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Old 05-04-2010, 09:16 PM
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DavidTrap (David)
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Focal Reducer Test

I bought a Televue Focal Reducer (0.8x) / Flattener earlier this year. Unfortunately, the weather in SEQ has hampered my efforts to test this until now.

I was also loaned a Baader MPCC to test as well - thanks Gerald.

The refractor is a William Optics ZenithStar 70mm Doublet. F6.2 and focal length 430mm. Without correction, its coma is pretty shocking...

I setup tonight and fired off 5x30sec exposures of the Orion Nebula. These were unguided, with the scope tracking in Alt-Az mode. Fortunately with Orion in the west and at ~45 degrees altitude, field rotation shouldn't be a huge issue with such short exposures.

I processed these with some darks in DSS and adjusted levels in PS - they're certainly not masterpieces!

The images in order are: without a flattener, MPCC and Televue.

As you can see, the coma is pretty shocking on a large chip (Nikon D200). It doesn't get much better with the MPCC (as expected for a device designed for fast newtonians). I'm pretty stoked with the results for the Televue. It is designed for 400-600mm focal length refractors.

It is expensive (over half the cost of the scope) and heavy - the focuser struggles to hold it and the camera up... However, the results are impressive.

I'd be keen to hear any learned opinions - especially regarding any visible collimation problems.

Thanks for looking,

David T
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  #2  
Old 05-04-2010, 10:16 PM
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Phoenix (Steve)
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Nice test David - very interesting in deed.

I have the WO Megrez 90 and have looked at the MPCC and Televue for flattening the field for imaging. The Televue in your tests speaks for its self but with a focal length of 621 for the Megrez 90 it sits just out side of the optimum range, 400 - 600mm, as you detail. I am also considering the new HOTECH flattener (not a reducer though), which has an extended focal length range.

Interesting to note the weight of the Televue and of course that it's considerably more expensive.

Nice comparison David.

Cheers, nix
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Old 06-04-2010, 05:25 AM
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Surprises me to be honest, as I had success with the MPCC on an 80mm Lomo triplet a while back. Yes the Televue worked as well, but no better (in my case), but what the MPCC really did was that it kept the native focal length, whereas the TV reduced as well as flattened, something I didn't want.
I did have to jigger about with the MPCC spacing, but not overly.
Gary
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Old 06-04-2010, 06:33 AM
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multiweb (Marc)
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Looks like the Televue did the job for you. I have tried the MPCC on my ED80 without much luck and managed to correct with the WO flat IV. I guess all refractors even for the same aperture are built slightly different and what works for one brand doesn't necessarily work for another. At the end of the day you get a nice widefield instrument with a corrected field and that's all that matters. Enjoy.
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Old 06-04-2010, 06:34 AM
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I also tried the MPCC on my refractor without favourable results. In fairness, I didn't try different spacings. Still, it's not designed for refractors.

I settled on the WO FF IV. Very happy with the results. It's not a flattener either.
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Old 06-04-2010, 07:37 AM
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If they are still available, the WOII works a treat with the ED80 range of scopes. For the WO scopes, I have a 72 FD (FL 430)and I used the WO III for that.

If you've not seen it i did a review of the WOII and the ED80 here: Refractor and Focal Reducer Trials
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Old 06-04-2010, 08:53 AM
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It's always a problem to find out which reducer works before you buy!!

I believe I've got the distance right for the Televue with a standard Tmount being about 10mm and the distance from the bayonet mount to the chip on the DSLR being 46.5mm. This should give the recommended distance of ~55mm. I think recommended distance is the same for the MPCC. I haven't any means to vary this so I can't say it won't get better if you fiddle.

Cheers

DT
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Old 06-04-2010, 09:04 AM
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Quote:
Originally Posted by DavidTrap View Post
It's always a problem to find out which reducer works before you buy!!

I believe I've got the distance right for the Televue with a standard Tmount being about 10mm and the distance from the bayonet mount to the chip on the DSLR being 46.5mm. This should give the recommended distance of ~55mm. I think recommended distance is the same for the MPCC. I haven't any means to vary this so I can't say it won't get better if you fiddle.

Cheers

DT
So true. You can't test before you buy with FF and reducers unfortunately. You make a decision based on a bit of research, buy then get lucky (sort of) . Spacing is a problem to vary too with solid rings. I got this made a while ago. Best thing I did. I use it all the time. Specs are here.
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Old 06-04-2010, 12:47 PM
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this is from last night, same scope, single 2min exp with the W.O pflat 3, doesn't do the job either.
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Old 06-04-2010, 01:12 PM
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Mick,

Is that through the ED80 or the ZS70? I think the William Optics flattener III is really designed for scopes more than 90mm in aperture.

H
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Old 06-04-2010, 01:48 PM
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I have a Borg #7887 FR/FF and it works perfectly with my full frame Canon 5DH on 80ED, 100ED, 150mm diameter F5 achro and the Tal200k. It is adjustable for different focal lengths by removable spacers between the two optic elements.

here
http://www.sciencecenter.net/hutech/.../html/7887.htm

You will also need the Borg Canon T-ring and nosepiece.

http://www.sciencecenter.net/hutech/.../html/5005.htm

http://www.sciencecenter.net/hutech/.../html/7425.htm

Not cheap but it works.

I would be willing to loan it for tests on anyones scope as I will not be needing it for quite a while. I am going to concentrate on widefields with the 300mm lens.

Bert
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Old 06-04-2010, 01:51 PM
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hi H, it was with the ZS70 but the pflat3 didn't work with my M90 either.
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Old 06-04-2010, 06:16 PM
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Quote:
Originally Posted by mick pinner View Post
this is from last night, same scope, single 2min exp with the W.O pflat 3, doesn't do the job either.
Hi Mick,

Can you post a higher resolution pick (up to 200K). I think I can see what you mean, but not with certainty.

I did a fair bit of research on the net before committing funds. I certainly found mixed reviews about the WO flatteners. It seems that flatteners may work with one scope but not another of similar size. Spacing is critical too.

DT
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Old 06-04-2010, 06:23 PM
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l've just ordered a TV 0.8 reducer going by your pic.
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Old 06-04-2010, 07:24 PM
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Mick,

Are you sure that's not field rotation??? The rotation looks like likes its going round in a circle, rather than out radially from the centre. I agree that you'd be unlucky to get bad rotation in a 2min exposure unless your alignment is WAAAAAAY out...

DT
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Old 07-04-2010, 06:55 AM
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Quote:
Originally Posted by DavidTrap View Post
Mick,

Are you sure that's not field rotation??? The rotation looks like likes its going round in a circle, rather than out radially from the centre. I agree that you'd be unlucky to get bad rotation in a 2min exposure unless your alignment is WAAAAAAY out...

DT
Very hard to get field rotation on M42 on 3min exposures. Maybe on 15min subs with a real bad polar alignment. That pattern is typical of over correction on the edges i.e. spacing is too big.
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Old 07-04-2010, 07:09 AM
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OK - still learning about all this stuff!

DT
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Old 07-04-2010, 02:46 PM
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On your three original shots the televue wins hands down so you've nailed it on the head. Your native scope has a field curvature of 36%, with the MPCC the corrected field goes down to 14% but then with the televue I have a value of 9.4% with the best FWHM of the three as well. Visually I cannot see any coma in your picture either.
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Old 07-04-2010, 02:49 PM
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Thanks for the image analysis. What software is that done with - CCDInspector?

DT
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Old 07-04-2010, 02:51 PM
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Yes - it is an invaluable tool worth every penny.
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