Can you please help me decipher what the results of CCDWare's CCDInspector mean?

[oymd]

Good evening everyone

I had some tilt on my WO Star 71 a week ago, and I was advised to tighten the screws that grab the focus tube, and it seems to have fixed the issue, as I no longer have play in the imaging train.

I was advised to use CCDWare's CCDInspector to analyse my images.

Can you please advise what the results mean? I had never used the software before, and a search on the forums did not help?

The way I used it was look up my best reference frame in the log file created by WBPP, and opened it in CCDInspector, and clicked: Measure ALL, then had a look at CURVATURE, and 3D plot?

But what do the results mean? DO the results have anything to do with the actual scope and setup, or just the camera's sensor?

I've also done the same with my image of IC1396 that I took with my Esprit100ED last month, and I chose the best reference sub from the stack according to WBPP.

Many thanks

William Optics Star 71. Image was IC 1848.

Curvature

 

3D Plot

 

 

Esprit 100ED. Image was IC 1396.

 

Curvature:

 

 

3D Plot:

 

 

Lastly, here's the analysis of the VEIL Nebula when the Star 71 had bad TILT, before I tightened up the imaging train:

 

Curvature:

 

3D Plot:

 

Obviously with the last 2 images, there is an obvious problem. Its been fixed now, but how do I interpret the results of the first 4 images?

Is 11% curvature OK? What should I look out for?

Thanks again

 

[oymd]

Anyone can shed some light please?

@vlaiv Need your help here!!

 

[vlaiv]

I'm not particularly versed in interpreting CCD inspector results, but I did look at first two images and I can't really say that there is something wrong with them in terms of tilt or curvature or anything.

CCD inspector results do depend on actual stars being imaged. You should really pick rather uniform star field without much in terms of other targets to asses your optics.

Another option would be to use artificial star.

You can asses things with artificial star without CCD inspector. Just place artificial star in center of FOV and take one image. Then slew scope so that star is in each corner and take another image (same exposure). Measure FWHM of all images and compare.

Ideally - you want numbers to be close and corners to be the same (maybe center will not be the same as corners - but as long as corners are roughly the same - it is ok).

Flatteners and focal reducers will not have same spot diagram in center and in corners. This can create "sense" of field curvature - where there is none (CCD inspector will see higher FWHM at edges than in center).

For example - here is Riccardi FF/FR:

RMS spot radius is 3 microns on optical axis - but 6 microns at 11mm away from center. This does not mean that field is curved and that there is defocus at 11mm - it simply means that design of optical element is not perfect (and never is) and that there are some aberrations that make star a bit larger at edge of the field (but much less than if actual field curvature was present).

As long as your stars are round over whole field and differences in FWHM are very small - you have good field.

[oymd]

@vlaiv

thanks vlaiv, very informative as always. 
 

What is an “artificial star” by the way?

Is it something you make or buy???

[vlaiv]

5 minutes ago, oymd said:

@vlaiv

thanks vlaiv, very informative as always. 
 

What is an “artificial star” by the way?

Is it something you make or buy???

You can do either - you can make it or if you don't want to mess with things - purchase one.

Artificial star is anything that will act as a point source to telescope. If you place it far enough it will act as true star.

It is used for different things - like scope collimation, checking optics of the telescope and in general - all the things that you would do with regular true star, but it can be used when cloudy and does not suffer from seeing.

There are several ways to make one:

1. small ball bearing will act as artificial star if placed far enough and illuminated by bright source (shiny little ball will have only one point that reflects light in direction of telescope and will look like single dot)

2. Strand of optics fiber cable with LED on one side will act as artificial star

Or you can purchase one:

https://www.firstlightoptics.com/other-collimation-tools/hubble-optics-5-star-artificial-star.html

above is quite "crude" one - at 50µm, but there are models with smaller hole like this:

https://www.teleskop-express.de/shop/product_info.php/info/p10781_TS-Optics-artificial-Star-for-Telescope-Tests-and-Collimation.html

22µm

or even smaller hole:

https://www.teleskop-express.de/shop/product_info.php/info/p7258_Pierro-Astro-PocketStar---Artifical-Star---White-Light---only-9-micron-diameter.html

9µm

I have TS one with 22µm hole. Here is example of test I did for Samyang 85mm F/1.4 lens:

from left to right: F/1.4, F/2, F/2.8 and F/4

Artificial star was at 5m distance, and test performed in basement

Here is another example - comparing lens edge vs center performance on different F/stop settings:

(this was done with mono camera and no filter - image enlarged to 400%).

[Adam J]

I think that the biggest issue is that I at least have never found any documentation properly explaining CCD inspector. It's like they want you to work it out yourself.