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Stacking focal reducers?

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#1 Abhat

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Posted 27 September 2016 - 06:19 AM

Is it possible to stack two F/6.3 reducers on C6 SCT? Will the scope reach focus, what would be effective focal length and how would it impact the image quality?



#2 alphatripleplus

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Posted 27 September 2016 - 07:27 AM

Yes, I have done that with twin Meade f/6.3 reducers that I use for EAA on a C8; I believe it would also work on a C6. In the following thread in the EAA forum, I experimented with stacked reducers for imaging/EAA live viewing  -  Set-Up 3 and Set-Up 4 specifically used just the stacked f/6.3 reducers:

 

http://www.cloudynig...ith-a-lodestar/

 

The effective focal length of  two identical focal reducers with zero separation is halved, so that the combination of two Meade f/6.3 reducers has an effective focal length of about120mm compared to 240mm for each reducer. I used the stacked reducers to form an image at a working distance (excluding the thickness of a SCT/T-thread adapter) of about 62mm to 73mm behind the reducer closest to the camera sensor, yielding f/3.9 to f/3.3. I got the best image in terms of least aberrations using the f/3.9 configuration (Set-Up 4) as shown in post#6 of the thread.

 

I haven't tried the combination visually, but you will probably see some aberrations and vignetting at the edge of the field. For images, the 1/2 Lodestar sensor I used, which has an 8mm diagonal, is probably as large as you can go without more noticeable aberrations at the edge of the FOV.

 

Edit: The Meade f/6.3 reducers I used were both the old made in Japan versions. I haven't tried the newer Chinese made Meade or Celestron f/6.3 reducers, but in principal the results should be similar if optical quality is the same. I would be interested to learn of your experiences if you try stacking.


Edited by alphatripleplus, 22 May 2019 - 08:13 AM.

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#3 Jim Davis

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Posted 27 September 2016 - 07:52 AM

Might be easier to find an old f3.3 focal reducer. They come up used every so often. I don't think anyone actually uses one anymore, they just have them sitting around. Put up a "wanted" ad in the classified.



#4 Eddgie

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Posted 27 September 2016 - 08:05 AM

Is it possible to stack two F/6.3 reducers on C6 SCT? Will the scope reach focus, what would be effective focal length and how would it impact the image quality?

You did not say you were going to image, but if you are going to use this visually, there will be heavy vignetting and possibly some aperture loss and if you are going to image, the fully illuminated field will only be suitable for very small chips.

 

You can make a great many things work   You can put skinny  bias ply tires on Corvette , but don't expect it to corner like a Camaro on good radials.   Just because you can do something does not mean you should do something. 


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#5 wargrafix

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Posted 27 September 2016 - 09:10 AM

Curiosity is killing me ow the images would come out.



#6 alphatripleplus

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Posted 27 September 2016 - 09:20 AM

Curiosity is killing me ow the images would come out.

 

Take a look at the EAA thread that I linked to in my earlier post #2. Set-up 3 and Set-up 4 show images on a small 1/2 sensor. As others have mentioned, if you use a larger sensor you will see more aberrations/vignetting at the edge of the FOV. For EAA, I find that Set-up 4 yielding f/3.9 acceptable. Just my 2cents worth.



#7 rmollise

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Posted 27 September 2016 - 09:25 AM

Might be easier to find an old f3.3 focal reducer. They come up used every so often. I don't think anyone actually uses one anymore, they just have them sitting around. Put up a "wanted" ad in the classified.

 

Oh, people still use them. They are still very much in demand by video users with 1/2-inch and smaller chips.

 

OP:  Because of the vignetting, this is only for imaging with small chip cameras. Same goes for stacked 6.3s...


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#8 Abhat

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Posted 27 September 2016 - 09:35 AM

Thanks alphatripleplus for your inputs. My interest was academic initially. I am losing the light pollution battle and also my ability to haul ridiculously sized telescopes. I do have plans to jump into Video Astronomy using R2 or comparable camera for almost real time viewing with live stacking.  I was wondering if double stacking a reducer would be feasible option for live viewing. That would allow me reduce exposure time and effects of field rotation.



