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New AT 6" and 8" Classical Cassegrain

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#1226 jgraham

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Posted 25 October 2020 - 08:54 AM

Always fun. All I knows is that I have the CC8 and RC8 and they both perform beautifully. The views through my CC8 and the images that I take through my RC8 are wonderful.

 

So much to be thankful for, so few clear nights...

 

:)


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#1227 quilty

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Posted 25 October 2020 - 09:00 AM

and if you took images through the CC and looked through the RC, would things change? You're really in the position to compare and find out differences apart from specs from the provider



#1228 Terra Nova

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Posted 25 October 2020 - 11:40 AM

Yes, I have followed along, and I've tried to sort out the claims versus the evidence & methodologies used to support those claims; and, I've asked questions about those.

 

Yes, if they did, GSO had a poor production constance.

 

Based on the 50 pages, I can agree that GSO's between-unit reliability is most likely low.  Not surprising for budget / economy production lines.  It's happened before with other designs, like refractors having a field stop / baffle close behind the objective that shrinks the CA down to a smaller Effective Aperture -- usually to hide a below-average optic.

 

When there are widely variable owner reports for a particular scope, poor quality control is often the culprit -- after factoring out observer experience, seeing at different locations, etc.  I read threads like this one, and try to get a consensus view, before investing my money to test the scope.  Sometimes I agree 100% with the consensus -- like the poor quality Criterion Dynamax 8; but with some scopes, like the Vixen VMC200L, my experience is different.

 

So far, these AT / GSO 6" & 8" CCs seem analogous to the TOWA scopes of the 1970s-1980s, your odds of getting an average or below-average example are higher than with other brands.

I’ve pretty much followed this thread since the beginning because I never had a CC, have never had an SCT that I liked or kept very long, love refractors and like Newtonians, and was really quite interested in acquiring one shortly after they were first announced. In fact I was on the Waitlist for several months until conflicting reports made me less warm to the idea, as well as all the production delays. Regardless of that, I have continued to read, and try to absorb and maintain interest in the comments. At any rate, i really don’t recall or at least I never picked up on quality control issues, inconstancies, and large product variability. It seems to me that the main issues all along have been (1) collimation (which has become less of an issue as owner have acquired more experience and shared how-to tips), (2) the need for extension rings and the need for re-collimation when said rings are exchanged, (3) a problem with the focuser not being square in all axes, and (4) the secondary being sized/placed in such a way as to ‘clip’ the primary’s light cone so that the primary’s full diameter is not accessed, thus reducing both light gathering AND angular resolution.

 

That said, I cannot understand the assertions made about the particular conic-section employed for the primary’s figure. As a CC, it should be parabolic. If it were an RC it would be hyperbolic. If it were a DK it would be elliptical. There are only four conic sections as any student of euclidian analytic geometry will tell you, the fourth being a circle, and that of course Is utilized in Schmidt and Maksutov designs whose primaries are spherical (circular cross-section). It seems to me a simple matter to determine the actual figure of one of the GSO CCs: use a Foucault tester on the primary. Has anyone done this? If they have, I must have missed it. If not, perhaps someone can perform such a test and post pictures of the results. Assuming that GSO is telling the truth and that these primaries are indeed parabolic, (and I have no reason to assume they would lie), I really can’t understand the possibility of large scale optical inconsistency. They have been making Newtonians for some time and their Newtonians get consistently good reviews. I had one of their 6” F6s for a while and it was quite good. The only reason that I got rid of it was that it was pretty redundant as I also have a 6” F4.5. So I really don’t see where this concept of inconsistent optical quality is coming from. It sounds more to me like inconsistent satisfaction based on people either liking or not liking the design, or not having enough experience with the design to make an informed evaluation. It seems to me that the most experienced users are the ones giving these CCs the best reviews. I appreciate that and have no trouble accepting it given a their more complex and uncommon design.

 

And with regard to the Foucault testing of the primary, this is not the first time I have asked this question in this particular thread:

 

https://www.cloudyni...29#entry9422903
 

https://www.cloudyni...28#entry9418813

 

And it has yet to be satisfactorily answered.


Edited by Terra Nova, 25 October 2020 - 11:53 AM.

