6 replies to this topic

[subhasism]

Hello CN ers

  recently, I bought an AZ GTE mount from this forum to configure a grab and go telescope for me. I use a sky watcher 72ed and asi224 camera and I use an ASI air plus to control the mount in eq mode, camera, polar align, plate solve and live stack. Everything seems to be going fine except within 3 to 5 minutes, I see the stacked image to have artifacts at the edges.

 

I have a good polar alignment and I do have the mount leveled and I also have a good set of calibration frames so I’m not sure what is causing this except I am thinking the mount has tracking issues.

 

 Will be looking for your experience using this mount for live stacking, and if you can think of any other reason, other than tracking error for a stacked image like this.

 

Thanks in advance

 

Subh

Edited by subhasism, 06 June 2023 - 10:51 PM.

[DarrylS]

I have an AZ-GTi (on an EQ head) with a 72ED and an ASI178, so a fairly similar setup, and I always get this artefact. I’m just guessing, but I think there is probably some flex or slop somewhere in the mount or EQ head to tripod connection.

[bmcclana]

Compare your pixel scale to the artifacts to get a measure of how much actual drift you have against the sky.

Also, plate solve the image to determine the rotation made of the drift/artifacts are aligned to the dev, RA or a combination.

My AZGTi has a pretty significant periodic error. I have measured it as ~40” arc-sec and one time when something was a little loose at more than 90” arc sec.

I have started using a guide scope when I want to expose longer than 15” or stack for more than 10 minutes to counteract all the tracking error.

[alphatripleplus]

You are seeing the effect of frame-to-frame drift in the livestack over a period of time - that's what is causing the areas of non overlap at the edges, as the FOV of later frames do not completely overlap with your initial frame . Frame-to-frame drift can result from periodic error in RA, flex in the mount, or even slight polar misalignment over time - the latter will accumulate over time. 

 

If you have tight polar alignment, you can reduce frame-to-frame drift, but it still accumulates over time if there is any slight PA error. Periodic error in the RA drive, depending on how large it is, can only be reduced by guiding or a mount with lower PE. 

 

Look at many of the uncropped EAA captures in this forum taken with equatorial mounts, and you'll see similar edge effects after a long livestack.

[alphatripleplus]

What seems interesting is that usually the frame- to-frame drift in a livestack shows as an area of overlap that goes right to the edge of the frame. In your capture, this isn't quite the case. I'm wondering if part way through the exposure the mount or scope was nudged one way, and then later it was nudged back to close to its original position. That might account for the unusual area of non overlap.

 

My suggestion would be the next time you take a livestack, alternate between looking at the individual subs and the stack while taking the exposure. Usually, you'll see individual the individual frames gradually shift (by looking at individual stars) relative to the livestack. If you suddenly see a big jump in an individual frame relative to the stack, that suggests a nudge of some sort.

[subhasism]

Thanks for all the insights. I am doing 10s 200 g subs and the live stack I shared was 30 min integration.

Definitely no nudge as the rig sits in my backyard (no one nearby) and I am in my kitchen table.

Great advice to compare the individual sub frames to check the drift. I like the idea to measure it using my resolution and pixel size. I guess the drift is significant enough to try a guided setup also.

Will report back after checking out with all the valuable recommendations.

Thanks once again to this wonderful group of people.

Subh

[subhasism]

Hello everyone, sorry for the delay in reporting back. The last 3 weeks went in welcoming my first grand daughter to this world and spending some exciting time with her. Now that I am back to the normal world of mine I tried a few things and got some real good success which I would like to share back.

 

1. First of all I realized that there is a lot (and I mean a LOT) of play in my DEC motors which led to bring in auto guiding to fight the backlash and dreaded drift. Here is my rig now.

1. I moved my ASI224MC to be the guide camera and made my ASI294MC to be my main camera
2. Converted my Astromania right angle finderscope to a guidescope and attached the ASI224MC
3. Setup auto guiding in ASIAIR (N-S calibration to move 25 pixels in 12-20 steps)

 

2. Second I learned a lot about how to make auto guiding work using ASIAIR Plus with a mount like mine that has so much backlash (thanks to other posts in this forum)

1. When I auto configured with PHD2 I got the RA and DEC max steps to be 2500 with aggression of 70% in RA and 100% in DEC. Also the DEC guiding mode set to be only NORTH.
2. But when I transferred the same to ASIAIR plus it did not work as good* so I changed the DEC to 10000 and aggression to 100% [Yes, I know it is absolutely extreme, but it worked]
3. However the most important step is to hit the UP (North) button until the image moves ie clear the DEC backlash completely in the NORTH direction.

 

* PHD2 guiding algorithms are much more comprehensive than ASIAIR plus when it comes to handling mounts with really bad backlash. So, step 2.3 however is critical to make the ASIAIR plus guiding work in this case for my low quality AZGTe mount.

 

As a result I was able to capture the image for 15-25 min total integration time (10s subs, 200 gain) with no or insignificant artefacts at the edges.

 

 

 

 

Thanks for all the suggestions/recommendations provided by many CNers. This forum is indeed a gold mine of information.

 

Subh

Edited by subhasism, 01 July 2023 - 02:27 PM.