C925 optimum optical path length

[Stu]

Wasn't sure what to call this thread, so that's what I chose!

I am trying to optimise the planetary views in my C925 and am looking at various cooling solutions currently. I know my collimation needs sorting so that is high on my list, but I also understand that the best performance of these SCTs is at a certain optical path length, I believe this is achieved using a 1.25" diagonal, is that correct?

I have been using a BBHS diagonal fitted to a filter wheel and with some extensions so it works well with my Tak and just interchange the two. Would I be better using my T2 Prism with as short a light path as possible?

Hope that makes sense? Any links to info on this topic would be appreciated.

[Mr Spock]

I'm sure, having spherical a mirror, the image in the centre would be unaffected by path length. I currently use a 2" diagonal and notice no difference in performance to a 1.25".

With a SCT collimation is critical - probably for the same reason.

[Peter Drew]

As Michael has said, collimation of a SCT is probably one of the most critical factors with regard to performance, second only to seeing conditions. I can't believe that the optical path length of the 9.25" is critical enough to restrict its use to a 1.25" diagonal.   

[Stu]

Thanks both! That knocks that one off the list anyway!

I had heard that abberations (not sure which!) were increased if you strayed too far from optimum, but obviously I heard wrong!  

[Alien 13]

I dont know how an SCT differs fundamentally from a Mak but I have noticed slight difference (cant say better or worse) when imaging depending on the position of the main mirror in relation to the secondary when using a direct connection or via a 2 inch extension, I suppose with the mirror further back the light cone hitting the secondary is less steep.

Alan

[Peter Drew]

9 minutes ago, Stu said:

Thanks both! That knocks that one off the list anyway!

I had heard that abberations (not sure which!) were increased if you strayed too far from optimum, but obviously I heard wrong!  

Spherical abberation is the one that comes into play with extreem variations in focal plane position. However, I currently use binoviewers exclusively for planetary viewing which have a significant light path adjustment on my 8" and 16" SCT's and the performance has been excellent. 

[Stu]

15 minutes ago, Peter Drew said:

Spherical abberation is the one that comes into play with extreem variations in focal plane position. However, I currently use binoviewers exclusively for planetary viewing which have a significant light path adjustment on my 8" and 16" SCT's and the performance has been excellent. 

Thank you Peter  

[YKSE]

Peter is right, the excessive back focus will increase spherical aberration.

For C8 and C9.25, my impression is that this excessive back rocus rate at around 150mm, comparing to 100mm back focus of stock 1.25" SCT diagonal, 150mm coresponds about 2" mirror diagonal with a SCT adapter.

I think I can find Ken Huchinson's SCT analysis for C9.25 about impact of back focus on vignetting, loss of aperture etc later on. Meanwhile you can have a read on the interesting discussions here.

https://www.cloudynights.com/topic/467365-sct-backfocus-animations-fratio-vignettingetc/

When using binoviewer with SCT, it's important to use GPC/barlow to minimize the back focus, Eddgie mentioned that Baader GPCs were designed by Roland Christen, the 2.6x GPC was specifically for totally restore the back focus introduced by maxbright binoviewer.

[Stu]

29 minutes ago, YKSE said:

Peter is right, the excessive back focus will increase spherical aberration.

For C8 and C9.25, my impression is that this excessive back rocus rate at around 150mm, comparing to 100mm back focus of stock 1.25" SCT diagonal, 150mm coresponds about 2" mirror diagonal with a SCT adapter.

I think I can find Ken Huchinson's SCT analysis for C9.25 about impact of back focus on vignetting, loss of aperture etc later on. Meanwhile you can have a read on the interesting discussions here.

https://www.cloudynights.com/topic/467365-sct-backfocus-animations-fratio-vignettingetc/

When using binoviewer with SCT, it's important to use GPC/barlow to minimize the back focus, Eddgie mentioned that Baader GPCs were designed by Roland Christen, the 2.6x GPC was specifically for totally restore the back focus introduced by maxbright binoviewer.

Thank you Yong, I hoped you would comment as well  

[CraigT82]

Ive been doing a bit of reading around this topic myself recently, admittedly I was searching for info for maksutovs but there isn't much on those, quite a bit on SCTs though over on CN. Not sure whether or not the SCT information is transferable to MCTs.

This site is quite interesting... http://www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Ft%3D8750

The general gist I've been seeing is that excessive back focus causes overcorrection and clipping of the aperture at the extreme. It's apparently advisable to measure what the optical length is with the supplied VB and diagonal and try to match that with whatever kit you plan to use, as thats the position where optimum SA correction occurs (by design).  Also I've read it's best not to worry too much, as small changes in optical length (<50mm say) isn't going to introduce enough SA to be noticeable.

2 hours ago, Alien 13 said:

I dont know how an SCT differs fundamentally from a Mak but I have noticed slight difference (cant say better or worse) when imaging depending on the position of the main mirror in relation to the secondary when using a direct connection or via a 2 inch extension, I suppose with the mirror further back the light cone hitting the secondary is less steep.

Alan

This page has a neat summary of the difference between SCTs and MCTs... http://www.lcas-astronomy.org/articles/display.php?filename=schmidt-cassegrain_and_maksutov-cassegrain&category=telescopes

[Alien 13]

3 minutes ago, CraigT82 said:

Ive been doing a bit of reading around this topic myself recently, admittedly I was searching for info for maksutovs but there isn't much on those, quite a bit on SCTs though over on CN. Not sure whether or not the SCT information is transferable to MCTs.

This site is quite interesting... http://www.microsofttranslator.com/bv.aspx?from=de&to=en&a=http%3A%2F%2Fwww.astro-foren.de%2Fshowthread.php%3Ft%3D8750

The general gist I've been seeing is that excessive back focus causes overcorrection and clipping of the aperture at the extreme. It's apparently advisable to measure what the optical length is with the supplied VB and diagonal and try to match that with whatever kit you plan to use, as thats the position where optimum SA correction occurs (by design).  Also I've read it's best not to worry too much, as small changes in optical length (<50mm say) isn't going to introduce enough SA to be noticeable.

This page has a neat summary of the difference between SCTs and MCTs... http://www.lcas-astronomy.org/articles/display.php?filename=schmidt-cassegrain_and_maksutov-cassegrain&category=telescopes

Thanks for that info it is very useful.

Alan

[newbie alert]

Collimation plays a massive part.. I always check now before imaging

[YKSE]

Here's the pictures of Ken's analyses:

1. Edge illumination

About 70% illumation at 46mm, clearly better than about 50% for C8, so vigetting at 46mm FS should be clearly milder than C8 (even though it's not bad in C8 for visual).

 

2. Focal length/ratio change becuase of different back focus:

Focal length changes 3.1mm for every 1mm backfocus change.

The increase FL can easily be verified by the TFOV change with a same 1.25" EP in stock diagonal and a 2" diagonal, in day time.

 

3. Aperture loss when BF is too much.

So a 200mm BF should reduce 9.25" to 9".

4. Optimun BF

It's 113mm after the rear port's flat surface, a little longer than C8's 101mm.

SA increase about 1/23 for every 25mm BF deviation from the optimum, with e.g. 213mm BF (100mm increase, i.e. about 2" diagonal with 20mm extension), you'll get 4/23, even if the scope is an very good 1/8 at optimum, the increase BF wiill throw SA to 1/8+4/23=2/7, that's under 1/4 diffraction limit, you should be able to notice more bloated on-axis bright star vs the stock diagonal.

[jetstream]

The central obstruction limits the C8 to about .8 Strehl (diffraction limit) regardless of optics. What is the effect then of typical back focus on the telescopes true performance level?