me still is, where is the stopping down occuring in two scopes with the same aperture but having different focal lengths? Sorry, I'm a bit dense with all this. Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:

I did state that ergonomics was a consideration for going short.

A 30" f/7 Cassegrain would be much shorter than a similar-apertured f/7 Newtonian.

C.

--- Greg Taylor <astronomus_maximus@yahoo.com> wrote:

...

Imagine a 30" f/7...

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--- Chuck Hards <chuckhards@yahoo.com> wrote:

Aperture alone determines flux density (througput). Focal length determines magnification- and now you just invoke the inverse-square law for brightness. Twice the focal length, twice the mag., 1/4 the brightness.

Sure, that makes perfect sense. But I'm still not quite grokking all of it. Let's say I have two scopes, an f/4 and an f/8, same aperture. My goal is to image, let's say, Saturn, and to have the resulting image of the planet the same size (that is, using the same magnification). How is the image brightness therefore different if the image size is the same (achieved obviously with different eyepieces)? (Sorry to tax the patience of the rest of you that do have a clue with my density here...) -Rich __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

My question obviously does belie my complete ignorance of astrophotography and the various methods used to image deep sky objects. When I read that a short focal length scope can photograph a given object X in "half" the time as a longer focal length scope of the same aperture, I see now what they didn't tell me is that the resulting image on the film is also 1/4 the size -- that's a big "duh". So I still fail to see the photographic advantage of a fast scope, unless what is imaged is huge (e.g., the veil nebula) and image size isn't all that important, right? Am I thinking straight here? At any rate, I'll quit pestering the list with my ignorance and go check out a few books on astrophotography and get a clue how this all works. -Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:

It's not different. It's identical, given identical apertures. You're confused because you're using eyepiece projection and making the image identical.

You get a shorter exposure in an f/4 scope than an f/8 scope of identical aperture because the image is only 1/4 as large at the focal plane. If you make the images of identical size through eyepiece projection, they are of the same brightness because they are now the same size (cover the same area). You are now no longer comparing an f/4 to an f/8. You've made them identical with the eyepiece, as far as brightness goes.

C.

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No, my bad, you did say half the size, I'm the one that got it mixed up with brightness. And I don't think Dave was too offended; he did add a smiley in there to let you know that. :-) Dave does have a great performer of a scope from what I remember at Wolf Creek by the way... Rich --- Chuck Hards <chuckhards@yahoo.com> wrote:

Rich, you are not a pest, and I'd be disappointed in you if you stopped asking these questions!

But you are correct, a fast scope is no real advantage if you are shooting eyepiece-projection, other than placing lower physical demands on your mount.

(Correction: I should have said 1/2 the size at the image plane, for half the focal length, or 4X the brightness...I'm just so upset that I ticked-off Dave, that I can't think straight!)

C.

--- Richard Tenney <retenney@yahoo.com> wrote:

...

My question obviously does belie my complete ignorance of astrophotography and the various methods used to image deep sky objects. When I read that a short focal length scope can photograph a given object X in "half" the time as a longer focal length scope of the same aperture, I see now what they didn't tell me is that the resulting image on the film is also 1/4 the size -- that's a big "duh". So I still fail to see the photographic advantage of a fast scope, unless what is imaged is huge (e.g., the veil nebula) and image size isn't all that important, right? Am I thinking straight here? At any rate, I'll quit pestering the list with my ignorance and go check out a few books on astrophotography and get a clue how this all works.

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Rich, Let me take a crack at this. I think the key is to remember that in a linear optical system, angles are preserved. This means that if an object subtends one degree in the sky, it will subtend one degree in a telescope. Here's an example. The moon subtends about 1/2 degree in the sky. For an 8" f7 scope (focal length 56 inches) a 1/2 degree image will be .49 inches tall. The image height for an 8" f4.5 (36 inch focal length) will be .31 inches. An 8" scope gathers 8 inches worth of light. You can spend the light any way you want to. FOR EXTENDED OBJECTS, all eight inches worth of that light is in the image. If, however, you form a smaller image because of a shorter focal length, that image will still contain all the light, but being smaller, it will be brighter. This is not true for stars. They are point sources, and thorough good optical systems of any f ratio their size is determined by aperture. A star in an 8" f7 is the same size as a star in an 8" f4.5. It is determined by diffraction limits. What's the result of this? Extended objects AT THE FOCAL PLANE will be brighter with faster scopes. Stars will be the same brightness independent of f ratio. An 8" scope will show the same magnitude star no matter what the f ratio is. If you run 50 power, the image is the same brightness in the f7 scope as it is in the f4.5 scope. You have magnified the image fifty times OVER WHAT THE EYE SEES in both cases, and you have done it with an 8" scope. The brightness will be the same in both. It is only at the focal plane where the brightness difference exists between the two focal ratios. Remember that power per inch of aperture determines visual brightness. This is th basic principle of what is happening. Other things will come into play (like magnification and non-linearity of the eye) that modify this, but the basic rule remains the same. Still clear as mud? Brent --- Richard Tenney <retenney@yahoo.com> wrote:

My question obviously does belie my complete ignorance of astrophotography and the various methods used to image deep sky objects. When I read that a short focal length scope can photograph a given object X in "half" the time as a longer focal length scope of the same aperture, I see now what they didn't tell me is that the resulting image on the film is also 1/4 the size -- that's a big "duh". So I still fail to see the photographic advantage of a fast scope, unless what is imaged is huge (e.g., the veil nebula) and image size isn't all that important, right? Am I thinking straight here? At any rate, I'll quit pestering the list with my ignorance and go check out a few books on astrophotography and get a clue how this all works.

-Rich

--- Chuck Hards <chuckhards@yahoo.com> wrote:

...

It's not different. It's identical, given identical apertures. You're confused because you're using eyepiece projection and making the image identical.

You get a shorter exposure in an f/4 scope than an f/8 scope of identical aperture because the image is only 1/4 as large at the focal plane. If you make the images of identical size through eyepiece projection, they are of the same brightness because they are now the same size (cover the same area). You are now no longer comparing an f/4 to an f/8. You've made them identical with the eyepiece, as far as brightness goes.

C.

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Rich, That's it. You can make the image larger, but you only have a limited budget of light. You need to balance that budget against film/CCD resolution. Film graininess will place a lower limit on the image size as will CCD element spacing. The balance is struck by choosing a resolution at the image plane that is within the diffraction limits of your scope and fit the image sensor, while giving you an acceptable exposure time. The sensor can be either film or solid state. The final balance usually requires pushing the sensor to the limit AND using darkroom techniques. Even after all this, many other things such as guiding error, processing error, dust, etc., etc., etc., make it so you only get a good shot on rare occasions. I have done a liimited amount of astrophotography and ended up wasting many yards of film. I have two or three good shots. John Dobson spoke well when he asked why an amateur would want to do this. The good photos have already been taken by the pros. The astrophotos I cherish the most are those locked up in my own memory, and refreshed regularly by observation and drawing. Once more, each person is different, but I believe most people share my experience. Maybe because they are mine! Brent --- Richard Tenney <retenney@yahoo.com> wrote:

Brent,

Let's see if I got this right now: Taking photos at the focal plane of a given object will be smaller and brighter in faster scopes, larger and dimmer in slower scopes for the same exposure times. Got it.

Therefore, photos taken at the focal plane of very small deep sky objects (e.g., a very small planetary nebula) still have to somehow be magnified to properly see the details, achieved either using some sort of lens elements beyond the focal plane (barlow, eyepiece projection or whatever, I don't know how this works yet exactly) to create that larger image on the film itself, or later in the darkroom by enlarging the very small negative on the film to make the final print. Am I getting close? Assuming I've got it right, what works better, making the negative image larger in the first place, or the print image large from a small negative image later on?

-Rich

--- Brent Watson <brentjwatson@yahoo.com> wrote:

...

Rich,

Let me take a crack at this. I think the key is to remember that in a linear optical system, angles are preserved. This means that if an object subtends one degree in the sky, it will subtend one degree in a telescope.

Here's an example. The moon subtends about 1/2 degree in the sky. For an 8" f7 scope (focal length 56 inches) a 1/2 degree image will be .49 inches tall. The image height for an 8" f4.5 (36 inch focal length) will be .31 inches.

An 8" scope gathers 8 inches worth of light. You can spend the light any way you want to. FOR EXTENDED OBJECTS, all eight inches worth of that light is in the image. If, however, you form a smaller image because of a shorter focal length, that image will still contain all the light, but being smaller, it will be brighter.

This is not true for stars. They are point sources, and thorough good optical systems of any f ratio their size is determined by aperture. A star in an 8" f7 is the same size as a star in an 8" f4.5. It is determined by diffraction limits.

What's the result of this? Extended objects AT THE FOCAL PLANE will be brighter with faster scopes. Stars will be the same brightness independent of f ratio. An 8" scope will show the same magnitude star no matter what the f ratio is.