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#1229 jgraham

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Posted 25 October 2020 - 01:27 PM

“...and if you took images through the CC and looked through the RC, would things change? You're really in the position to compare and find out differences apart from specs from the provider”

 

Welll... I actually did quite a bit of imaging with my CC8. I was so happy with the results that I decided to pick up the RC8. After the RC8 arrived the CC8 went back to being a visual scope. I liked them both so much that I upgraded the focuses to Moonlites. That’s the Sign that I have decided a scope is a keeper... it gets a Moonlite. :)

 

I am planning on taking some more images with the CC8 and to spend some time looking through the RC8, something that I haven’t done yet. If I get a chance I’ll set them both up at the same time and configure them as close as I can get with the same effective focal length (magnification).

 

I’ve been enjoying this hobby for 58 years and the to-do list just doesn’t get any shorter... and that’s a good thing.


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#1230 Bomber Bob

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Posted 25 October 2020 - 02:52 PM

So I really don’t see where this concept of inconsistent optical quality is coming from.

 

The mirror sets may be consistently good between units, with the inconsistencies due to assembly problems.  We've seen that with SCTs, such as correctors off-center, secondaries off-center, etc.  New owners who lack experience with this scope type may not know what to check, and/or how to correct issues.  There are Newt owners who struggle with collimation, and it's a simpler reflector type.



#1231 Garyth64

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Posted 25 October 2020 - 03:53 PM

Gary, the primary measures approximately 150 mm but we figured the scope to be working at 135-138 mm due to the secondary reducing the primary size by being undersized and the light cone missing some of it. I am not talking about the light it blocks by size. So it's the thru put of light that should be advertised not the primary physical size.

Mike

As I said, it doesn't work like that.

 

Sure if the secondary is too small, then the light from the primary is reduced.

But after going thru this very long discussion, I have found many, many errors, assumptions, and comments made with no facts to support them.

I am burned out, there is just so much to try and explain and comment on.  There is a lot of misinformation in those pages. I'll just pick a couple.

 

Larry Carlino's review has been misread.  This is what he said:

 

"With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable. The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches."

 

What he was saying in that in order to increase the image contrast, he would want to reduce the size of the secondary, and that would reduce the effective aperture, because not all the light of the primary would hit the secondary.  Many people want to subtract the size of the secondary from the primary and then come up with a "effective" aperture.  In that sense it doesn't work that way.  I think many people got confused over this.

 

Another thing, I need to correct.  This following statement is wrong:

 

"Basic rule is if you have an F4 for primary the secondary will be a f3 to make it at f12 system
To figure out moving the secondary you need to times it by itself and that will tell you how far most the back focus so 3×3 = 9 so for every 1 mm it should move the back focus 9 mm. So doing the math 18×9 it moves you back focus 162 mm."

 

It doesn't work like that either.  It is not a ratio.  The primary f-ratio times the amplification of the secondary (not it's f-ratio) gives the overall f-ratio of the system. 

In the 8" f/12 CC, the tube is only 21" long.  Some thought that the primary would be an f/4.  It cannot.  It is most likely a f/3.

As an example, I created a system that will work within the 21" of tube.  If the primary is f/3, and the amp of the secondary is 4x, then the system becomes a f/12.  What could be the position of the secondary?  Let's pick 7" inside the image plane of the primary, this is p.  If so, then the image plane would be 11" behind the primary.  The mirrors are separated by 17".  The distance from the secondary to the image plane behind the primary is 28", this is p'.

p'/p equals the amplification of the secondary.  With p = 7", and p' = 28" the radius of curvature of the secondary would be 18.6". 

What size secondary should there be?  Thanks to Texereau, I come up with a secondary diameter of 2.48".  So the secondary for this example is a f/3.76.

Now that I have all those parameters, I can figure out how much the image plane will move when I move the secondary.

 

The above quoted statement I said is wrong.  In my mock up system, if I move the secondary .1" toward the primary, the image plane behind the primary will move 2".  If I move the secondary .2" the image plane will move 3.88".  It is not a ratio!  (if you want the formula, I will give it.)

 

And this "effective" aperture concept, where the physical size of the secondary is subtracted from the physical size of the primary, should be discarded. It is misleading and confusing to many. 

I can see dividing the area of the secondary, by the area of the primary, to come up with percentage of light lost. 

In the advertisement, for the 8" f/12 CC, it is said that the CO is 33%.  That means the secondary has a diameter of 2.6", which only blocks 10.5% of the incoming light to the primary.

 

Sorry for coming so late into the game.


Edited by Garyth64, 25 October 2020 - 03:54 PM.


