SCT FR/FF on a refractor?

[james_screech]

I'm thinking if buying a Altair Lightwave 66mm F6 ED-R Refractor for wide field imaging and was wondering if my Celestron FR/FF would work with it to flatten the field, ir the the field curvature of a SCT very different from that of a refractor.

James 

[alan potts]

I would think not, these things are made for different focal lengths, if you look at only refractors there are many covering F4 right to F8, some cover sort 600mm-800mm or maybe more but a SC reducer flattener will be for a longer F/L. I tried my Meade F10 reducer in my Mak F15 and it was no good at all, I stand to be corrected of course.

Alan

[brantuk]

As Alan says - you can get FF/FR's for general ranges of refractors - but you can also get them optically matched for the figure of a particular scope. Because the light is bent differently through an SCT or Mak, and exhibit different types of aberration, a different FR/FF is required. With Altair I usually find it's best to talk with Nick/Ian to find out which one is best for the particular scope I'm looking at.

[Astroboffin]

Well, as the celestron and Meade 6.3 reducers work in the same way, I would be right in saying that the reducer is made up of two sets of two lens, 4 in total, (I know this as I have dismantled one to fit in a new 2" cell) one set is the reducer and the other is the flattener, the reducer is nearest to the scope when fitted, so the flattener is actually only flattening the reduced f6.3 image, and not the original f10 image, so in my opinion it should work on an f6 scope.

This conclusion is from my logic as I have not tested, although I might just try it, but can't see any problems, and I am sure I have seen a test done on here with a few FR/FF on a frac scope, and the celestron was one of the FFR that was used in the test

Hope that helps

AB

[james_screech]

Thanks for the replies, I thought it might not work I'll just have to get a field flattener designed for f6 refractors instead.

James 

[Astroboffin]

Thanks for the replies, I thought it might not work I'll just have to get a field flattener designed for f6 refractors instead.

James

Did you read my reply???

[DaveS]

Well, you could *try* it but I think it wouldn't work as the optical requirements are different in the two cases. For refractors the optics flatten and reduce together, you cannot say that one group flattens and another reduces. For example, my WO FR IV only has two lenses to do the job, as does my Hotech flattener, which doesn't reduce.

[Astroboffin]

Well, you could *try* it but I think it wouldn't work as the optical requirements are different in the two cases. For refractors the optics flatten and reduce together, you cannot say that one group flattens and another reduces. For example, my WO FR IV only has two lenses to do the job, as does my Hotech flattener, which doesn't reduce.

Completely disagree, it doesn't really matter how many optical elements there are, at the end of the day one reduces and the other flattens, and the reducer is always first, so nearest to the back of the scope, so the flattener is flattening the reduced f ratio.

Field curvature is field curvature, whether it be from a refractor lens or a curved mirror, so the way I see it the flattener needs to work with the reduced f ratio image, so an f6 image from a refractor should be able to be flattened with any field flattener that is designed to work with f6.

Sorry to disagree, but logic tells me otherwise

AB

[Astroboffin]

Well, you could *try* it but I think it wouldn't work as the optical requirements are different in the two cases. For refractors the optics flatten and reduce together, you cannot say that one group flattens and another reduces. For example, my WO FR IV only has two lenses to do the job, as does my Hotech flattener, which doesn't reduce.

By the way I also have the WO IV FFR and you are correct there is two elements to it, BUT each element is made up of two lens (four pieces of glass In total) which are then stuck together with the correct space between them.

I also have the Meade and celestron FFR and they have two elements made from four lens also stuck together in two groups of two, but in this FFR the glass is thicker.

I do t think there is a Barlow or reducer out there that consists of only one piece of glass, they have a minimum of two, which means that a FFR will, have a minimum of four, set out in two sets of two

Regards

AB

[ollypenrice]

I've seen all sorts of efforts to use this flattener with that scope and the success rate is uncertain. There are successes but forget the theory. Does it work? Given the fact that it is fairly common for manufacturers to make flatteners for their own scopes which don't work (naming no names) it is asking quite a lot for them to make flatteners which will work with scopes they have never seen. All sorts of unexpected problems arise. A common one is the internal reflection. Given its quality and relatively affordable price lots of folks want a reducer flattener for the TEC140. TEC are not stupid, they know this, but they don't make one. Alarm bells should sound in the ears of anyone thinking, Aha, I'll buy a Riccardi. That should work. From what I've seen - it doesn't. And that's about it. It might work, it might not. Light is extreme stuff. It has ways of being unpredictable. The only safe way is to see what has been proven to work - but, when you do this, pay careful attention to the chip size bcause if your chip is larger then it may not be clean to the edge.

Olly

[DaveS]

By the way I also have the WO IV FFR and you are correct there is two elements to it, BUT each element is made up of two lens (four pieces of glass In total) which are then stuck together with the correct space between them.

I also have the Meade and celestron FFR and they have two elements made from four lens also stuck together in two groups of two, but in this FFR the glass is thicker.

I do t think there is a Barlow or reducer out there that consists of only one piece of glass, they have a minimum of two, which means that a FFR will, have a minimum of four, set out in two sets of two

Regards

AB

Unless you can show me the optical diagram I'll have to disagree with you there.

[Astroboffin]

Unless you can show me the optical diagram I'll have to disagree with you there.

Optical diagram of what ??

The focal reducer flattener, ??

this is the Meade and celestron one, the two lens on the left are the reducer parts, and the two on the right are the flattener parts, cemented together into two lens cells.

The two lens in each cell are made up of whats called a "flint and crown glass", the crown being the oval shaped lens, and the other being the Flint.

The white area In between is a spacer, which is a set width, and can't be altered.

AB

[DaveS]

Well, yes obviously.

Thanks for showing me that, it is, as you say, of the Meade / Celestron FR. BTW, I *do* know about optical glasses, including today's anomalous dispersion varieties.

Not all FR are that simple, here are two from the Borg collective, courtesy of out favourite retailer, first a triplet:

http://www.firstlightoptics.com/borg-collective-other/borg-07x-triplet-multi-super-reducer-with-camera-rotator.html

And this one is a quadruplet, though I had to find out from other sources:

http://www.firstlightoptics.com/borg-collective-other/borg-f4-ed-super-reducer-set-including-filter-box-camera-rotator-and-spacer-rings.html

And from the APM site, a selection of FR designed for different focal ratio telescopes. Note the optical diagrams.

http://www.apm-telescopes.co.uk/apm-reducer.html