If you run 50 power, the image is the same brightness in the f7 scope as it is in the f4.5 scope. You have magnified the image fifty times OVER WHAT THE EYE SEES in both cases, and you have done it with an 8" scope. The brightness will be the same in both. It is only at the focal plane where the brightness difference exists between the two focal ratios. Remember that power per inch of aperture determines visual brightness.

This is th basic principle of what is happening. Other things will come into play (like magnification and non-linearity of the eye) that modify this, but the basic rule remains the same.

Still clear as mud?

Brent

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Greg: You're not being silly at all, I'd personally hate to climb a tall ladder in the dark! And going with a huge Cassegrain will put tremendous demands on the mount- suddenly it's a trailer-mounted beast, and if going Dobsonian on the mount, good luck lifting into the rocker-box alone! Clyde Bone has addressed the problem of huge apertures & moderate f/ratios in portable instruments without altitude sickness. See his article in S&T of some years ago. Pricey, if going with commercial instruments (you need a TV refractor AND a big Dob), but I think it can be done much more ecconomically by the sly ATM. Relay lenses & Naysmith foci, convex secondaries, the sky is the limit if we don't slavishly build clones of the typical Dob. Loved your remarks; heck, I need the oxygen just driving to the observing site! C. --- Greg Taylor <astronomus_maximus@yahoo.com> wrote:

I know, just being silly.

Greg

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Brent I apologies for my post going public and subjecting you to praise and harassment. I thought I grabbed your personal email address. I also apologize to everyone for cluttering the list with my personal request for tutorage from Brent; I will have to be more careful. Chuck Yes, I have been very influence by Brent and I would say that it has all been good. I really enjoy the Thursday night class Dave and Dave Thanks for watching out for me. This list is cool. We can all learn neat stuff together. Jim Brent Watson <brentjwatson@yahoo.com> wrote:50 inches and 40 mm gives 32X. With a wide field, that is a bit over two degrees. With 87.5 inches and a 40 mm, you have 55X and about 1.5 degrees. Still great Milky Way views. With the f7, you can also get stunning, high power planetary views with longer focal length eyepieces (more eye relief) and easier viewing. You don't have to try and look through as small a hole. I still would take the f7 over the f5, although f5 is beter than f4.5. Brent --- David L Bennett wrote:

Don't forget considerations of field of view! My 10" f/5 Cave with a 40mm Tele Vue Wide Field gives some very impressive Milky Way shots ;)

On Friday, February 21, 2003, at 09:58 AM, Chuck Hards wrote:

...

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department! :)

C.

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Jim. (& Chuck & Jim & Dave & Dave) But I am grateful that it 'went public' This has been every educational for me. For reasons I don't know and I have no idea where I got them, I have been under the apparent misunderstanding that a lower f number brought in more light. I know nothing of astronomy (which is why I joined) and am certainly no photographer. So I am grateful to have seen the exchanges. Maybe if I get to see enough of them I might learn enough to figure out even what kind of equipment would be best for my goals and abilities. And it may be due to the same ignorance but I was not aware of anyone being harassed. You all sound very experienced and significantly intelligent to me. Thank you, Rog Jim Gibson wrote:

Brent

I apologies for my post going public and subjecting you to praise and harassment. I thought I grabbed your personal email address. I also apologize to everyone for cluttering the list with my personal request for tutorage from Brent; I will have to be more careful.

Chuck

Yes, I have been very influence by Brent and I would say that it has all been good. I really enjoy the Thursday night class

Dave and Dave

Thanks for watching out for me. This list is cool. We can all learn neat stuff together.

Jim

Brent Watson <brentjwatson@yahoo.com> wrote:

50 inches and 40 mm gives 32X. With a wide field, that is a bit over two degrees. With 87.5 inches and a 40 mm, you have 55X and about 1.5 degrees. Still great Milky Way views. With the f7, you can also get stunning, high power planetary views with longer focal length eyepieces (more eye relief) and easier viewing. You don't have to try and look through as small a hole.

I still would take the f7 over the f5, although f5 is beter than f4.5.

Brent

--- David L Bennett wrote: > Don't forget considerations of field of view! My > 10" f/5 Cave with a > 40mm Tele Vue Wide Field gives some very impressive > Milky Way shots ;) > > On Friday, February 21, 2003, at 09:58 AM, Chuck > Hards wrote: > > > Everyone should, unless there is a specific reason > for > > going "short". Of course, legitimate reasons > include > > transportation and ergonomics. > > > > Brent's always been right-on in that department! > :) > > > > C. > > > _______________________________________________ > Utah-Astronomy mailing list > Utah-Astronomy@mailman.xmission.com > h tp://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy

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I get harrassed so much I don't even notice it any more. HUFF - HUFF ;) Brent --- Jim Gibson <xajax99@yahoo.com> wrote:

Rog\ You wrote: but I was not aware of anyone being harassed.

