GSO RC8 field flattener recommends

[DJT]

Hoping the collective brains here will help.

Am running an RC8 with an STL6303, using a 2.5 " moonlite focuser with adapter to the STL

I notice that when self guiding with the STL, the guide sensor is above the main sensor and with no flattener this means the guide stars have an element of coma

I am not sure if this impacts guiding when using centroid method in CCDOPS but suspect I should probably get a flattener at some point.

The astronomics dedicated 2" flattener has limited 55mm back focus and as I will want to put an MMOAG into the mix at some point for NB this wont be effective.

Wondering what other flattener options people are using with this sort of combo. I know Paul H has used a TAK flattener for a 130 frac of some sorts to good effect and I think the AP FF may suit but interested in other people's experiences ( including " get over it, it should'nt impact guiding.." )

Thanks

[alpal]

Hi David,
Someone told me that they were using an Astro Physics flattener with an
RC8 & it also worked on the RC10 & gave a much flatter field.
(They may have also been talking about the reducer as well.)

Of course such a part would probably cost an arm & a leg.
It might be worth a Google search.

cheers
Allan

[graham.hobart]

I know on here recently I was recommended an AP CCDT67 telecompressor for my RC 8. I haven't used it on this yet but it works well on my C8.
There is a diagram on the AP site about the amount of compression you get the further forward you are in the light cone.
Graz

[DJT]

Thanks Graham and Allan

I have the AP reducer flattener as well but would like to keep this scope native. Some feedback from a fellow ISer suggests I should revisit collimation first before trying to resolve the field flattness issue with more "stuff", so am off to beat the C<"p out of the piggy bank to get enough for a Tak Collimator (and associated takettes..though joy of joys, I have one takette that can be reused for once).The colimator would be useful to have anyway.

[Meru]

Hey David,

Teleskop express offer a wide range of FF which would suit your needs, they can be found here - http://www.teleskop-express.de/shop/index.php/cat/c138_Astro-Imaging----Reducers-Converter-Correctors.html

Also yes collimation is extremely important, but it's never affected my guiding when I dont use correctors. I've gotten 60min sharp subs at the native FL without a FF or good collimation. You do not require any expensive equipment for collimation either (Infact, absolutely nothing except a screw driver and allen keys). I managed to get it pretty much bang on with just those tools and this guide - http://www.deepskyinstruments.com/docs/DSI_Collimation_Procedure_Ver_1.0.p df

FYI I personally have the Orion field flattener which works beautifully, especially for $149 new. It does only have 55mm backfocus though. I think the WO P-Flat worked quite well (not sure of its back focus). Lastly as per Graham's post, I also have the CCDT67 it works beautifully on both my RC8 & 80ED (And my 120ED when I had it). I know you're interested in imaging at the native FL but just thought I'd put it out there :)

[Peter.M]

Though I have never used them, I always wondered about using an RC8 and these two with a 2.5 inch moonlite

http://www.teleskop-express.de/shop/product_info.php/info/p2646_TS-Universal-BIG-field-flattener-for-refractors-from-f-5-to-f-9-for-full-size-sens.html

http://www.teleskop-express.de/shop/product_info.php/info/p4089_Adapter-from-MoonLite-2-5--Crayford-to-TS--2-5--Flattener.html

It is attractive because it would be all threaded, has room for an AO unit if wanted and would hopefully provide a large corrected circle.

[DJT]

Thanks Peter for the links. I tried a new HA filter the other night which confirms I really should go the OAG route so will see what what transpires on that side of things. It would be interesting to see what TS can do for the various adapters for the OAG, whichever one I end up with. Moonlight do one for the 2.7" AP adapter that comes with the MonsterMOAG which is a fair bit cheaper than the TS one, which is good to know.

Jury remains out on the flattener still though, at least until I get another chance to look at collimation.




Thanks Meru
I had a good look through the DSI link. Really pleased with that as it provided a very good explanation of what is going on with the RC as well as the procedure. Again, waiting for clear skies to try this out but thanks for the link.

The original point on this I suppose is that with the internal guide sensor being above the imaging one, its picking up stars which show some astigmatism for calibration and guiding purposes and the concern is whether or not this impacts guiding. Am only just getting to grips with CCDSoft for acquisition anyway so as I get through the learning curve this may not be a big issue after all but if it is then at least there are options for correcting the image by the looks of things.

Collimation actually seems pretty well "balanced" based on the link by the way, but always worth another look.:)

Thanks for the responses everyone, very helpful indeed.

[clive milne]

David,
One small correction....
Off axis images in an rc8 will not suffer from coma at all.... not even a teeny bit.
The aberrations inherent in the design are astigmatism and defocus due to (steep) field curvature.

[DJT]

Cheers Clive, Noted

[alpal]

You have the AP reducer flattener!
I don't think any other reducer flattener would be better -
from what I heard.

[DJT]

Hi Allan
Looking at the AP site they state ,

"At 0.67x, the CCDT67 will cover the chip of the ST-10 and ST-8300. It can never cover the chip of an STL series camera or the newer large size digital SLR chips, because its clear aperture is no larger than their CCD chips." Dont fancy losing the real estate if i can help it. Their recommend is for the 2.7" flattener.

I originally bought the CCDT67 to use with the 60Da, pre ccd days, but for now I am still aiming to stay native. Fix collimation and crop I reckon for the time being.
cheers