#9 alphatripleplus

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Posted 27 September 2016 - 09:55 AM

Thanks alphatripleplus for your inputs. My interest was academic initially. I am losing the light pollution battle and also my ability to haul ridiculously sized telescopes. I do have plans to jump into Video Astronomy using R2 or comparable camera for almost real time viewing with live stacking.  I was wondering if double stacking a reducer would be feasible option for live viewing. That would allow me reduce exposure time and effects of field rotation.

 

Glad to hear you are interested in entering Video Astronomy / Near real time viewing. Yes, stacking reducers is common usage in  near real time viewing, although I had not heard of anyone stacking two f/6.3 reducers until I tried it. Stacking reducers (not just  twin f/6.3 reducers) is a feasible way to reduce the effective f/ratio and use shorter exposures. In general, however, image quality suffers with greater reduction, but the approach works well for EAA.

 

If you are using an alt-az mount and are worried about field rotation, live stacking of short subs using software like SharpCap or other EAA oriented camera software is definitely the way to go. Check out the EAA forum (under Observing) for opinions on these topics. Good luck!


Edited by alphatripleplus, 27 September 2016 - 09:59 AM.

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#10 noisejammer

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Posted 28 September 2016 - 06:24 AM

Years ago, I asked the late Doc Greiner about stacking reducers. (Doc G was active on the MAPUG list.) He commented that although it's possible, the f/3.3 reducer would produce a better image. I eventually got a f/3.3 reducer; with a bit of modification to the housing, I was able to reach f/2.5 on my LX200/12.

 

Anyway, I tried stacking over the weekend - but not on my Cat, I put a pair of Celestron f/6.3 reducers on my APM 115/805 (natively f/7.)   My scope has about 265 mm available back focus.

 

The intent is to play with modest resolution video so I examined the image with a 30 mm eyepiece.

 

A curious result - the first reducer generated a lot of pincushion with some colour fringing around the edges. but there was no visible vignetting. However, two reducers seems to work better than one! The pincushion was not visible and neither was the fringing. Of course, this could easily be because the image scale was quite small (around f/3.) There was slight vignetting and I ran out of back focus so I don't know how it would look at greater reduction ratios.


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#11 ensign

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Posted 29 September 2016 - 10:59 AM

I routinely stack reducers - a .63 and a .5 - on  a C8 for use with a Mallincam.  I haven't measured too carefully, but I'm guessing the final focal ratio is in the neighborhood of F3.

 

This is ideal in terms of image scale and speed for my purposes. 

 

With my Skyraider DSc, I haven't noticed any obvious aberrations, nor have I had any issues reaching focus.

 

All bets are off for visual, though.    Even using a standard C8 with a .63 reducer, with no additional reducer, never produced views that satisfied me.  I suspect that the issues were a combination of field curvature and vignetting.  While the vignetting was not starkly obvious, I believe that it had the impact of effectively reducing light gathering - in essence, aperture reduction.  Some objects that are a cakewalk with my 9.25 Edge HD were utterly invisible in the C8 with the .63 reducer.


Edited by ensign, 29 September 2016 - 11:00 AM.

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#12 wargrafix

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Posted 06 October 2016 - 01:42 PM

saw the results!!!

 

 

Must...resist..buying..another..reducer....



#13 core

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Posted 10 October 2016 - 11:12 PM

Just to add another data point, I routinely stack a 0.63x visual back FR with a 1.25" 0.5x FR for EAA work.  I prefer this over a single Meade 0.33x FR as I have it in a configuration with a flip mirror assembly for fast and easy scope setup (requiring visual eyepiece for initial alignment, not needing to switch in and out and re-focus which is required when using the 0.33x FR).

 

The 1.25" filter rings are just empty spacers, fyi.

 

gallery_40204_3321_20060.jpg


Edited by core, 10 October 2016 - 11:29 PM.


#14 mclewis1

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Posted 11 October 2016 - 11:14 AM

Peter,

 

Very nice setup (great pic too). 

 

What type of C - T thread adapter is that? (curious about the internal 1.25" filter threads).



#15 roelb

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Posted 11 October 2016 - 08:50 PM

Hello Peter,

Thanks for the nice picture.

I'm interested to know what distance you have between the f/6.3 and 0.5x FR,
and also the distance between the 0.5x FR and the CCD.

Also, what is the total focal ratio that you achieve with these setup?

And at last: what scope are you using with this image train?

Thanks in advance.