#1232 quilty

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Posted 25 October 2020 - 03:56 PM

“...and if you took images through the CC and looked through the RC, would things change? You're really in the position to compare and find out differences apart from specs from the provider”

 

Welll... I actually did quite a bit of imaging with my CC8. I was so happy with the results that I decided to pick up the RC8. After the RC8 arrived the CC8 went back to being a visual scope. I liked them both so much that I upgraded the focuses to Moonlites. That’s the Sign that I have decided a scope is a keeper... it gets a Moonlite. smile.gif

 

I am planning on taking some more images with the CC8 and to spend some time looking through the RC8, something that I haven’t done yet. If I get a chance I’ll set them both up at the same time and configure them as close as I can get with the same effective focal length (magnification).

 

I’ve been enjoying this hobby for 58 years and the to-do list just doesn’t get any shorter... and that’s a good thing.

That's what my question was about. So after a while you really can tell, apart from all advertising stuff, if the two of them differ significantly visually and at imaging power. 

Terra proposes the Foucault test to tell if the mirror is hyper or para. I don't mind that and I think both are fine as long as the overall design fits. But the torch test is so easily done and if everyone owning a CC or RC scope did it we would properly know about CO and aperture and consistency at that. And till now I seem to be the only one who tried to directly measure or read the primary focal length which is still puzzling me a bit. It's not that complicated, we're facing full moon soon and you'd just read at what distance from the primary the moon displays sharply on a white card. Combined with the torch results (consistency) we knew more about too small secondarys or too long primarys, me at least. And if this applies to the CCs only or to both.

And Jóhn, fine that you're happy with your two cassegrain scopes. Maybe this can be sorted out by knowing better about themcool.gif  

Anyone thinking to be at good health needs to see the doctor, even your president



#1233 quilty

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Posted 25 October 2020 - 04:37 PM

 

 

Larry Carlino's review has been misread.  This is what he said:

 

"With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable. The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches."

 

What he was saying in that in order to increase the image contrast, he would want to reduce the size of the secondary, and that would reduce the effective aperture, because not all the light of the primary would hit the secondary.  Many people want to subtract the size of the secondary from the primary and then come up with a "effective" aperture.  In that sense it doesn't work that way.  I think many people got confused over this.

 

 

and this is nonsense. 1. the RCs are even shorter and CO doesn't seem to be an issue. The longer the scope the less that problem, so why castrate the CC and not the already existing RC?

2. When aperture already is controlled by secondary size, its further reduction doesn't reduce CO but aperture

3. the 8CC CO is 36% as far I know, the sec dia is quoted as 67 mm, assuming the primary baffles of 6 and 8 inch scopes are the same.

 

I read no one who just substracted sec size from primary size to get the aperture. But I think, CO reduces not just contrast but definition as well. 

F numbers and mirror distance. Most logical for f/12 is that the primary is f/3 (it is about that) and the secondary 4x. When moving the secondary the focus shift may be 4 sqare or 12 (3x4), you might know exactly. I shifted my secondary by 1mm (roundabout) to find a backfocus reduction of 15mm. My point was if I would increase the mirror distance by 18 mm which are necessary to completly catch the primary inside the secondary the focus would be shifted into the tube, so not feasible unless the secondary curvature increases to about 4.3x at the same time. I don't see what should be wrong about that. 

Once agan: My primary seems to be rather f/3.3 or f/3.2 than f/3 at I'm still waiting for a clue to that.



#1234 Spacedude4040

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Posted 25 October 2020 - 06:29 PM

Hey late to the game smile.gif .................read what I wrote in red. I'm not talking about surface area here.

 

"Spacedude4040, on 24 Oct 2020 - 8:09 PM, said:

    Gary, the primary measures approximately 150 mm but we figured the scope to be working at 135-138 mm due to the secondary reducing the primary size by being undersized and the light cone missing some of it. I am not talking about the light it blocks by size. So it's the thru put of light that should be advertised not the primary physical size.

    Mike"

As I said, it doesn't work like that.

 

Sure if the secondary is too small, then the light from the primary is reduced.

But after going thru this very long discussion, I have found many, many errors, assumptions, and comments made with no facts to support them.

I am burned out, there is just so much to try and explain and comment on.  There is a lot of misinformation in those pages. I'll just pick a couple.

 

Larry Carlino's review has been misread.  This is what he said:

 

"With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable. The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches."

