ASTAP vs CCDI - Forums

Topic starter

Jan 24, 2023

I've had plenty of tilt problems, and so I use both ASTAP and CCDI to measure my tilt in subs and see how (bad) I'm doing.

Ok, yesterday I measured a particular sub in ASTAP (taken with ASI294/RASA 8). It showed my tilt to be "mild" at 18%. Then I measured the same sub in CCDI. It showed a whopping 42% tilt under "curvature". How could they be so different? That's well over a 100% difference!
I see this happen every time I compare the same sub in both, CCDI shows MUCH higher tilt every time. I can only conclude that they must be measuring different things.
Can anyone here with any knowledge of these softwares tell me what is different about the measurements produced by these 2?

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Josh Jones

Jan 24, 2023

So they can and will certainly be different. The measurements may be calculated differently, but also the amount of stars used to calculate could be different in each software as well. ASTAP for example if you hit F5 lets you select extra stars which pull in more data for more accurate results will change results just within itself. Do you have screenshots of both? Biggest false readings I see from these tools is the lack of a good starfield and absence of nebula in the images. They really like loaded starfields (parts of milky-way) but with no nebulosity in them or it can throw them off. (just my experience)

Regardless of the value of tilt, do both correlate to each other in terms of direction for the most part?

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Paul Wilson

Topic starter

Jan 24, 2023

Paul Wilson

Topic starter

Jan 24, 2023

Here they are. These are of the SAME frame, #190 in both ASTAP and CCDI. These are a different frame than the one I started with, but same deal.

I included the finished image above, I think it's a bit strange how the diffraction spikes all lean in a diagonal. It was a test image on a starfield as I try to learn NINA, coming over from ASIAIR Pro in order to use the tools.

Josh Jones

Jan 24, 2023

thanks for the info… so I know your original question was what's the different ways they are calculated? Which I don't know, but if you like I can give a couple things I would do if this was my setup and how to start down the road to getting things looking a tad better?

1. I would shy away from shooting a cluster in the center. A more flat field of consistent stars might be a better approach to getting consistent measurements. here.
2. At first I looked and thought a ton of guiding issues going on there, but it's those diffractions for sure and that's just cabling and easily solved. A nice arch pattern will help… or a complete 90 degree cable guide setup for a nice 4 point diffraction spike. Diffraction spikes like you have can lead to smaller stars falsely appearing to be 'rice' or 'eggs' but in fact they are just diffractions. I would shoot to maybe work out the cable guiding as well here, get round stars.
3. In ASTAP you might turn on the 4-corner inspection by hitting F5. Gives a bit more realistic sensor modeling in my opinion.

I think some simple steps there, may lead to some better results with the tools to begin with… others may have some good tips here.

Back to your original question though, notice CCD is using 1647 stars, ASTAP is using 2744 stars… One is using FWHD and the other HFD. I guess my point there is expect the software to be different. I just wouldn't get to hung up on the different values, but really the interpretation of that data to be consistent and to help move in the direction of the best image you can get.

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Joe Linington

Jan 24, 2023

Those diffraction spikes look like a Bahtinov mask. Is this common with the RASA scopes?

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Paul Wilson

Topic starter

Jan 24, 2023

The spikes haven’t always been this weird. I don’t mind spikes but leaning like they are is a diffraction distraction

Paul Wilson

Topic starter

Jan 25, 2023

Josh Jones:
thanks for the info... so I know your original question was what's the different ways they are calculated? Which I don't know, but if you like I can give a couple things I would do if this was my setup and how to start down the road to getting things looking a tad better?

1. I would shy away from shooting a cluster in the center. A more flat field of consistent stars might be a better approach to getting consistent measurements. here.
2. At first I looked and thought a ton of guiding issues going on there, but it's those diffractions for sure and that's just cabling and easily solved. A nice arch pattern will help... or a complete 90 degree cable guide setup for a nice 4 point diffraction spike. Diffraction spikes like you have can lead to smaller stars falsely appearing to be 'rice' or 'eggs' but in fact they are just diffractions. I would shoot to maybe work out the cable guiding as well here, get round stars.
3. In ASTAP you might turn on the 4-corner inspection by hitting F5. Gives a bit more realistic sensor modeling in my opinion.

I think some simple steps there, may lead to some better results with the tools to begin with... others may have some good tips here.

Back to your original question though, notice CCD is using 1647 stars, ASTAP is using 2744 stars... One is using FWHD and the other HFD. I guess my point there is expect the software to be different. I just wouldn't get to hung up on the different values, but really the interpretation of that data to be consistent and to help move in the direction of the best image you can get.

*** Gosh turns out I had the cable router sitting unused right in one of my parts boxes! I am heading out to the garage to install it

Paul Wilson

Topic starter

Jan 27, 2023

Just a follow up for people needing help on this issue in the future.

I quickly installed the simple, curved cable router on my RASA (which I meant to do ages ago).
It amazingly eliminated the "weird" slanted diffraction spikes. The spikes made it look like I had star streaks from bad guiding (or something).
I was stunned at how effectively the router was able to give my stars a very much nicer appearance.
Thanks for reminding me!

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