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Old 19-03-2020, 09:16 AM
Bruceaust
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Focal Reducer/Field Flatener ZWOasi294

Hi,

I use a ZWOASI294mc on a LX200Classic with a Meade zero shift focuser mounted to visual back.
As Lx200 users would be aware there is limited space to fit a FR in the imaging train. When a Meade 0.63 FR is screwed to the visual back, it makes the Camera foul on the forks when approaching -90 dec (LX200 on wedge).
Is there a FR available that can be screwed onto the nose of the ASI294 without introducing coma or stretched stars at the edges of the image field. I have tried a 2" GSO FR/0.5......lt is fine in the center of the image but distorts stars at the edge of the field.


Any suggestions welcome.


Bruce
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Old 19-03-2020, 11:31 AM
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mental4astro (Alexander)
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Bruce,

If you want an aberration free image for ANY camera, you need to use a focal reducer/flattener designed SPECIFICALLY for the scope you are using. No ifs, no buts.

That is why that 0.5X 1.25" reducer isn't doing the job. Nothing wrong with it. It is designed as a general purpose reducer for refractors and cassergrains, and the slower the f/ratio the better. And these same modest reducers are no good in Newts from the very start - they are a complete optical mismatch.

The only option you have for your fork mounted scope is to use a diagonal in the optical train. It is the only way you will get the camera to clear the yoke. Unless if you defork the scope and put it on a GEM...

Alex.
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Old 19-03-2020, 12:41 PM
Bruceaust
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Thanks Alex,
Yes I understand that.
I am using a diagonal as well as a filter wheel so i am short of room.
If I use the Meade reducer i am limited to about -72 Dec before it hits. The Meade Reducer needs about 100mm back focus to the chip so it has to be installed on the Visual Back. If i could get a reducer that has the usual 52mm(approx) back focus AND it works with the LX200 it could solve my problem?


Bruce
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Old 19-03-2020, 12:49 PM
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mental4astro (Alexander)
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Your SCT I assume is a standard SCT, not an ACF. This would mean you can only use the f/6.3 reducer/corrector that is available for both Meade and Celestron SCTs.

What size diagonal are you using, 2" or 1.25"? A 1.25" will give you more room. But the filter wheel could still get in the way...

Alex.
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Old 19-03-2020, 01:48 PM
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Outcast (Carlton)
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You could try the f3.3 reducer...

Works well with small sensor camera's (I've used it with 224MC), back spacing is considerably less... not sure if you can get these new anymore though...

Won't come to focus with a DSLR (not enough inwards travel) but, fine with dedicated Astrocams where the sensor depth is only 12mm or so...

I have a spreadsheet Ken (Merlin66) sent me that shows you what's what with regard to spacings... but, apparently can't upload it here..

Send me a PM with your email address if you would like a copy, might help you out with deciding if the f3.3 is of any use to you..

Cheers
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Old 19-03-2020, 03:00 PM
Bruceaust
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Actually the image I get with no FR is not bad . but it does have a bit a star stretch at the extremities. What i have done in the past is just crop the image. Maybe i will try the F/0.63 and put up with not being able to image below -72Dec. There is not much there anyway! But at least it will prove the viability of the reducer.
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Old 19-03-2020, 04:32 PM
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The "stretch" that you refer to is the coma that these scopes display, which is what these f/6.3 reducer/correctors fix, and what the ACF and HD SCT designs correct for in the first instance. As visual instruments, this coma is not optically significant. But for photo it is.

These ACF and HD SCT's as a result cannot make use of the standard f/6.3 reducers because these scopes are already corrected, and attempting to use these correctors will only introduce aberrations! Instead a different reducer needs to be used with these particular scopes. Again, like I said in my first post in this thread, the corrector/reducer needs to be matched to the scope being used.

Newtonians are no different with the coma corrector used, depending on the focal ratio. Coma correctors are designed for a range of f/ratios, so one optimized for f/4.5 to f/6 won't clean things up so well in an f/4 Newt.

Alex.
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Old 19-03-2020, 05:38 PM
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Thanks Alex,
I am stuck with what a have, i believe. So i will learn to get the best out of what i have.
Thanks for your help.


Bruce
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Old 19-03-2020, 06:01 PM
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I found my Celestron C925 to exhibit both coma and significant vignetting if the spacing was too great with the 6.3 reducer. You could try reducing the spacing if you are able and see if that improves the coma (It will probably always have some, mine still does) and if it helps, it will also improve your clearance. Mine worked out as about an F7 reducer where I was happiest with the image.

The remaining coma actually makes sorting out the collimation easy, it is visible at the edges of the field in short exposures and if the tails on stars in the corners were not pointed into the corners, and halfway along the edges pointed straight at the respective edges then it needed a tweak.
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Old 20-03-2020, 07:38 AM
Bruceaust
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There has always been some confusion as to the correct Back Focus for the Meade FR/6.3. What complicates thing for me is, i am using a Meade zero shift focuser as well.

So do you think I can get away with the following arrangement:
Visual back, FR/6.3, Meade zero shift Focuser, ASI294.


I have found imaging problems when i have used the following arrangement (and it still does not give me clearance thru the forks)

Visual back, FR/6.3, Meade zero shift Focuser, 2"Diagonal, ASI294.


I have thought of modifying the FR so it screws on to a standard 2" nose barrel. that way It could slide into the Focuser thus eliminating the space taken by the FR when screwed onto the Visual Back.
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Old 20-03-2020, 10:20 AM
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To achieve F6.3, back spacing to the sensor is 105mm, I honestly don't believe there is any confusion on that however, I note there are other figures out there but, 105mm if you go looking is the predominant figure & tends to be backed up by evidence...

You may actually find with your current setup you describe, you may in fact be slightly over that...

Your diagonal would add pretty close to 100mm to the image train, I think most of the ZWO's sensor depth is 12.5mm so, even with just those items in the train you might be slightly over the 105mm.

How much does the zero shift focuser add to the train?

Maybe you just need a small extension tube to make it up to 105mm which may or may not give you the clearance you are looking for...

As I said in an earlier post, if you PM me your email address, I will send you a spreadsheet one of the 'very smart & experienced' members on here sent me that shows how different back spacing effects the f ratio reduction you achieve... this might provide you with information to look at & compare to see if there is a spacing that delivers you the clearance you are looking for together with an acceptable f ratio reduction

Cheers
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Old 22-03-2020, 12:03 PM
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Tulloch (Andrew)
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I recently bought the Celestron T-adapter (55 mm total length) for my Evolution alt/az mount, and now have the same issue as you when I add my Meade f6.3 reducer.

At the time, I was considering this adapter from Sirius Optics but eventually went with Celestron one. Would this be a solution for you? Maybe you would not get the full f6.3 reduction, but at least it might fit ...

http://www.sirius-optics.com.au/siri...r-for-sct.html

Andrew
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Old 22-03-2020, 12:20 PM
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Quote:
Originally Posted by The_bluester View Post
I found my Celestron C925 to exhibit both coma and significant vignetting if the spacing was too great with the 6.3 reducer. You could try reducing the spacing if you are able and see if that improves the coma (It will probably always have some, mine still does) and if it helps, it will also improve your clearance. Mine worked out as about an F7 reducer where I was happiest with the image.

The remaining coma actually makes sorting out the collimation easy, it is visible at the edges of the field in short exposures and if the tails on stars in the corners were not pointed into the corners, and halfway along the edges pointed straight at the respective edges then it needed a tweak.
Not to hijack the thread, I am looking for spacing options for the c9.25/0.63 reducer/asi294 setup. What parts are you using to solve this?

Cheers,

Trent
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