 

What he was saying in that in order to increase the image contrast, he would want to reduce the size of the secondary, and that would reduce the effective aperture, because not all the light of the primary would hit the secondary.  Many people want to subtract the size of the secondary from the primary and then come up with a "effective" aperture.  In that sense it doesn't work that way.  I think many people got confused over this.

 

This is what GSO said in blue which is a reduction of primary mirror! Larry wrote the black text before and after it.

"With the advertised high-reflectivity mirrors, the GSO should have roughly the same magnitude penetration and image brightness as the C-8. I contacted the dealer, Agena Astro, to explain the situation, and they immediately queried GSO for the technical details. The response was detailed and quite interesting.

The design of the telescope is a compromise that gives priority to high-power lunar and planetary performance. With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable. The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5mm). According to the factory specs, the 100 percent illumination circle is 15mm. The sparkling planetary views are the result, but light grasp obviously suffers. This is reminiscent of the situation with the Synta 127mm Mak-Cass that, from numerous reports and measurements, has a clear aperture of about 118mm."

 

Another thing, I need to correct.  This following statement is wrong:

 

"Basic rule is if you have an F4 for primary the secondary will be a f3 to make it at f12 system
To figure out moving the secondary you need to times it by itself and that will tell you how far most the back focus so 3×3 = 9 so for every 1 mm it should move the back focus 9 mm. So doing the math 18×9 it moves you back focus 162 mm."

 

It doesn't work like that either.  It is not a ratio.  The primary f-ratio times the amplification of the secondary (not it's f-ratio) gives the overall f-ratio of the system. 

In the 8" f/12 CC, the tube is only 21" long.  Some thought that the primary would be an f/4.  It cannot.  It is most likely a f/3.

As an example, I created a system that will work within the 21" of tube.  If the primary is f/3, and the amp of the secondary is 4x, then the system becomes a f/12.  What could be the position of the secondary?  Let's pick 7" inside the image plane of the primary, this is p.  If so, then the image plane would be 11" behind the primary.  The mirrors are separated by 17".  The distance from the secondary to the image plane behind the primary is 28", this is p'.

p'/p equals the amplification of the secondary.  With p = 7", and p' = 28" the radius of curvature of the secondary would be 18.6". 

What size secondary should there be?  Thanks to Texereau, I come up with a secondary diameter of 2.48".  So the secondary for this example is a f/3.76.

Now that I have all those parameters, I can figure out how much the image plane will move when I move the secondary.

 

The above quoted statement I said is wrong.  In my mock up system, if I move the secondary .1" toward the primary, the image plane behind the primary will move 2".  If I move the secondary .2" the image plane will move 3.88".  It is not a ratio!  (if you want the formula, I will give it.)

 

And this "effective" aperture concept, where the physical size of the secondary is subtracted from the physical size of the primary, should be discarded. It is misleading and confusing to many. 

I can see dividing the area of the secondary, by the area of the primary, to come up with percentage of light lost. 

In the advertisement, for the 8" f/12 CC, it is said that the CO is 33%.  That means the secondary has a diameter of 2.6", which only blocks 10.5% of the incoming light to the primary.

 

Sorry for coming so late into the game.

The rest of the quoted statements are not mine and it sure looks like you make it that way. I don't believe that anybody was reducing the primary light gather ability by deducting the area of the secondary if my memory serves me well.

Also I cant believe you powered thru those pages to make a point lol.

Mike
 


Edited by Spacedude4040, 25 October 2020 - 06:43 PM.


#1235 Garyth64

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Posted 25 October 2020 - 06:48 PM

For discussion, let's keep the primary at f/3, and to discuss the changes you want to do, I still can use my example parameters as it will give you a clue as to what to expect.

 

So what you want to do it to increase the separation of the mirrors, you can only do that by moving the secondary away from the primary.  In my example, p was 7", and p' was 28", the amp is 4x, the separation of the mirrors was 17", RoC sec is 18.6".

 

Now p will be 6.25".  Where will p' be?

 

p' =  -RoC x p        =  19".    The focus will still be inside the tube.  So that will not work.         

         RoC - 2p

 

You said, "When moving the secondary the focus shift may be 4 sqare or 12 (3x4)".

 

-No, that is not how you find the amount of focus shift.  You would be moving the secondary 3/4", and the focus shift would be 9" into the tube.



#1236 Garyth64

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Posted 25 October 2020 - 06:58 PM

Hey late to the game smile.gif .................read what I wrote in red. I'm not talking about surface area here.

