Celestron & Meade marriage...

[bojan]

Well, only here in my backyard - I attached new Meade 4000 0.63 FR to C11.
The working distance is not quite right (I have yet to determine is it too long or short), vignetting and out of focus is visible on the attached crops (trapesium in the centre and star in the corner, from the same frame)..

But the difference between "with" and "without" in terms of illumination is pretty huge (well, ~2x... both exposures were 15sec).
All images are single frames, taken at ISO6400.
Weather was OK before clouds rolled in (with rain), all 6 stars in Trapesium were clearly visible, both visually and on photo.

[Sunfish]

A shotgun wedding surely.

I purchased an 0.63 reducer for my little C8 for no other reason than field of view at the time and a vague notion that faster was better. I think it was more expensive also. But it has worked all round.

Good to see that it works well on larger SCT. Other than the pain of reflections early on I don’t think I take it off. It also seals up the back of the old scope and protects the sensor.

Are there some small DSO targets where it would be better without. .? Perhaps eyepiece projection in that case..?

[raymo]

Oodles of small planetary nebulae, many of which are quite bright, and
some have interesting structures.
raymo

[Sunfish]

Thanks Raymo.

I think there are some of those high in the North I can point at when it clears up.

I did find a galaxy above the trees but it was small and faint , particularly with the moon out.

Bright planetary nebulae in particular would stand out.

[redbeard]

Just for fun I have the opposite, a Meade LX200GPS and a Celestron FR 0.63. :)

There were a few brands 0.63 focal reducers that were very similar at some stage, Meade, Celestron and a couple of others. I use mine most of the time as faster and wider. Not for planets though. Getting the right location in the imaging train helps a lot. Here is some info I found on the web a while ago and had saved on the HD. I think the site is gone but here is the text.
Cheers, Damien.




********
"
The information on this page page has been gathered from from many sources such as Russel Croman's white paper on proper placement of focal reducer for RC scopes. My purpose is to capture best practices when I find them, and publish them here. Personal web sites come and go, so I don’t want to link to it and not find it at some future date.

The relation between the reduction factor R, the focal length Fr of the reducer and the distance D between the lens and the focal plane is:

R = 1 - D/Fr

The Meade/Celestron F/6.3 reducer has a focal length of about 230 mm. Therefore, the nominal distance between the lens and the CCD focal plane is about D = FR*(1 - R) = 230*(1 - 0.63) = 85 mm. It is only at this distance, that this reducer gives its nominal reduction factor of 0.63x. The design point of the reducer is matched the the type of scope and the focal ratio. If the reducer does not have the right spacing, it changes the reduction factor and starts introducing coma distortion.

The real point being made hear is that there is a correct distance from the CCD focal plane to the center of the Focal Reducer. This is not dependent on focal length of the scope or the aperture of that scope. It is specific to the focal distance of the focal reducer.

SCT scopes can not use image scale calculators commonly found on the Internet because the imaging train is fixed to the back of the scope, and the primary mirror is moved to achieve focus. The primary mirror moves to bring the scope into focus, thus the focal length of the scope actually changes when focusing the scope. A 2000mm focal length scope may actually vary from 2000 to 2250mm focal length over the range of focus. When the focal length changes, the point of focus behind the scope also changes.

There are two very important considerations on the SCT scope, the distance between the focal reducer and the CCD chip and where the focal reducer is placed relative to the back focus point of the scope. The first distance is fixed as calculated using the above formula. The scond calculation says that the focal reducer is placed in front of the focal point of the scope by some measure based on the reduction factor of the reducer. On refractors this is no a big problem, because the whole equipment train changes in unison with the focusing process.

The SCT has unique issues in that the focal length changes when the primary mirror is moved to bring the image in focus. My experience based on measuring the image scale shows a reduction factor closer to 7 when the imaging train is fixed to 85mm on my Celestron C-8 and the focal reducer attached directly to the back of the scope.

The distance between the camera and the reducer needs to be adjusted until the proper image scale is achieved. According to my calculations the focal reducer need to be about 1/2 inch inside the scope to achieve the proper reduction. In order to get the proper reduction I need to push the distance between the reducer and the camera closer to 95mm. The point is, you have to measure to know the proper spacing. The other thing I learned is the focal reducer needs to be as close to the back of the scope as possible to work. This is true for both the C8 and the LX200 10"

The correct distance from the focal reducer to the CCD focal plane based on image scale measurements, was closer to 95mm. My measurements are based on the center of the focal reducer not the leading edge. The center of the Celestron focal reducer is about 11mm from the back edge of the reducer. If the reduction factor is not 6.3, the focal reducer is not placed properly within the imaging train and the scopes focal point.