#16 core

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Posted 12 October 2016 - 12:18 AM

I bought the T-C adapter from ebay a couple of years ago (still have a spare! :grin:); there was a chap machining them out of Delrin, unfortunately I don’t find it with a quick ebay search now.  I’ll have to dig thru my emails for the transaction.  It’s really a versatile and neat little piece of gear; besides the internal 1.25” filter threads, the OD is 2”, so it can slip into drawtubes if needed.  In the picture, the 1.25" eyepiece extension tube is required for the eyepiece to be about parfocal with the camera.

 

Scopestuff has a similar item, but not T-threaded:  http://www.scopestuff.com/ss_c2bf.htm

 

Unfortunately I’ve lost (misplaced) some of my notes on my testing; this is a partial bit of what I have:

 

Procedure:

 

- C6 OTA in the daytime, pointed at a telephone pole ~0.5mile or more away.

 

- With standard SCT T-adapter and the Delrin T-C, I measure a part of the telephone pole on the LCD screen (horizontal beam part)

 

- I then mix and match the pieces, and each time I measure the same part of the pole to get an effective FR.  Baseline measurement with standard SCT 0.63x FR confirms initial results.

 

- I’ve forgotten if the test were done with a SDC435 or MC-micro

 

Conventions:

 

T-C - the Delrin T-to-C adapter (the C-ring on the camera is screwed all the way flush to camera body)
0.63 - SCT f/6.3 FR
644 - Meade flip mirror body
T10/T20 - Supplied Meade T-extentions
0.5x - 0.5x 1.25” FR
Spcr - 1.25” filter ring as spacer
CS - the C to CS spacers provided in Samsung C-mount cameras

 

With that in mind, the FR combos I get (from the notes I have on hand):

 

0.63⇨644⇨T10⇨T-C[0.5x⇨UV/IR]              f/4.3
0.63⇨644⇨T20⇨T-C[0.5x⇨UV/IR]              f/4
0.63⇨644⇨T20⇨T-C[0.5x⇨UV/IR⇨Spcr]     f/3.5
0.63⇨644⇨T10⇨T-C[0.5x]⇨CS⇨CS            f/3.3

 

I’d also gotten different combos (without the 644, not enough backfocus) with a Meade SCT f/3.3 FR, it really gets bad around f/2.7 from my notes.  I’d also tried out other combos (diagonals, vixen flip mirror, 1.25” to C adapters, etc) on various other scopes.

 

Keep in mind the test I did was mainly for my EAA use, with its much smaller sensor size.  I’ve also had a Pentax Q7 for a while that I’ve been meaning to get into the mix as well (sensor size 1/1.7 inch (7.6mm x 5.7mm)).

 

I mainly used the setup with a C6 for outreach (requiring fast setup), but I’ve also used it with C8, C14 and 80-100mm refractors (does require OTA to have more back focus though).

 

I’ve been on a astronomy hiatus for the past year+, I really do need to sit down one nice weekend afternoon and repeat the experiment and get the results all tabulated (again!).


Edited by core, 12 October 2016 - 12:29 AM.

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#17 core

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Posted 12 October 2016 - 12:38 AM

All bets are off for visual, though.    Even using a standard C8 with a .63 reducer, with no additional reducer, never produced views that satisfied me.  I suspect that the issues were a combination of field curvature and vignetting.

Using it with a 2" setup? iirc, the FR reduces the C8's ~150mm back focus down into the 100mm range - resulting in aperture loss if using a 2" diagonal.



#18 ensign

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Posted 12 October 2016 - 10:46 AM

 

All bets are off for visual, though.    Even using a standard C8 with a .63 reducer, with no additional reducer, never produced views that satisfied me.  I suspect that the issues were a combination of field curvature and vignetting.

Using it with a 2" setup? iirc, the FR reduces the C8's ~150mm back focus down into the 100mm range - resulting in aperture loss if using a 2" diagonal.

 

Yes - using it with 2" gear.  The idea was to get the Cat to have wider-field views.  While this was accomplished, the other effects dampened my enthusiasm for the combination.

 

So now I'm using an 9.25 Edge HD dual mounted with an Equinox 80 on an ES Twilight II mount.  With my Pentax 40 XL the true field with the Edge is 1.1 degrees, which could not exactly be considered a wide field.  The little 80 will give a 5 degree field with the same eyepiece and it also acts as a finder.