 

"Spacedude4040, on 24 Oct 2020 - 8:09 PM, said:

    Gary, the primary measures approximately 150 mm but we figured the scope to be working at 135-138 mm due to the secondary reducing the primary size by being undersized and the light cone missing some of it. I am not talking about the light it blocks by size. So it's the thru put of light that should be advertised not the primary physical size.

    Mike"

The rest of the quoted statements are not mine and it sure looks like you make it that way. I don't believe that anybody was reducing the primary light gather ability by deducting the area of the secondary if my memory serves me well.

Also I cant believe you powered thru those pages to make a point lol.

Mike
 

I quoted from Larry's post, and thanks for posting the rest of it.  I didn't think I had to.

 

The other quote is all I quoted from you, ""Basic rule is if you have an F4 for primary the secondary will be a f3 to make it at f12 system
To figure out moving the secondary you need to times it by itself and that will tell you how far most the back focus so 3×3 = 9 so for every 1 mm it should move the back focus 9 mm. So doing the math 18×9 it moves you back focus 162 mm."

 

That is the only statement of yours I said was incorrect.

 

Yep, I read (suffered) thru it all.  You tasked me to do it my saying the questions I asked were all in the back pages.  I powered thru it because of the errors that should be corrected.


Edited by Garyth64, 25 October 2020 - 07:00 PM.


#1237 Garyth64

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Posted 25 October 2020 - 07:30 PM

I just hope that some of you will be able to learn and figure out all the math on Cassegrains and Gregorians.

 

You can even figure out the secondary size needed in those systems and Newtonians, either for visual or photography.

 

All the formulas and explanations are in Texereau's book.

 

My recent interest in this thread is because I will be buying the 8" f/12 CC, whenever they are in stock again.


Edited by Garyth64, 25 October 2020 - 07:32 PM.


#1238 Spacedude4040

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Posted 25 October 2020 - 07:33 PM

I quoted from Larry's post, and thanks for posting the rest of it.  I didn't think I had to.

 

The other quote is all I quoted from you, ""Basic rule is if you have an F4 for primary the secondary will be a f3 to make it at f12 system
To figure out moving the secondary you need to times it by itself and that will tell you how far most the back focus so 3×3 = 9 so for every 1 mm it should move the back focus 9 mm. So doing the math 18×9 it moves you back focus 162 mm."

 

That is the only statement of yours I said was incorrect.

 

Yep, I read (suffered) thru it all.  You tasked me to do it my saying the questions I asked were all in the back pages.  I powered thru it because of the errors that should be corrected.

OK so we have a f4 primary with a 3x mag secondary. How much would the back focus change if you moved the secondary 1mm towards the primary?

Mike



#1239 Garyth64

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Posted 25 October 2020 - 07:38 PM

OK so we have a f4 primary with a 3x mag secondary. How much would the back focus change if you moved the secondary 1mm towards the primary?

Mike

What are the other parameters?  What is the distance of the secondary inside the focus of the primary.  What would be the current distance between the secondary and the focus behind the primary?    What are p and p"?

 

Or do you want me to make up another example?

 

It won't take me too long to answer you back.


Edited by Garyth64, 25 October 2020 - 07:39 PM.


#1240 Spacedude4040

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Posted 25 October 2020 - 07:48 PM

Make up another example please



#1241 Garyth64

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Posted 25 October 2020 - 07:55 PM

Already did the math.  Do you want to see all the math?

 

if you move the secondary 1mm, or .039" towards the primary, the focus will shift .27".

 

You can move the secondary a little, but if it gets moved too much, you could end up messing with the original parameters that the system was originally designed for.

 

That's what happened to my Gregorian years ago.  In my design, I didn't have enough back focus.  So I moved the secondary so  the back focus increased by about 8".  In the Gregorian, the secondary is nulled with light source at p, and KE or ronchi at p'.  I changed the parameters and it changed the correction on the system.  In my other thread, I showed that I adjusted the mirror cell, and changed the focuser.  Hopefully, this will be enough to get it back where it should be.

 

I also have an 8" Cassegrain optics set, that I have yet to assemble as a complete Cass.  It has a f/4.5 primary, the system is a f/18.  The optics were made in the basement of the Alder Planetarium back in the 80's.  I took the Gregorian optics out of it's tube, and installed the Cass primary to use it as a f/4.5 RFT.  It would be a convertible system.  My avatar shows the RFT.  You can see the two focusers.  I never installed the Cass secondary.  I just recently decided to reinstall the Gregorian optics into that tube, it's original tube.


