First Light pics GSO 8RC

[rat156]

Hi All,

Other than the Jupiter shots I took a few nights ago I got some of NGC 6744, mainly to test out the field flatness etc.

Here's the results of 6 x 10 minutes exposures, stacked.

As can be seen there's a bit of a problem in the upper LHS of the frame.

There's a crop from the lower RHS and upper LHS as well so you can see the difference.

What is the most likely cause of this?

Cheers
Stuart

[renormalised]

Focuser looks like it probably has a bit of play in it. Check your collimation, though.

[gbeal]

Hard to say Stuart, but I agree with Carl, focus is slightly soft on the image, and this won't help. It is the bog standard focuser? Peter (prmd) has already had a vent about the standard one, so maybe link up with him.
It seems accepted that you need to "upgrade" the focuser.
Looks nice though.
Gary

[Peter Ward]

Stuart,

CCDinspector says collimation.

Rgds
Peter

[rat156]

Hi Peter,

Thanks for that. I should really get around to using CCDI...

Could this be the focuser out of collimation? The planetary pics with a much lighter setup looked pretty good. I did notice that when I loaded everything into the focuser it was struggling to hold the weight.

Cheers
Stuart

[Peter Ward]

I very much doubt it.

The trick is to use the extension rings so the focuser extends as
little as possible.

I found the friction adjustment on the focuser to be very usable, it was able to lift a STL with no problems, though I did find the 10:1
slipped when working against a heavy load.

[rat156]

No extension rings used at all, couldn't reach focus with my setup with any in. I don't think that the focuser was out much at all. So this is probably the scope's collimation then?

Cheers
Stuart

[AlexN]

I agree with Peter on that... use as many extension tubes as you can so you only have to rack the focuser out the shortest distance possible.. I got mine to the point where I was only racking the focuser out 4mm to achieve focus and had very little focuser slop problems. if any..

Your image shows great detail in 6744.. Thats a really tough nut to crack in my experience..

As im sure you are aware, collimation is paramount to success with the RC optical design.. The first nights I had mine I spent a few hours making sure it was as good as I could possibly get it.. once complete, the images the scope produced were VERY pleasing.. The ST10XE I had at the time gave a fantastic field of view! You should have a lot of fun with this setup!

[Peter Ward]

Stu...if you are using an AO7, try imaging without it.

[AlexN]

He's running AO-L Peter.. His problem is that the total distance to the CCD is rather long with ST10 - CFW - AO-L - MOAG

[rat156]

Using an AOL, so no mirrors (other than the scope).

I suppose if I image directly at zenith, then either side of it if the picture changes then it'll be slop in the focuser, if not then I'll collimate it. The star test through an eyepiece was pretty good though. Do these scopes hold collimation well? Can I use a laser?

Cheers
Stuart

[Peter Ward]

1) Yes.
2) Yes, but don't shine it at PolAir 1 :)....lasers are OK for coarse alignment,
but nothing beats an airy disk ;) CCDinspector is pretty good too.

[Paul Haese]

Just seen this.

It could be that the sensor is not square to the optical line too. It would only need to be a little out to create this effect. Checking the collimation would narrow down the problem though.

[rat156]

Hi all,

I checked the collimation by taking more photos.

I took additional car not to screw up the focus lock too tight, as I'm pretty sure that was moving things about a bit. If the camera was pivoting about that point it may have sagged in the focuser towards (or away from) that point.

A also used Peter's 10-20% focus method.

Here's the result (yes I have since cleaned the clear filter, I'd rather do that than take flats for every filter).

CCDInspector of the subs reveals that the collimation value varies from 7.7" to 1.9" in totally opposite directions, unsurprising IMHO, I've never gotten a consistent result from CCDI when I was close to being in collimation, it bounces around. To be fair it was complaining that I had too few stars in the frame.

Anyway, I have a focuser fix on the way from the states that will satisfy me and allow me to use my Robofocus setup.

Cheers
Stuart