focusing a SCT with a f/6.3 reducer

[Tony_]

I recently bought a celestron f/6.3 reducer.
The instruction for focusing (for imaging) is to get a sharp focus at the centre of the image then turn the focusing knob 1/24th turn anti clockwise.
I guess this is to give a uniform focus? However, it mean the focus is out at the centre (?) where the main DSO will be. The centre spike of the bahtinov mask moves a fair way from the centre.
I thought the idea of a reducer/corrector was to get a uniform, in focus, image across the entire field.
Is there a better method - or does this sound okay?

Regards,
Tony.

[Merlin66]

The SCT reducers, both Meade and Celestron IMO are average performers. I don't think they do their job as effective "flatteners"
The Optec Nextgen reducers I found to be MUCH better.

[Tony_]

That's a pity - looks like more money to spend and they aren't cheap.
I had a Hirsch f/6.3 reducer that wasn't too good so I bought a Celestron brand hoping it would be better.
It is better - but as with the Hirsch the collimation drops off rapidly from the centre of view. I suspected the field can't be too flat if the collimation isn't good over the whole field. My first image wasn't too bad but my next too were useless.
I'll try again. Otherwise, as difficult as it is, I'll go back to imaging at f/10.\ until I decide to buy another reducer.

Tony.

[5ash]

Hi Tony ,
I've used a Meade f6.3 with my c9.25" and have been happy with the result., however the c9.25" has a slightly different combination of f ratios for its primary and secondary to regular SCT 's which may make a difference. What is your scope? Can you post one of the pictures you took?
Regards philip

[Tony_]

Here's one I was reasonably happy with (no worse than my efforts at f/10).
The field seems fairly flat but I'm not sure about the center focus.
The seeing wasn't great so I wouldn't expect a really sharp image.
My SCT is a 9.25", the coma isn't too bad without a flattener.

[5ash]

Hi Tony ,
Field doesn't look too bad , I suspect some asymmetry as on some sides stars look more round than others. Also some "chromatic" effects across some brighter stars . Is this an LRGB , OSC or a DSLR image? Have you checked your collimation? I have always focussed a star in the centre of the field using a mask then taken my pictures of DSO's in the centre of the field.
Regards philip

[Tony_]

I think the collimation wasn't too bad. There was a liquid on the secondary mirror (this could have affected the optical performance). It wasn't until a few days later when I took the corrector plate off that I discovered the liquid had dripped down from the focuser - it was the lubricant.
I will have to relubricate it at some stage - not sure how.
I wouldn't be surprised if my corrector plate is out - I have the rotational direction close enough, but the first time I took the plate off (a few years ago) I didn't realise there were small spacers in there. These came out and I basically had to put them back randomly - not sure how much difference it makes. This could account for the chromatic effect on some of the stars - but I haven't noticed it before. The lubricant on the secondary mirror wouldn't help.
This image was taken with a CDS canon 60d at about -15C. 90 x 30 secs @ ISO1600. I've had trouble guiding lately so I'm down to short exposures.

[5ash]

I assume when you took the corrector off you cleaned the secondary? The photos were taken with a clean secondary ? If the corrector is not in the correct position the telescope will not perform very well , this might be your problem not the reducer? Lubricant getting on the mirrors is reputed to be a problem with some c9.25" made in china in recent years. I have not left my scope pointing down as a precaution .
Regards philip

[Tony_]

Can anyone in Australia re-align the optics of an SCT? If not I may have to do the best with how it is.
I haven't noticed the chromatic effect on stars before. This image was taken before the corrector was removed and the secondary was cleaned.
I have checked a few previous images - images I have taken without the reducer do not have this chromatic effect - so I think it may have something to do with the corrector or imaging train.
I don't think the position of the corrector would be out by more than a fraction of a mm - I am a bit skeptical whether it would have been that precise to start with?

Tony.

[5ash]

Hi Tony ,
Collimation of a SCT is achieved by looking at an outer focus star in the centre of the field of view and adjusting the 3 secondary screws to get a symmetrical image. Look up the instructions on the web, only small adjustments are needed , if needed! Do not loosen any of the screws by more than half a turn.
Regards philip

[Tony_]

Thanks Philip,

I know how to do the collimation - I have Bob's knobs to make it a lot easier.

What I wanted to know is if anyone in Australia can realign the corrector plate - if it needs to be done. I suspect this is not so easy.

Regards,
Tony.