David Bennett was teasing Brent because Brent had the good since to favor longer focal lengths. It wasn't enough to send Brent off in a huff.

Jim

RLKelm <rlkelm@attbi.com> wrote:Jim. (& Chuck & Jim & Dave & Dave) But I am grateful that it 'went public' This has been every educational for me. For reasons I don't know and I have no idea where I got them, I have been under the apparent misunderstanding that a lower f number brought in more light. I know nothing of astronomy (which is why I joined) and am certainly no photographer. So I am grateful to have seen the exchanges. Maybe if I get to see enough of them I might learn enough to figure out even what kind of equipment would be best for my goals and abilities. And it may be due to the same ignorance but I was not aware of anyone being harassed. You all sound very experienced and significantly intelligent to me. Thank you, Rog

Jim Gibson wrote: Brent I apologies for my post going public and subjecting you to praise and harassment. I thought I grabbed your personal email address. I also apologize to everyone for cluttering the list with my personal request for tutorage from Brent; I will have to be more careful. Chuck Yes, I have been very influence by Brent and I would say that it has all been good. I really enjoy the Thursday night class Dave and Dave Thanks for watching out for me. This list is cool. We can all learn neat stuff together. Jim Brent Watson <brentjwatson@yahoo.com> wrote: 50 inches and 40 mm gives 32X. With a wide field, that is a bit over two degrees. With 87.5 inches and

a 40 mm, you have 55X and about 1.5 degrees. Still great Milky Way views. With the f7, you can also get

stunning, high power planetary views with longer focal length eyepieces (more eye relief) and easier viewing. You don't have to try and look through as small a hole. I still would take the f7 over the f5, although f5 is beter than f4.5. Brent --- David L Bennett wrote:

...

Don't forget considerations of field of view! My 10" f/5 Cave with a 40mm Tele Vue Wide Field gives some very impressive Milky Way shots ;)

On Friday, February 21, 2003, at 09:58 AM, Chuck Hards wrote:

...

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department!

...

:)

...

C.

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Yes, wide-field is a consideration, but that wide-field is hardly even close to diffraction-limited. If you want performance, you have to set aside "wow" factors and whatever is in vogue at the moment. Put on your engineers hat and take off the baseball cap. C. --- David L Bennett <dlbennett@mac.com> wrote:

Don't forget considerations of field of view! My 10" f/5 Cave with a 40mm Tele Vue Wide Field gives some very impressive Milky Way shots ;)

On Friday, February 21, 2003, at 09:58 AM, Chuck Hards wrote:

...

Everyone should, unless there is a specific reason for going "short". Of course, legitimate reasons include transportation and ergonomics.

Brent's always been right-on in that department! :)

C.

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Dave: OOPS! Sorry to have ruffled your feathers, didn't mean to at all! Please accept my humble apology! I was writing about image quality re: resolution, not aesthetics. I want to differentiate what people claim as a "pleasing view" from a diffraction-limited, truly high-quality view. The fact is that people spend a lot of money for a wide-field, that really has nothing at all to do with image quality. Aesthetics are a consideration, but must be set-aside when talking about resolution, energy distribution, and all that other dry, boring optical bench stuff. In the end, the best telescope for anyone is the one that makes them the happiest, be it f/4 or f/15, home-made or mass-produced, same for eyepieces. I too have had over 34 years of consistently pleasing and sharp images, and, good gosh, none of those views had anything to do with f/4.5 scopes and three hundred-dollar eyepieces. In fact, the absolute best views in terms of resolution that I've ever had have all been with scopes of 10" aperture or less, and $9 eyepieces. Go figure! (I'm fully prostrate now, grovelling in my best supplicant pose...am I forgiven, or shall I reach for the hair shirt and head for the ash pile?) C. --- David L Bennett <dlbennett@mac.com> wrote:

Chuck,

I would never consider 17 years of consistently pleasing and sharp images 'vogue' or simply a 'wow' factor. Patient collimation and a great optician are paramount of course. The best planetary images I've ever had have been through Siegfried's Clark, Brent Watson's 10" f/10 newtonian, Dave Bernson's 12.5" f/5 dob and my Cave. I'll keep my amateur astronomer 'baseball cap' on thank you.

Dave Bennett

P.S. And you thought you'd get through a whole day without ruffling anyone's feathers?! ;-)

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