Which field flattener for the WO 71 Gran Turismo? - Forums

Topic starter

May 2, 2021

Hi all,

I'm looking for a small refractor for wide field targets such as the heart nebula, orion nebula, etc.
Right now I'm gravitating towards the William Optics 71 Gran Turismo. I noticed that the dedicated field flattener that WO recommends for this specific scope, is also a 0.8 reducer (the Flat 6AIII). Point is that I rather not have the focus length of the WO 71 GT reduced to a focal length that is actually less than that of the WO Zenithstar 61.

Then I noticed that there's also the WO Flat73A flattener. This is a purely a flattener and not a reducer. However, this flattener is only recommended for the WO Zenithstar 61 and 73 scopes.

So I was thinking "can't I just use the Flat73A on the WO 71 Gran Turismo, or might they be incompatible?" On Astobins Great Wall I normally see the combination WO 71 GT + Flat 6AIII.

Does anyone use the combination WO 71 GT + WO Flat73A flattener?

Thanks in advance and CS!

Douwe

Engaging

Joe Santacroce

May 16, 2021

Like others, as you mentioned I have the WO GT71 (and a Z61) and I use the 6Aiii. For some targets, that's more than enough Focal Length but others fall far short. The FL is 336mm with the 6Aiii. I thought of trying it without the Reducer\Flattener for some targets then if needed crop out the starts along the edge but that might not provide the best results. I have the 6r flattener for the Z61 and it is also a .8 reducer.

I would also be interested in seeing if anyone either used no R\F or used the Flattener only.

Nicole & Ryan

May 16, 2021

We've got the WO 71 with dedicated flattener, tried to get "into" Horsehead and Flame a little by taking off the flattener, results were not good, didn't even keep all of Flame after processing, and sure was not anything to share here.(Alas, still trying to get the F6Alll to be flat

, our fault not it's)

Nikki

Steven

May 20, 2021

I've tried my best on google. but I can find surprisingly little.
But, with the 73 being a doublet. And the 71 (and any other listed for the 6AIII) being triplets.. I have my doubts..

every time the 73 Flattener is listed, it is specifically and only listed for the ZS73.
Not like some other flatteners that are listed as: "Apo's between F5-F7 etc etc".. so ... I'm not sure.

I did find a list

And it says at the bottom that the ZS73 flattener is only listed for the ZS73. "some William Optics field flatteners are fixed devices for particular telescopes like the ZenithStar 61 and ZenithStar 73"
And some other flatteners are listed for multiple scopes, as they are adjustable in length, showing the required length next to it.

Making it appear that the 73A flattener is fixed in 1 position..? But.... the ZS73 flattener (P-FLAT73A).. is adjustable.... but it is still only listed for the ZS73. so.. I'm confused.

I'm not an expert on flatteners and reducers anyway. So maybe there is a reason why it would work together, mainly with one being a doublet, the other a triplet.. So the flatteners are built different to deal with that. but.. no idea.

Douwe79

Topic starter

May 20, 2021

Thanks all.

In the meantime I’ve asked WO if the Flat73 is compatible with the GT 71.

I noticed they were very reluctant to answer this question. The only quote I got out of them was:

Flat73a/r are made for ZS73.
Flat73a/r are the flatteners you have to use with ZS73.

No explanation why.

CS!

Larry S

May 20, 2021

The above is the current info from the WO website for the 6AIII. I have it on the GT71, and I find with smaller sensors (ASI183, ASI533) a setting of 10.1 works better than 9.1. When I tried it with an APS-C sensor (ASI2600MM) and had radiating star trails indicating the sensor was too close to the reducer. I found that 13.1 worked best. In all cases the back focus from reducer to sensor was 55mm. I tried the GT71 once without the FR, and the results were not good.

Helpful

May 24, 2021

Hi,

Have you looked at the HoTech SCA Field Flattener.

Telescope field flattner lens, SCA field flattener (hotechusa.com)

I have this installed on my GT71

Best wishes

G

Douwe79

Topic starter

May 24, 2021

@GW thanks for the tip!

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