Edited by Garyth64, 25 October 2020 - 08:22 PM.


#1242 Spacedude4040

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Posted 25 October 2020 - 08:22 PM

So if we move farther out like 10mm does it equals 2.7" or does it increase. If you don't mind



#1243 Garyth64

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Posted 25 October 2020 - 08:25 PM

Hang on, it will only take me a few minutes.



#1244 Garyth64

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Posted 25 October 2020 - 08:32 PM

Moving the secondary 10mm, or .39", the focus shift is 3.79"


Edited by Garyth64, 25 October 2020 - 08:32 PM.


#1245 Garyth64

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Posted 25 October 2020 - 08:40 PM

RC = 2 x p x p'

            p' - p

 

also, RC = 2 x p x amp

                     amp - 1

 

Amp = p'/p

 

position of the secondary:   p =  fl + b  

                                                   amp -1

 

Secondary diameter =  (D - a)  x  p     + a

                                              fl

 

diameter of focal plane = fl x .009

 

RC = radius of curvature of the secondary

p = distance of secondary inside the focus of the primary

p' = distance of the secondary to the focus behind the primary

amp = amplification of the secondary

fl = focal length of the primary

b = distance behind the primary from it's surface to the focus behind the primary

D = diameter of the primary

a = diameter of the image plane at the primary focus  (for visual only).  This will be larger for photography.

 

That's about it.


Edited by Garyth64, 25 October 2020 - 08:43 PM.

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#1246 doug mc

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Posted 25 October 2020 - 08:55 PM

Took this from Sidgwicks handbook. Could fix all our cassegrain problems. Anyone done this?

Using a transfer lens to reduce CO and gain full aperture. 

Attached Thumbnails

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Edited by doug mc, 25 October 2020 - 08:59 PM.


#1247 Garyth64

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Posted 25 October 2020 - 09:12 PM

Why are you losing full aperture?

 

The full aperture is only reduced if the secondary isn't large enough to collect all the light from the primary.



#1248 quilty

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Posted 26 October 2020 - 06:19 AM

For discussion, let's keep the primary at f/3, and to discuss the changes you want to do, I still can use my example parameters as it will give you a clue as to what to expect.

 

So what you want to do it to increase the separation of the mirrors, you can only do that by moving the secondary away from the primary.  In my example, p was 7", and p' was 28", the amp is 4x, the separation of the mirrors was 17", RoC sec is 18.6".

 

Now p will be 6.25".  Where will p' be?

 

p' =  -RoC x p        =  19".    The focus will still be inside the tube.  So that will not work.         

         RoC - 2p

 

You said, "When moving the secondary the focus shift may be 4 sqare or 12 (3x4)".

 

-No, that is not how you find the amount of focus shift.  You would be moving the secondary 3/4", and the focus shift would be 9" into the tube.

50 pages now. Isn't it great for such a simpe scope?

Gary, I don't completely understand your equations, I'm just using "ray set" (is that correct?)

 

Do you agree that if the effective aperture is 14 mm short, a mirror distance shift of +18 mm wll sort that out? (Or maybe 21 (7x3) mm), in my memory it was just 18 mm but that's beside the point

If so, then it was my fast estimation to increase the power of the sec to the same extent for compensation. Maybe it doesn't work that easily and you know better, but can you agree, that the curvature of the secondary must then be increased to an extent that the focus is behind the tube again?

So, what extent would that be? What sec power is needed to achieve that?



#1249 Spacedude4040

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Posted 26 October 2020 - 06:20 AM

Moving the secondary 10mm, or .39", the focus shift is 3.79"

Thanks for the math Gary, if I was designing a system that's the way I would go. When I'm setting up the optics knowing the mirror spacing and back focus I just use the secondary magnification times itself to guide me to move the secondary a mm or two to bring the moon into focus. Works for me smile.gif

Mike

 

Thinking about it now.................. I would search out a Cassegrain spreed sheet because I'm lazy smile.gif


Edited by Spacedude4040, 26 October 2020 - 06:47 AM.


#1250 quilty

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Posted 26 October 2020 - 07:04 AM

@ spacedude

Mike, I remember you did a detailed sketch of the 6 inch CC with exact dimensions which we finally agreed and which I cannot find again. It was a thread, would you share the link? My notes were not planned to endure 50 pages smile.gif




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