StarTools

Hey Everybody,

Can I get some advice on processing images in Star Tools that were captured on an Alt-Az mount? I have a Celestron Evolution and was wondering of there was a general guideline for the length of each exposure and total amount of exposures that would work well for processing before the field rotation begins to adversely affect the results.

Thanks,

Patrick

Hi Patrick,

since no one else responds, here are my two cents ... the ST guidelines for stacking recommend to stack with "Intersection Mode" for example in case of using DSS:

https://www.startools.org/links--tutori ... r-settings

This means, that all areas in your resulting stack are based on the same/full number of input sub frames, resulting in a constant SNR throughout the whole data set.

Field rotation therefore means, that the intersection area will become smaller the longer your imaging session lasts. So it's up to you to decide what's "too small"

Please take this with a grain of salt. Hope that helps?

Best regards, Dietmar.

that makes sense - thank you Dietmar!

Hey Patrick,

Dietmar is spot on. Plan ahead for the rotation, and either address it with counter-rotation during pauses, or just know ahead of time to center your target and have the intent to do major cropping of the sweeping overlaps. With a lot of field rotation you'll be down to just the center.

How long to go on individual subs will depend on just how good your alt-az tracking is, and your focal length. You'll have to take test shots and see how the stars look.

There are some who can make it work, but it takes a lot of effort. There's a guy who uses a 10 or 12 inch goto dob and makes some great deep space images. I'm not sure if he crops or rotates, maybe both. But after testing on his target he takes quite short subs, like maybe 8 to 15 seconds depending on what kind of trailing he saw in the stars, but he takes lots of them. Like hundreds, if not thousands. Because total integration time is still the key. Of course dark skies and fast focal ratio can help you on that front also.

As you can imagine, that puts a lot of pressure on your storage capacity, and your computer's ability to do major stacking jobs on so many files.

Take some test shots and let us know what you find about trailing and field rotation over time.

keeping it east or west below 60 degrees helps with field rotation and keeping the focal length down helps, plus keeping the weight down. I find each time I image I have to experiment to see what exposure length works best that night even if I anticipate getting 20 seconds I still test first.

A good article on how to estimate exposure times based on your latitude and the targets altitude and azimuth. The technique is applicable to any alt/az mount. https://telescopemount.org/short-exposu ... t-results/

About 2/3 down the page you'll find some charts the author created with approximate exposure times. You pick the chart that has your latitude and use the rows and columns to determine the exposure time for your target. I use the chart very frequently with my DSLR and alt/az AP rig and found it to be quite accurate.

It's a lot easier to fix field rotation problems if they aren't there to begin with.

Clear skies.