GSO RC 8 Flattener

[poppasmurf]

Can anyone suggest a suitable field flattener that suits my GSO RC8?

Shane

[jwoody]

I think a lot of users use this one

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p4006_TS-2--Korrektor-fuer-Ritchey-Chr-tiens-ohne-Verkuerzung.html
or
http://www.teleskop-express.de/shop/product_info.php/info/p4955_Astro-Physics-CCDT67-0-67x-Reducer-2----e-g--for-GSO-RC.html

Log in to get the non-European price (ex VAT)

[poppasmurf]

Thanks Jeremy will look into them. I have a Bintel branded reducer for the RC, but have not tried it yet as I am in the middle of an imaging project. After some more research I have come to the conclusion that the problem I am seeing in my images is either flexure or slight field rotation.

Shane

[kens]

As regards the CCDT67 they also have the CCD47
http://www.teleskop-express.de/shop/product_info.php/info/p8932_TS-Optics-Optics-2--CCD-Reducer-0-67x-for-RC---flatfield-telescopes-ab-F-8.html
As far as I can tell it's identical but EUR 40 cheaper. But it isn't described as a field flattener

[poppasmurf]

Cheers Ken, the only difference I can see between the two is Brand, one being AP the other TS.

Shane

[Logieberra]

I'm loving the AP CCDT67 on my GSO 12"
https://www.optcorp.com/astro-physics-ccd-telecompressor-for-f9-f18-telescopes.html

[Steve Pattie]

Hmmm. I understand the reason for the flattener, but isn't placing lenses in the light path prone to chromatic abberations. The RC was designed for full field flatness across a large image circle without having to use correctors.

Steve

[DJT]

Hi Steve

There's a wiki on this that is quite interesting.

The RC design helps eliminate coma but it's still a bugger in terms of providing a flat field. The RC8 still needs a flattener to cover say an APS-C sensor on a DSLR.

You can chose between adding a piece of glass to act as a flattener or cropping or using a smaller sensor or even tinkering with lens correction in PS. Personally I crop.

https://en.m.wikipedia.org/wiki/Ritchey–Chrétien_telescope#

[Star Catcher]

I have both the CCDT67 and the TS field flattener. I get lovely round stars for my OAG and internal guide chip using the TS field flattener in my RC10. Not so with my focal reducer but that is to be expected as they have different purposes. :)

Ted

[Steve Pattie]

Thanks David for the info.

[Atmos]

Just to reiterate in this, technically the RC design gives a large flat field... flat in the sense that it doesn't suffer from coma (field curvature) BUT only suffers from astigmatism. For science this is fine because without field curvature stars remain in their correct astrometric positions and it doesn't matter if their size (FWHM) changes. A corrector is needed to correct for astigmatism to give what us astrophotographers would consider a flat field in that stars remain uniform in size across the inaging field.

[bratislav]

Well... flat field means EXACTLY that, flat. That is, Petzval curvature is zero. RC's like any Cassegrain follow a very simple rule - remaining field curvature is directly proportional to a difference in radii between primary and secondary mirror. Now, nearly all commercial RC's have different radii of curvature, hence focal plane is not flat, but curved (concave to the sky). Without field flattener, both astigmatism AND field curvature remain.

BTW, coma has nothing to do with field curvature. Typical exmaple is a standard Gauss camera lens, which covers enormous field in comparison to telescopes, and is FLAT yet has heaps of coma in corners.

For further reading I suggest look up Seidel aberrations.