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Good quality reducer for 80mm scope?

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8 replies to this topic

#1 mistateo

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Posted 14 February 2020 - 06:06 PM

Let me start by saying that I have spent about 2 hours scouring forums looking for the answer before posting, and didn't find anything definitive.  Perhaps that is a hint that one doesn't exist...

So anyway, I have a Stellarvue SV080T-25FT, which was Stellarvue's best 80mm triplet before the SVX series took over.  It has a strehl rating of .98 according to the report given with the scope.  I purchased it about 2 years ago, when I knew practically nothing about imaging.  It came with the SFFR-80 .8x reducer, and the performance of that reducer is shockingly poor.  I tested it last night with the 4/3 sized sensor in the ASI1600MM.  The recommended spacing for that reducer/flattener is 55mm from the t2 interface on the back.  Since I am using 3mm thick astrodon filters, I hear I am supposed to add an additional 1mm, bringing it to 56mm of spacing, which is what I went with.

 

The corner stars were absolutely horrible, as were the left and right sides.  The stars in the top center were a little better than the bottom center, which suggests there is also a small tilt issue.  All connections were threaded, no wobbling, so I may need to adjust the focuser slightly.  I can try fussing around with the back spacing a little bit, but I am doubting it will help much with how bad the stars looked.  My imaging scale was about 2 arcseconds per pixel, guiding was between .5 and .6 total RMS error, with RA and DEC error being very similar.  I ran a drift alignment to .5 arc min error on each, and I was shooting only 30 second subs, so I don't want to discuss tracking issues.

I noticed that when Stellarvue put out the SVX series, they had a new dedicated reducer for their SVX80, claiming the old one wasn't all that great (no kidding).  But NOW when I go to the website, they now mention "Please Note: We no longer offer a reducer-flattener for this system. This is the sharpest visual 80 mm f-6 refractor and when used with the included SFF4 photographic field flattener it is an outstanding, full frame 480 mm f-6 astrograph."  This is on the 3 inch focuser version.... What gives Stellarvue?

In the past I have tried a TRF-2008 which had seagull stars in the sides/corners, the orion .8x reducer (rebrand of long perng I believe, used by astrotech, et al) which gave elongated stars in the corners, but surprisingly better than the TRF-2008.  Does ANYONE make a decent reducer/flattener for an 80mm f/6 scope that is worth while?  I am looking for one that will give excellent performance for at least a 28mm circle to fully cover APS-C sized sensors.  One that does 42mm would be even better, but I won't hold my breath.


Edited by mistateo, 14 February 2020 - 06:08 PM.


#2 Astrola72

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Posted 14 February 2020 - 06:24 PM

Yeah, I've got the same scope and I'm resigned to just imaging at 480 f/6. I do have and use the "large photographic field flattener" SFF3-25-48.

 

Joe



#3 vnc1218

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Posted 14 February 2020 - 06:45 PM

Wow! I didn't know about the problems with this flattener, I bought one just last week...  Would you mind posting the image you had problems with, I'd like to know what I'm getting into with this reducer/flattener?  I would do it my self, but the forecast shows cloudy skies for the foreseeable future.  One possible avenue of recourse is calling Vic from Stellarvue, he's a genuinely excellent guy, and might help if something is defective.  Helped me work out spacing and other problems for another scope of mine.  Hope someone chimes in with a solution for you, I'd be interested too.



#4 bobzeq25

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Posted 14 February 2020 - 07:00 PM

The problem is, not all 80mm F6 scopes can or should use the same reducer.  Unless you find someone with good results for your particular scope, it's a crapshoot.

 

The fact that _Stellarvue_ can't find a good one for their scope is telling.  They know people want one.



#5 TareqPhoto

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Posted 15 February 2020 - 08:01 AM

Post examples of your issues, i feel like i can't follow here, i saw others with same scope you have and using reducer with successful results. 



#6 Tapio

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Posted 15 February 2020 - 08:40 AM

It would be nice to see images.
There are better reducers but how much are you willing to spend ?

#7 Jdrasberry

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Posted 16 February 2020 - 03:46 PM

Well, it’s been two weeks. Certainly by now the OP has called Vic at Stellarvue and gotten this all straightened out.

 

Joe



#8 Jared

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Posted 16 February 2020 - 06:14 PM

The problem is, not all 80mm F6 scopes can or should use the same reducer. Unless you find someone with good results for your particular scope, it's a crapshoot.

The fact that _Stellarvue_ can't find a good one for their scope is telling. They know people want one.


If you said not all 80mm scopes should use the same flattener or reducer/flattener I would agree with you, but all refractors of a given focal length (ignoring those with built in correctors of some type) should have essentially the same field curvature and so should need the exact same flattener. The radius of curvature of a doublet or triplet refractor is about ⅓ of the focal length.

That being said, getting the back focus correct can be tricky and the tolerances can be pretty tight if you are using a decent sized camera chip, and a 4/3” camera chip would count as “decent sized” for a 480mm focal length scope. I have found that sometimes scopes’ native focal length is a bit above or below the specified value, thus changing the appropriate backfocus. I have also found more than once that the engineering drawings listing backfocus for the camera and/or filter wheel are based on preliminary drawings not production values, so back focus can be wrong. Sometimes the mechanical backfocus for the camera does not account for the glass for the chamber, which adds to the optical backfocus requirement just like a filter wheel. In other words, getting a good match in spacing for a particular camera/scope combo doesn’t seem to be quite as simple as “order the right length adapter” from Precise Parts.

I don’t happen to know of a good quality reducer/flattener for 480mm focal length refractors, but if you can find one it should work for ALL 480mm focal length unflattened refractors once the spacing is correct.

#9 Tapio

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Posted 17 February 2020 - 01:38 PM

https://www.teleskop...ig-sensors.html

https://www.teleskop...Vollformat.html


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