Field flatteners

[Mokusatsu (Australia)]

Visual astronomer here who has never had a need of a field flattener, hence the possibly unsophisticated question...

I've acquired a gorgeous William Optics FLT 132 refractor from a deceased estate. It's a scope that would take some extremely pretty pictures if I were to go down that route.

William Optics have a few field flatteners and reducers which they promote as compatible with the scope.

https://www.testar.com.au/products/f...56-gt102-gt153

https://www.testar.com.au/products/f...-flt132-flt156

https://www.testar.com.au/products/f...-flt132-flt156


Question: are field flatteners very specific to specific makes and models of scopes, so if I need a field flattener I should buy the WO one, or would there be a variety of models I could choose from?

[Hans Tucker (Hans)]

Originally Posted by Mokusatsu

Visual astronomer here who has never had a need of a field flattener, hence the possibly unsophisticated question...

I've acquired a gorgeous William Optics FLT 132 refractor from a deceased estate. It's a scope that would take some extremely pretty pictures if I were to go down that route.

William Optics have a few field flatteners and reducers which they promote as compatible with the scope.

https://www.testar.com.au/products/f...56-gt102-gt153

https://www.testar.com.au/products/f...-flt132-flt156

https://www.testar.com.au/products/f...-flt132-flt156


Question: are field flatteners very specific to specific makes and models of scopes, so if I need a field flattener I should buy the WO one, or would there be a variety of models I could choose from?

"Are field flatteners very specific to specific makes and models of scopes" Answer .. yes and no. I believe the WO FLT 132 has a native Focal Ration of F7 which is common amongst APO's. You can look at what TEC has on offer or look at the Riccardi 0.75x APO Reducer and Flattener. The issue is adapting the Reducer/Flattener to the scope. Staying with the WO Brands would reduce that headache. What is your motivation for looking beyond what WO has on offer? Price? or Just testing the waters? WO has a good range and the Testar prices seem really good. You can have a Flattener and keep the native F7 or you can have a 0.72X or 0.8X Reducer/Flattener. One question you do need to ask yourself before you chose is what Camera would I be using with the FLT132. Just my 2 cents worth. Happy hunting.

[Mokusatsu (Australia)]

I'm happy to buy the WO one, cost is not too much. I guess first thing was just curiosity, and secondly if a FF shows up in the IIS classifieds at a bargain price what is the likelihood of it working.

[OzEclipse (Joe Cali)]

I've bought two on classifieds, one non-reducing flattener and one reducing flattener, both work well. People change gear, sell stuff.

Field flatteners are usually designed/matched to a certain focal length range, ie to flatten a particular curvature of field. So if you change focal lengths, you need to change flatteners. There are a whole lot that work between 400-600mm covering many of the popular 70-90mm ED refractors. Others work around 800-1000mm covering 4-5inch photo refractors. Yours is a 132mm f7. So, in addition to the matched flattener from WO, the Televue reducer Model RFL-4087 would work on your refractor and give you an effective focal ratio of f5.5 and an effective focal length of 728mm
https://www.televue.com/engine/TV3b_...id=71&Tab=_RFL

Originally Posted by Mokusatsu

Secondly if a FF shows up in the IIS classifieds at a bargain price what is the likelihood of it working.

So, in the words of that great Australian philosopher, Mick Dundee, "Better than average!"

[PRejto (Peter)]

An important consideration not mentioned is that field flatteners require a specific back focus distance. If you go with the WO product you will know exactly what that distance is. If you go with a different brand there will likely need to be experimentation required to dial in the correct BF distance. That exercise can be tricky, expensive, frustrating and time consuming. I'd strongly advise you to go with the same brand unless you have a compelling reason not to....such as brand X has a longer BF so my equipment will fit okay, but will not fit with the WO solution. (e.g., you want to use an ONAG between the FF and camera).

Peter

edit: If you look at my equipment on my TEC140 I'm using an ONAG which is a beautiful setup. I was forced to use the TEC FRC with a BF of 115mm which just allows the ONAG to fit. The standard TEC FF has a BF of 85mm and therefore could never work with an ONAG. BF varies quite a lot between different brands/OTAs.