Odd star shapes in SGP

I have an issue with my camera on one of my setups. It is the ASI 1600 mono through a Stellarvue 102 and dedicated flattener and the ZWO focus motor on a steel track style of Feathertouch focuser.

The issue is, when focused the star shapes are unusual in SGP, when in comparison to CCDCeil they are round.

Any thoughs?

Same camera & scope?
Same day & time?
Same focus points?
When & how did you focus?

That looks like a diffraction pattern. Spider web on the front of the scope?


There is only a few points difference between auto focus in CCDCiel and SGP.

It was done on the same night at the same time as I have had difficulty getting this PC to run SGP properly. I am now close and getting autofocus to run effectively in SGP is my only hurdle, hence the tests between CCDCiel and SGP.

CCDCiel tends to give a slightly sharper focus, a few points different to SGP, which is when I noticed the star shapes in SGP were not round.

If it were diffraction from something in front of the lens, I thought it would occur in both software programs, as it is on the same night at the same time, give or take a few minutes here or there.


Some off-the-cuff thoughts:

In your images above, are you showing at 100%. If the images are downscaled it could be the display is showing some minor artifact from a (possibly different) downscaling algorithm. I cannot imagine that SGP is distorting the actual captured image files unless the imaging system is itself capturing somewhat distorted files.

As both SGP v4 and CCDCiel have options to use ASTAP to calculate AF image focus metrics, how do the HFR/HFD focus metrics actually compare? For a real detailed comprison maybe you should load your AF images into a common program so that as closely as possible you are comparing like with like.

The left image looks very much like the result of an astigmatic in focus telescope. It might be that between your first tests with SGP and the following tests with CCDciel the lens had the necessary time to reach thermal equilibrium.

There is no way for an otherwise perfect refractor telescope to produce the stars on the left side by just defocusing it.

Kind regards,

Make the same subframe with both applications without touching the focus and without waiting more the 5min between frames, then you will know if it focus or software related.

You have a camera with a well depth of 20K pushing gain and when the 20K is scaled to the 65K you get what I found out some time ago. Use your Dynamic range. I have a 1600MM and very rarely use it beyond the 0 gain Dynamic setting.

Hi Everyone,
Welcome JoeRome77 to the forum!

I’ve used CCDCiel with this setup for a long time now, I have a spare licence for SGP hence the comparisons to transition to SGP.

The above star comparison image is two side by side images shot at full camera resolution and the same gain, one in SGP the other in CCDC zoomed to the stars you see, in Pixinsight.

I followed all the focusing setup calculations and instructions to setup the auto focus feature in SGP and after my first shot, immediately did not like the look of the stars compared to CCDC. I took quite a bit of time doing this with different step sizes, all with successful focuses but with the odd star shape.

There is a difference in the end step position when auto focusing between the two programs, but each time with SGP I was unhappy with the star shape. Finally I auto focused in CCDC, took a 2 second image, immediately closed CCDC booted SGP and waited for the same chip temperature took a 2 second image at the same focus point as CCDC had focused at.

Realising here it wasn’t the difference in focusing but how the image is being taken by SGP as far as I can tell, and please note there was no spider or spider web moved or not moved in the production of these images!


This could be a slight stretching difference. May be CCDC has less stretching applied.

Open in SGP, the image you took with CCDC and the one you took with SGP (both images taken at the same focus point). Apply the same stretch in SGP to both images and compare again.

Also make sure you’re using the ASCOM driver for both. I’m not sure if CCD Ciel has native ZWO support or not but that could be another point of divergence. SGP does literally nothing to the raw data off of the camera. It goes from the driver, into the fits with no alterations. We do this very purposefully.