CCDInspector

[kinetic (Steve)]

Hi,
I have read a recent post about the GSO RC-8" scopes
and field flatteners and people using CCDInspector to
check the flatness/ collimation of the OTA /CCD image train.

Could someone possibly please tell me if these two images
which are, pretty much, optimum resolution due to very good seeing
show any sign of field curvature or collimation defects? EDIT: see below

Thanks in advance.

Edit: sorry , didn't realise there was a free trial...
Here is a screengrab....there are differences in the two images.

Does this mean collimation is out? Or is this a field flatness issue?
I'm not sure why there would be differences either?



Steve

[avandonk]

Steve you need to take images of a part of the sky where the stars are both faintish and about uniform in brightness. You can see that the bright bunch of stars in the bottom right hand corner of the second image gives a false indication of more curvature with CCD inspector.

Bert

[TrevorW]

I've used CCD and had readings that varied from image to image.

Personally I try to avoid such things as they then too add confusion, if it looks Ok then it probably is

[multiweb (Marc)]

Originally Posted by kinetic

Hi,
I have read a recent post about the GSO RC-8" scopes
and field flatteners and people using CCDInspector to
check the flatness/ collimation of the OTA /CCD image train.

Could someone possibly please tell me if these two images
which are, pretty much, optimum resolution due to very good seeing
show any sign of field curvature or collimation defects? EDIT: see below

Thanks in advance.

Edit: sorry , didn't realise there was a free trial...
Here is a screengrab....there are differences in the two images.

Does this mean collimation is out? Or is this a field flatness issue?
I'm not sure why there would be differences either?



Steve

Hi Steve, as Bert suggested you need to take the shortest exposure you can of a uniform star field devoid of any nebulosity. If there are any bright star around try to keep it in the center of your field. Otherwise the information you'll get from CCDIS will be affected. Why short exposures? Because field rotation or any other tracking issues will be translated into different aspect%, FWHM and it's going to pollute your data. Then you can start tweaking things. The advantage of centering a bright star too is that the halos/reflection will give away any misalignment.

[kinetic (Steve)]

Trev,
I would tend to agree with you mate...but on digesting Bert and
Marc's very informative advice I can see I need to do some
homework with CCD Insp.
Thanks guys, I really didn't think it would matter, I thought both
images would be good candidates just based on the sharpness of
stars (these are several of the sharpest EVER results with this setup)

But I can see what you mean now Bert/ Marc....thanks for the tips.

Steve

[multiweb (Marc)]

Another point you may want to consider. The orientation of your camera. Assuming your CCD is rectangular (like most) keep in mind it's easier to center the secondary sideways than up or down. What I mean is that you have total control about the centering of the spider square with the focuser but closer or away from the focuser is tricky and that's where most of the secondary tilt will occur once you're collimated. So if you can live with your camera being rotated 90 degrees so the short side of the CCD is parallell to the tube it'l be easier to get good stars on the edges in that configuration. If this makes sense?