Celestron focal reducer and coma

[MarkS]

I may have found the reason for why I still get coma away from the image  centre when using Celestron F6.3 focal reducer/flattener:  its because my distance from CCD to FR is wrong.

The reducer came with no instructions saying what the optimal FR-CCD distance should be so I looked on the web (this was 15 months ago when I bought the scope).  Various people quoted that the Celestron F6.3 focal reducer has a focal length of 285mm and so the FR-CCD distance should be 105mm (to achieve F6.3).   The standard Celestron SCT T-adapter that I use gives this spacing exactly so I thought everything was O.K.  Until last night that is.

I actually measured the focal length last night and it came out to 230mm, NOT 290mm.  At 230 mm the FR-CCD distance needs to be 85mm not 105mm (to achieve F6.3). Using a spacing of 105mm actually gives a focal reduction of F5.2 not F6.3 so it appears I've been using it an a non-optimal distance i.e. the wrong distance to give field flattening.  I've independently confirmed that I am getting F5.2 by measuring the reduction I get between 2 images - one with and one without the FR.   Why did I never do this before?

Revisiting the web I find that some people quote the Celestron F6.3 focal reducer as being 285mm and needs a spacing of 105mm.  Others say it  230mm and needs a spacing of 85mm.  Maybe there are two versions? Who knows?

In any case I have the second version of the FR and I now need a SCT T-adapter with an optical length of 30mm instead of the Celestron standard one which is 50mm.  Bizarrely, Celestron don't list one.

Moral of the story:  don't believe what you read on the web and don't assume the manufacturer is selling you a compatible set of components. 

Does anyone know where I can find a SCT T-adapter with an optical length of 30mm?

Mark

[Rocket Pooch]

Hi,

If your putting this onto the camera get some T extension tubes, its going to be easier to find than the SCT ones.

Chris


P.S. Good find by the way, at least with the AP Kit they tell you the exact spacing required.

[Mike]

Mark,

if you can't find one get one made. There is an engineering guy in Bexleyheath that would make one for you.

[RobertM]

You could try Baader, they do lots of different adaptors and the quality is good.

[JohnP]

Well found that man.... Does that mean your images are going to get even better.... I can't wait to see your first light when you get the correct spacer....

Good Job, John

PS - I should look at my WO 0.8X flatterner - I've never bothered with spacing just put it in anywhere but then again the chip on Atik16ic is so small I doubt it makes a difference. I know Fay still has issues with coma using her's with the EOS. Out of interest how did you accurately measure focal length...?

[MarkS]

To measure the focal length, use the lens to project the image of a distant object (sun, moon, streetlamp) onto a wall or a sheet of paper.  When the image is in focus, the distance of the lens from the wall or sheet of paper is the focal length.

Mark

[JohnP]

cool - that's what I thought - just wasn't sure how distant object had to be... thks

[Rocket Pooch]

John,

I don't know if you remember this but Olly had a ruler in his observatory he used it to prefocus i.e. if the scope was 500mm he would measure from the front on the lens to the focal plane of the camera, and bingo the image was 90% in focus, I showed him this a couple of years ago, saves mucking about.  Does not work for SCT's though.