HOTECH SCA Field Flattener

[pmrid]

Hi to all users of these devices. Because there was no documentation with my new HOTECH and nothing on their web site about this issue, I wrote to them recently asking for clarification of the distance that had to be maintained between the SCA and the imaging plane of the CCD/DSLR etc. They have sent a reply which I thought I would share for the benefit of IIS members:

" ..... The SCA Field Flattener requires 55mm distance from the shoulder of the T-thread to your QHY8. You will have to use additional T-thread ring to add up the difference for optimal imaging result. ... "

OK. there you have it.

Peter.

[h0ughy]

but that is the standard requirement for the QHY8? the DSLR is fine and works well. so it would be the same if you were using a canon lens on the qhy8

[pmrid]

Hi David. not sure I follow what you're saying. Wouldn't the 55mm distance be a function of the flattener and not the CCD/DSLR? And wouldn't the 55mm requirement be the same for any CCD or DSLR - not just a QHY8?

Peter

[h0ughy]

true - if the distance was already 55mm(or thereabouts) for the canon cameras for example then to be the same the qhy8 - if it were to be parfocal so to speak. so if it was designed for the dslr primarily then the qhy8 would have to be similar - hope i am not talking in circles:shrug:

[pmrid]

It's simple provided I think in circles and these days, I think that's what I seem to do most of the time.
Peter.

[allan gould]

Is the Hotech SCA field flattener the same beast as the GSO Focal reducer being sold by Andrews Coms? Part no CO-001? Or the AstroTech 2" field flattener?

[h0ughy]

no

[allan gould]

Oh!

[lookus]

would this flattener work with a zinithstar 66 and an ed80 and a dslr?

thanks.

[mldee]

Just finished putting together a little compendium of what I could find on some generic FR's. It may be of help to others.

Just the low-hanging fruit of a few Googles.

Interesting that Meade drastically changed the Focal Length of their f6.3 in 2006 according to one of the references.

All care, no resp :P

[allan gould]

Mike
I think that some of your distances are incorrect as Ive been verifying my Meade f6.3 FR and can state that the correct distance for this is 87mm from chip to back element of lens. See http://timosastro.1g.fi/tools/focalreducer.html
This FR works from 80-106mm with acceptable results. Best is at 87mm. Ive confirmed this and have seen the distance at some sites quoted as 106mm.
The spacing for the f3.3 remains the same at 57mm.
Also the GSO 2" Coma Corrector cant have a spacing of 53mm as it comes with a 2" adapter which is already at 42mm from the back element and a T adapter on a Canon camera would add another ~45mm to the chip. Just measured it on the one I purchased from Andrews. (edit - you're right as I miss read FR for coma corrector).
Also note that the GSO and AstroTech Coma Corrector are not the same beast - many are assuming they are.

[mldee]

Hi Allan,
Thanks for the inputs. Let's hope we can eventually come up with a set of reliable numbers! It would be nice if others also donated any info on other types of FR's that are known.

Small comment on the Meade info, the default Meade FL from the Timo calc is 260mm, and shows a FR of .67 at 87mm, needs 95mm to achieve .63. No big deal.

Also note comment from http://www.wilmslowastro.com/software/formulae.htm about much shorter FL of post 2006 Meade .63's. Just trying to verify that comment, would you consider your Meade to be pre 2006? I have one of unknown vintage, so will have to suck it and see.

I used 154mm FL (post 2006) as was mentioned as the FL in the other link to get the 57 at .63FR for a Meade. Would be good clarify this for newer post 2006 Meades.

Let's hope we can build on the list, now that there are some easy to use tools to do it.

I put my C8 on the mount yesterday, so will play around this week and post my results.

Cheers,

[allan gould]

Mike
My FR is definately pre 2006. Was purchased in 1996 from Bintel.

[mldee]

Good Lord, last century! I'm unsure if I was even born then :D!

[pmrid]

Hi Mike.
I may have missed something but I didn't see the Hotech FR in your list and, since this thread is about the Hotech, I thought this a tad amusing. I don't know the FL of the Hotech but the manufacturer says the optimal separation is 55mm.
Peter.

[mldee]

Hi Peter, Yep, sorry for the unintended hijacking. My intention was to supply the info so folks could work out what they needed. The Hotech appears to not be a Field reducer, but a pure flattener, so the tools probably don't work. I've added the Hotech info to the list anyway, and posted again below.

Cheers

[telemarker]

Peter,
I noticed that you are using the Hotech FF with the ED127. Could you give us an appraisal of its performance at the edges of the QHY8. Is it a flat field?

[pmrid]

It's not bad. My separation is not quite 55mm (it's 54.2) but it's near enough I think. The edge coma I saw without the FF seems to have been corrected. Here are 3 images from Monday night. See what you think. all 3 are ED127/QHY8 and between 2 and 3 hours of 10min subs. The 4th image of NGC6559 was taken a few weeks before and without the FF.
Peter

[telemarker]

Thanks Peter, looks like another item for the shopping list. :lol: