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  #21  
Old 26-05-2022, 01:32 PM
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Originally Posted by bojan View Post
With the help of Astrometry.net, I calculated the actual focal reduction (standard SC-T2 camera adapter, backfocus 105~114 mm ) to be 0.589 (F/5.9).
So - next attempt will be with 85 mm backfocus.
Hi Bojan,

Also that reduction ratio of 0.589 you determined is very consistent with the ratio I would expect from the reported image circle diameter of the Celestron Reducer ( assuming it to be the same/similar to your Meade reducer). i.e: As per my previous post quoting a "24mm fully corrected image circle" with the Reducer versus presumably (Celestron don't say) a Full Frame Image Circle of the native C11 Telescope making the reduction ratio something like Full Frame (42 to 42.3mm) down to the 24mm stated or ....

24/42 ~= 0.57 (Close enough for Government Work )

Best
JA
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  #22  
Old 26-05-2022, 02:15 PM
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Yes..

I definitely overdid it (by going for 105 mm backfocus).

I am expecting acceptable performance at ~F6.3.
However, for very small and low surface brightness objects, f5.5 will be quite OK so all this was actually a useful exercise :-).
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  #23  
Old 26-05-2022, 03:13 PM
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redbeard (Damien)
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Hi Bojan, here is the pic.


The distance from the FR to the sensor on my rig is about 125mm. Would suggest even trying a few mm less as experiment. Not saying it's in the best position, but that is what the later photos uploaded earlier is using.
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  #24  
Old 26-05-2022, 03:58 PM
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Originally Posted by redbeard View Post
Hi Bojan, here is the pic.
The distance from the FR to the sensor on my rig is about 125mm. Would suggest even trying a few mm less as experiment. Not saying it's in the best position, but that is what the later photos uploaded earlier is using.

Damien, Thank you..
So, ~125 mm backfocus in your case and good result.. and mine is awful with this distance. Maybe they are different after all...


BUT - on the image it says 105 mm is sweet-spot for FR.


I will try to use Astrometry.net to see what is your camera FOV to calculate f/number.

Last edited by bojan; 26-05-2022 at 04:18 PM.
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  #25  
Old 26-05-2022, 04:59 PM
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According to Astrometry.net, your f/number is f/5.32 (assuming the FL of our scopes are identical, which they are not.
This is consistent with ~125 mm backfocus..

C11
Size: 27 x 18 arcmin
Radius: 0.270 deg
Pixel scale: 0.312 arcsec/pixel


C11_FR 114 mm
Size: 48.5 x 32.3 arcmin
Radius: 0.486 deg
Pixel scale: 0.561 arcsec/pixel
Reduction: 0.55
F/5.5


C11_FR 103 mm
Size: 45.8 x 30.6 arcmin
Radius: 0.459 deg
Pixel scale: 0.531 arcsec/pixel
Reduction: 0.589
F/5.9


Meade 11, Celestron 6.3 FF/FR, backfocus 125 mm
Size: 50.8 x 33.8 arcmin
Radius: 0.508 deg
Pixel scale: 3.74 arcsec/pixel
reduction: 0.5315
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  #26  
Old 26-05-2022, 09:04 PM
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That's interesting.
Nice work!


These are the specs for my scope:

Clear aperture........................... ...................254mm (10")
Focal length ................................... .............2500mm
Focal ratio (photographic speed) ................f/10
Resolving power ................................... .......0.45 arc sec




I wonder if coma gets worse the higher the F.
I think they are designed to give the best flat field at 6.3.



I kinda like the idea of F/5.32 or something faster than 6.3 so next outing, I'll redo some pics and then use the shorter Moonlite flange and put the FR back into the train at the sweet spot if possible and see how that looks. I'll do tests with the Hirsch one too. It could be months though before I get to the scope.


Cheers,
Damien.
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  #27  
Old 26-05-2022, 09:56 PM
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I noticed on your image I submitted to Astrometry that star shapes corresponds to upper case (on attached picture).. so distance is too large (you will have to decrease it somehow to get closer to ideal value.. not sure how to do ity with your focuser without serious modification.. maybe I am wrong).

So, taking into account your FL is 2500 mm, your effective f/number is closer to 6... but still not 6.3.. (submit to Astrometry one frame taken without FR to get exact frame size) which is irrelevant because it is image quality in corners that matters (IMO), and your combination is way better than mine as it is now.


Anyway.. time is on our side :-)


CS,
Bojan
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  #28  
Old 27-05-2022, 04:30 PM
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I made the low profile adapter for 85 mm backfocus with FF-FR and Canon camera..
Tube is 3D printed some time ago "just in case" (dimensions were taken from standard Celestron eyepiece adapter), M42x1 thread was glued in place (originally came from ebay (M39 -> M42 ring adapter)), and M42-EOS adapter I had already. This should be good for test, the "real thing" (it will require some machining to adjust the length) is on its way.
Weather looks good for Sat evening..
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Last edited by bojan; 27-05-2022 at 04:51 PM.
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  #29  
Old 29-05-2022, 07:32 PM
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Comparison between Meade (mine) and Celestron (courtesy of JA) FF-FR's at 85 mm back focus.

Centre is OK, but in corners of the APS frame Celestron is way better, similar to Damien's specimen.
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  #30  
Old 30-05-2022, 06:59 AM
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I also found this page.. Author did much better job, testing more brands.
Celestron 0.63 test result is consistent with Damien's specimen, but my result is different.

I also found that behavior in frame corners (APS.. FF will be even worse) is not much dependent on working distance...

