FWHM and Eccentricity, What Are Typical Values?

[jerryyyyy]

If you are interested, I have a new Takahashi 180ED that I have been collimating.  The data are out here:

http://www.astrobin.com/full/78328/F/

I am going to get some more as soon as the clouds clear here.

Jerry, did you finish your work on the collimation? I note that the link you provided showed eccentricity values around 0.5 which is above (i.e. worse) that the 0.42 that I've seen recommended for round-looking stars. It's very rare that I get much below 0.5 with any of my scopes (all fairly fast refractors). I've never been able to determine whether that last little bit to get to 0.42 and below has to happen through guiding or better optics (or focus, etc.).

Since we're now imaging at 5 micron pixels and below I'm beginning to wonder whether it is even possible to routinely achieve eccentricity values at or below 0.42 (uniformly across a wide field). Geoff posted some really good numbers, but I suspect that is for a fairly narrow field of view (i.e. probably not much more than one degree on a side).

Not sure I will ever be done, but this is an image from last night.  I posed a discussion over on a file from pfile on Astrobin.  If you look at the colored output from CCDInspector you can see the small gradient I am still trying to erase.  Am making one small change per night.  The PI output also shows the gradient.  Hopefully you can see the FHHW numbers (which are different in the two pieces of software... same image). 

[jerryyyyy]

This is the CCDInspector output.  Do not seem to put two images up together.

[Geoff]

Geoff,

What settings to you use for your auto focus routine for the CDK and are you using PWI or some other software for focusing?  I image with a CDK12.5 as well, but your FWHM numbers are coming in substantially better than mine (mine push against the 4" mark) so I would appreciate understanding your method.  CCD is an F16M and I use 600 sec iterations as well.  My eccentricity is fine, typically in the 3.5 range, so its just the focus that's the issue.  Could just be my skies, but I am curious.

Thanks,

Jim

Hi Jim
I use the PWI software for focusing with the default settings (attached), which requires the use of MaximDL.  The camera pixel size is 7.4 microns, but I usually bin 2 x 2 , giving an image scale of 1.2 asp.  I know the CDK is supposed to have very stable focus, but my experience has been that you still need to refocus if the temperature changes by more than 2 or 3 degrees celsius. 
Skies can be a problem--there have been times when nothing I did could get me under 4" FWHM.
Geoff