Placement of Focal Reducer

Y=D/R

The focal reducer must be placed a distance Y in front of the native focal point of the telescope. This distance is simply the ratio of the working distance of the reducer to the reduction factor.



Actual Working Distance

In practice it is difficult to achieve the exact working distance needed to yield the specified reduction factor. Recalling that the reduction factor is just R = 1 - D/Fr, the above equation for Y can be rewritten as:

Y=(Fr*D)/(Fr-D)

For example, here is the calculation of the working distance for my setup:

Working distance is 85 mm with a Fr =231

Then:

Y=(231*85)/(231-85)

Y=134.48

or about 5.29".

Thus the reducer optics must be 5.29" in front of the native focal point of the telescope, according to Celestron the back focus is about 5.25", which puts the reducer in side the scope to place it properly. The distance from the base of the scope cell to the center of the reducer’s optics is about 30mm, which pushes the focal reducer back even further. This offset has to be taken into consideration when calculating the proper position for the focal reducer. A more practical approach may be to place the reducer as close to the back of the scope, set the working distance from the CCD to the focal reducer, measure image scale and keep adjusting unto the proper scale is achieved. In order to get the proper image scale, I actually had to move the CCD focal plane back another 10mm beyond the calculated distance. This is with the focal reducer directly attached tot he rear cell of the C-8.

As the working distance is increased the image size on the CCD chips decreases and vignetting occurs. So it is possible to change the working distance and or reduction factor because of the equipment you are using. There are consequences for not achieving the proper spacing and proper placement of the focal reducer.

"
******


Also:
http://www.astrotarp.com/Focal_Reducer_Distance_to_Sensor_Pa ge.html

[Sunfish]

Thank you for that. Very interesting.

Measuring my celestron visual back with Pentax adaptor it is 60.3 and the same with a Meade Canon t adaptor attached. So I get a focal distance to face of adaptor of 105.76 with a Pentax which corresponds with closely with Astrotarp link but more than your text suggests. It would be troublesome to adjust without cobbling together a bunch of adaptors.

[bojan]

As was obvious on the test image I attached earlier in this thread, my combination has a problem with working distance.
This was partly because of very confusing and non-standardised terminology used by amateurs and equipment suppliers, partly because I was using adapter combination similar (or same) to this one (https://www.ebay.com.au/itm/Newest-SCT-to-2-Adapter-2-to-M42-0-75-Adapter-Canon-Camera-Mount-Adapter/173143895500?hash=item28502e2dcc:g: KBAAAOSwj1haeVei).
The required (maybe?) working distance for Meade series 4000 0.63x focal reducer is ~61mm (for Canon camera), and the one I have has ~68mm total length.

I could not find the variable adapter neither low profile T2-EOS adapter (they are all ~11mm thick), so it occured to me it should be possible to modify one of my existing low profile M42-EOS adapter (https://www.ebay.com.au/itm/M42-Lens-to-Canon-EOS-EF-Mount-Adapter-Ring-1100D-600D-60D-550D-7D-Metal-DC703/182163573415?hash=item2a69cb8aa7:g: gBQAAOSwivlcbQQU&frcectupt=true)s, by replacing the yellow part with T2 insert from thick adapter.
I need to machine the black insert to mach the yellow insert dimensions.
Will post the result of this operation when I am done.


EDIT:
Another possibility is this adapter (https://www.ebay.com.au/itm/M42-to-T-T2-M42-1mm-thread-pitch-T-T2-mm-0-75mm-Coupling-Ring-Adapter-For-Lens/351690349440?ssPageName=STRK%3AMEBI DX%3AIT&_trksid=p2060353.m2749.l2649)... It is not as thin but it may be worth trying.

[Merlin66]

Damian,
The reducer also impacts on the final effective f ratio, as does the back focus positioning.
The attached spreadsheet was developed a few years ago to make things easier.

[bojan]

Ready for machining...

[bojan]

Finished..
Now I have distance (flange-to-flange) 59mm minimum (it was ~68), and it is variable to some extent.
Now I have to experiment a bit (with shims, spacers..) to optimize the working distance.

[bojan]

After covid era and my C11 in Mt Pleasant (while I was stuck in Melbourne), I finally fount the time to test various distances (on star field near Proxima).
Images were taken with above mentioned adapter, with 4 mm spacer and standard (11 mm) EOS adapter, so the flange-to-flange distance varied between 59 and 68 mm.

I can't see any significant differences in image quality at corners of APS sensor (it is terrible) and 1/2 way off the centre (so~so) so I'm guessing I will use it only for faint and small objects and crop out the rest.
Are there any good experiences with this FF/FR?

[bojan]

After having some similar reports (from other forums), it looks like this FF-FR is only useful for small, dim targets, and only with with heavy cropping of everything outside of 1/4 distance from APS centre (so useful field diameter is ~15 mm or less).