It seems that the "working distance" value for FF-FR everybody is concern with only ensures f/number (which is pretty unimportant parameter actually) but not the flat field (it actually worsen it).

I think in the next step of wasting my time I will try a simple doublet from old binoculars (FL =~21 cm).

Last edited by bojan; 30-05-2022 at 07:34 AM.
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  #31  
Old 30-05-2022, 12:00 PM
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Quote:
Originally Posted by bojan View Post
Comparison between Meade (mine) and Celestron (courtesy of JA) FF-FR's at 85 mm back focus.

Centre is OK, but in corners of the APS frame Celestron is way better, similar to Damien's specimen.
Hi Bojan,

That's an interesting comparison. :

Some observations based on the Centre image:
1. I would say that the Meade is a little sharper or possibly(?) slightly better focused. The stars are definitely smaller, more than the ~5% difference (see below) in magnification/FOV would suggest.

2. Assuming that the images are shown at the same screen magnification, is that the Meade offers an approximately 5% wider field of view, based on my measurements between a couple of reference stars I chose (92mm on the Celestron Left Hand Side Image V 88mm on the Meade Right Hand Side), which equates to ~<5%.
Some observations based on the Corner image:
1. I agree with you on the issue of definetly better corner performance on the Celestron with the APS sensor, but that may simply be because 85mm is not the sweet spot for the Meade reducer, whereas it appears to be with the Celestron. Maybe you could continue to tune reducer/sensor distance to try to find the optimum distance for the Meade. This could be more easily achieved if the M42 spacer you made was adjustable like a helicoid focuser. There are infact M42x1 helicoid focusers on eBay in the $30-40 range which have either 12-17mm, 17-31mm or (32-55mm?) of adjustment range. You could somehow mount your Schmidt Cassegrain Female coupling ring to the helicoid focuser and have such an adjustment system.
I'm Always curious: One thing I notice is that there is a definite difference in contrast between the Meade and Celestron Images. Were the same exposure settings used for all images? It is possibly caused by more light scatter or perhaps some nearby cloud scattering light - or different lighting conditions ...possibly changing between the Meade and Celestron images ???.

Best
JA

Last edited by JA; 30-05-2022 at 12:26 PM.
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  #32  
Old 30-05-2022, 12:26 PM
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Quote:
Originally Posted by JA View Post
...Were the same exposure settings used for all images?
Yes, but I started soon after sunset so there was still a bit of sky glow, sorry :-)

Also there was a bit of wind from time to time so this could also contributed to stars diameter, along with focus.. which was different between C and M.

As to sweet spot.. Celestron FR didn't show much difference between 85 and 115 mm back focus (comparison attached below).. I was considering buying the adjustable focuser, but decided to try with fixed tubes first.

I will do some more work on the next opportunity, this time I had to stop because of clouds rolled in (when I closed the ROR they went away).
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  #33  
Old 30-05-2022, 12:35 PM
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Yes, but I started soon after sunset so there was still a bit of sky glow, sorry :-)
Ah...now I see

Good luck with the project.

Best
JA
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  #34  
Old 01-06-2022, 05:10 PM
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I stumbled on this discussion when searching..

It occurred to me that my FF/FR was not assembled properly...
Worth to check on first occasion...

Well, I finally checked above after seeing this discussion on CN....

After softening the glue around retaining ring with acetone I found the convex surface of the front element was inside and flat surface outside towards the scope.
Hoping for the best now (when weather permits).
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Last edited by bojan; 01-06-2022 at 05:29 PM.
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  #35  
Old 01-06-2022, 06:42 PM
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After softening the glue around retaining ring with acetone I found the convex surface of the front element was inside and flat surface outside towards the scope.
Hoping for the best now (when weather permits).
That's possibly great news - I hope it works out well.

Best
JA
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  #36  
Old 01-06-2022, 08:49 PM
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Originally Posted by JA View Post
That's possibly great news - I hope it works out well.
Best
JA

Yes, I hope so too. Corners of the APS frame were way out of focus, so something must have been very wrong, very much outside normal tolerances. Maybe I have problem with rear group as well.. but now it is easy to dismantle retaining ring in case performance is still not OK.
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  #37  
Old 02-06-2022, 05:45 PM
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Hmm..
Looking at Telescope Optics website (relevant image from there attached), this OSLO simulation doesn't correspond with published FR-FF cross section in Meade data sheet (visible also on list of surface data).
According to simulation, Meade's "back" group should be placed to front, and "front" group should be placed to back and reversed.
Anyway, proof is in the pudding, they say..
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Last edited by bojan; 03-06-2022 at 10:28 AM.
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  #38  
Old 03-06-2022, 11:26 AM
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Laser test..
Meade FR groups configuration based on reflections from surfaces is now consistent with data sheet (Thick-Thin, gap, Thick-Thin).. but not with simulation..

Celestron configuration is not so obvious (probably due to different glass and coating applied), but with some effort it appears similar and closer to simulation model.
They are not optically identical design, that is for sure.
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Last edited by bojan; 04-06-2022 at 09:19 AM.
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  #39  
Old 20-06-2022, 05:49 PM
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Finally, clear night on Saturday..
Yes, the front element of my Meade FR was flipped over, and now after repair it behaves significantly better.
It is quite usable inside ~15mm circle (no visible coma) but outside of this the filed is seriously warped. So - cropping is still the only option to deal with this.


Celestron FR-FF is way better (in terms of field flatness.. possibly in general as well).
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Last edited by bojan; 20-06-2022 at 06:14 PM.
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