Hi Paul,

Could I get your opinion about this field curvature map.

I am trying to tune a new Philipp Keller scope.

Imaging train:

500mm classical cassegrain F9/dedicated field flattener/AOL/MOAG/STL11000

I spent a lot of time adjusting collimation.

Focus was not optimal I discovered afterwards and seeing was moderate at best.
Image taken at aprox 72 deg altitude.

Many thanks,

Tim

Keller20inch_Curvature_map1.jpg (102 KB, 67 downloads)

Hi Tim,

Looks pretty good! Better focus and seeing may help get more precise results, but I see nothing to worry about in this map. Tilt is 4% which translates to about 0.1 arcsecond which is not noticeable, and collimation is very good.

Curvature is small enough also not to be noticeable, but the number will likely get better with better seeing conditions and better focus.

Looks like a great scope!

Regards,

-Paul

Hi Paul,

Thanks very much...It was far easier to collimate the scope using your software....
Also, this scope has three collimations screws at the back end as the primary mirror is the one adjusted, although the secondary can also be adjusted , but only if necessary....it was interesting though, how the single star defocused seemed to require markedly different collimation depending on how out of focus the star was...in the end it was much better to use the multiple star collimation for this scope....

Cheers,
Tim

Hi Tim,

Glad to hear the multi-star collimator is working well for you.

The primary mirror is the one to adjust on a Classic Cassegrain like yours. Secondary is usually adjusted on systems with a movable primary, such as most SCT.

I'm curious about your experience with the single star collimator. There are a few things to keep in mind that could affect the outcome:

1. The star should be defocused enough to occupy at least 50 pixels diameter, more is OK.

2. The star should not be near saturation or in the non-linear region of the CCD anywhere in the star image. This usually means keeping the ADU count under about 45,000 for most NABG cameras. For an ABG chip it should be kept a bit lower. As the star image gets closer to good focus, it may become saturated or cross into the non-linear region, so be careful when adjusting focus or changing binning.

On the other hand, S/N ratio of the star should also be kept reasonably high. This means that the star shouldn't be defocused so much as to produce low S/N ratio. I prefer to keep the ADU of my defocused star around 30,000-40,000 with my NABG ST10xme.

3. The star should be kept close to the center as much as possible after making adjustments. Designs with strong field curvature and other aberrations will distort the defocused star enough to cause skewed readings when the star is not near the center of the optical path.

Please let me know if any of these may have played a part in your testing.

Regards,

-Paul

Hi Paul,

I didn't use your single star collimation...I was doing that by eye only ...when that was problematic I switched to CCDI and did the multi-star collimation which was very good to use....

Cheers,
Tim

Hi Tim,

Aha! I misunderstood. I thought that by now nobody would be using the outdated visual method!

Regards,

-Paul

quote:

Originally posted by Tim-Jewel Box Observatory:
...it was interesting though, how the single star defocused seemed to require markedly different collimation depending on how out of focus the star was...