 

I'm very happy with this setup.


Edited by ensign, 12 October 2016 - 11:00 AM.


#19 Adun

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Posted 04 August 2018 - 05:27 PM

The effective focal length of  two identical focal reducers with zero separation is halved, so that the combination of two Meade f/6.3 reducers has an effective focal length of about120mm compared to 240mm for each reducer. I used the stacked reducers to form an image at a working distance of about 64mm to 73mm behind the reducer closest to the camera sensor, yielding f/3.9 to f/3.3. I got the best image in terms of least aberrations using the f/3.9 configuration (Set-Up 4) as shown in post#6 of the thread.

 

Edit: The Meade f/6.3 reducers I used were both the old made in Japan versions. I haven't tried the newer Chinese made Meade or Celestron f/6.3 reducers, but in principal the results should be similar if optical quality is the same. I would be interested to learn of your experiences if you try stacking.

 

Hello alphatripleplus.

 

I have one of the newer Chinese Meade reducers, and I just measured it. I has about 130mm focal length.

 

if I got another one, would the FL of the dual stack still be halved? That would be like 35mm distance from last lens to camera sensor, quite short.

 

I have a C6 that works fine with the F6.3 reducer for visual (using prism diagonal) but I want to get it down to ~F4 for EAA with my RT224 (6mm diagonal).

 

Would you say it's likely to work stacked with another one?

 

Other people are stacking the 6.5 with a cheaper antares/gso 0.5 reducer, and I don't know what's best.



#20 gnowellsct

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Posted 07 August 2018 - 09:53 PM

 

 

You can make a great many things work   You can put skinny  bias ply tires on Corvette , but don't expect it to corner like a Camaro on good radials.   Just because you can do something does not mean you should do something. 

That was my thought.  Bite the bullet and get a fast apo.  Or maybe one of these f/3.3 babies.....or hells bells, sell it and get an Edge HD and use it in fastar configuration.  


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#21 mclewis1

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Posted 08 August 2018 - 07:51 AM

I have one of the newer Chinese Meade reducers, and I just measured it. I has about 130mm focal length.

 

if I got another one, would the FL of the dual stack still be halved? That would be like 35mm distance from last lens to camera sensor, quite short.

 

I have a C6 that works fine with the F6.3 reducer for visual (using prism diagonal) but I want to get it down to ~F4 for EAA with my RT224 (6mm diagonal).

 

Would you say it's likely to work stacked with another one?

 

Other people are stacking the 6.5 with a cheaper antares/gso 0.5 reducer, and I don't know what's best.

Both choices can would work with your 224 based camera. 

 

It seems that when you stack focal reducers you don't want to "push" the reducer's optically, that is you don't want to set them up to work at their limits (max reduction factor). In a practical sense this means when using two SCT reducers keeping the spacing distance down a bit and stacking the two reducers together without any spacers (this is from practical use examples, but I don't think anyone has tried adding spacers between the two reducers). So that would be SCT reducer > SCT reducer > 80-100mm back to the camera's sensor.

 

For the SCT and .5x reducer combination that would be SCT reducer > 1.25" visual back >.5x reducer on a 1.25" nosepiece > camera. The spacing from the back of the reducer to the sensor should be under 40mm (which means the reducer will be working in the .6 to .7x range). The nosepiece of the camera (and the reducer on it) can be moved within the visual back to change the spacing back from the SCT reducer. In this case just playing with it to see what works for your particular setup.

 

Some other general thoughts ...

 

- These are all just examples of what has worked for me or a few others here on CN. There are also many other ways of coupling the camera to a reducer (or reducers). There's not much practical info on a wide variety of using two reducers together so trying different configurations is just as valid as trying some of these examples.

- None of this produces astrograph quality images, it's been proven to work for some folks for EAA with small (type 1/3 or 1/2" sensors) and maybe a bit larger but not much else.

- the more aggressive you make the setup (the higher the reduction factor) the more likely you are to have problems coming to focus even with an SCT (which has a huge amount of focal point range compared to other scopes).

- The SCT reducer should be mounted as close to the rear of the scope as possible. So using it behind an external focuser is usually not recommended (less likely to come to focus).


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