I am thinking now about DIY - I have couple of surplus objectives from binoculars (20~25 cm FL, ø40-ø50 mm).
Mounting them inside the camera adapter tube should not be the problem.

[bojan]

I stumbled on this (https://www.iceinspace.com.au/forum/showthread.php?t=175400) discussion when searching..

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

[JA]

Hi Bojan,

I have a Celestron reducer one of these....
https://www.bintel.com.au/product/celestron-reducercorrector-f6-3/?gclid=EAIaIQobChMIgYSOh9H59wIVCZ1L BR1ZjQo1EAQYAyABEgK6h_D_BwE

You could borrow it if you like to compare with the Meade optics to see if there is any difference. I notice in the details in the link it says...
"Produces a corrected field of 24mm diameter. The back focus is 85mm, from the reducer’s back element to the cameras sensor"


Best
JA

[bojan]

Hi JA,

Thank you for the offer, yes, I would really like to try another sample.
I will PM you.

BTW, mine is this one on the link below (I bought it from that mob 3 years ago).
https://agenaastro.com/meade-series-4000-f-6-3-focal-reducer-field-flattener.html

[bojan]

In order to obtain recommended (by Bintel) 85 mm distance from FF to sensor (with Canon camera) I need to machine 27 mm long spacer, with 2"-24 and M42 threads, to fit it to FF/FR on scope side and M42x1 for M42x14 mm spacer + M42 EOS adapter.
Or, I need this adapter (https://www.ebay.com.au/itm/185306432416?ssPageName=STRK%3AMEBI DX%3AIT&_trksid=p2060353.m2749.l2649) from ebay (bought it just now), plus some M42 extenders...

[redbeard]

Some comparison shots for you Bojan.

The Orion-CANON and ETA photos were taken when I did not realise the FR needed to be in the correct place.

The Orion-SONY and EAGLE photos were taken with the Celestron 6.3 FR in roughly the rightish place ish.


Sorry bout the resolution.

[bojan]

Hi Damien,

Thank you for the photos :-)

The first pair look similar to what I am obtaining now (working distances used for my test here were 103~117 mm (105 is recommended by Meade and some other users, this is what you get using "standard" camera adapter (which is 58 mm long, + T2-> EOS adapter, as shown on attached drawing)).
Do you remember what was your first and "rightish" distance?

Celestron (? Bintel) recommends 85 mm.. 18mm less than I am currently using, huge difference...

[bojan]

Reading the Agena add for Meade 4000 series FF/FR (https://agenaastro.com/meade-series-4000-f-6-3-focal-reducer-field-flattener.html), they are now recommending 85 mm working distance (backfocus), just as Celestron (Bintel).

Picture of image train on Agena site is misleading and actually plain wrong (attached from their website). It should look more like the second picture (with ~30mm extender, not ~60 mm)


I am pretty sure, 3 years ago (and recently on Agena (https://agenaastro.com/articles/product-types/focal-reducers-guide.html)) suggested value was 105 mm (also mentioned on CN (https://www.cloudynights.com/topic/776170-focal-reducer-effect-on-back-focus-distance/) couple of times, even last year).

[bojan]

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.

[JA]

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 :D:D)

Best
JA

[bojan]

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 :-).

[redbeard]

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.

[bojan]

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.

[bojan]

According to Astrometry.ne (https://nova.astrometry.net/user_images/5930952#annotated)t, 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

[redbeard]

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.

[bojan]

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

[bojan]

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 (https://www.ebay.com.au/itm/203066920175?hash=item2f47bb4cef:g: mj8AAOSwzqVfKOMQ&frcectupt=true))), and M42-EOS adapter I had already. This should be good for test, the "real thing (https://www.ebay.com.au/itm/234549188471?ssPageName=STRK%3AMEBI DX%3AIT&_trksid=p2060353.m1438.l2649)" (it will require some machining to adjust the length) is on its way.
Weather looks good for Sat evening..

[bojan]

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.

[bojan]

I also found this page (https://stargazerslounge.com/topic/128928-sct-reducercorrector-test/).. 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).

[JA]

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

[bojan]

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).

[JA]

Ah...now I see :thumbsup:

Good luck with the project.

Best
JA

[bojan]

Well, I finally checked above after seeing this (https://www.cloudynights.com/topic/748192-disassembly-of-meade-4000-63-focal-reducer/) 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).

[JA]

That's possibly great news - I hope it works out well.

Best
JA

[bojan]

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.

[bojan]

Hmm..
Looking at Telescope Optics website (https://www.telescope-optics.net/miscellaneous_optics.htm) (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..

[bojan]

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.

[bojan]

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).