angryowl

Got the OAG spacing done inside using an artificial star and now just waiting for a clear night to test. Supposedly tomorrow night should be clear so fingers crossed.

A while back @souls33k3r had an issue with elongated stars in his EdgeHD and we had a late-night troubleshooting session trying to determine the cause of this with no success. At first, we thought it may have been guiding related, but after trying several PHD settings nothing improved. I seem to remember the issue was fairly similar to mine wherein the stars would sometimes come out acceptably round but the image would always slowly drift out of the FOV. Don’t know if this is in any way related to the issue at hand, but thought I’d add that here in case it proves to be of any significance.

Did not have much luck in trying all the things I wanted to last night and eventually when I couldn’t get the OAG spacing right I just gave up. Managed to get some subs with the EdgeHD but they’re not in great focus as the Bahtinov mask I have is much larger the scope’s aperture so had to be held by hand. These are three 10 minute consecutive unguided exposures and while the stars themselves look relatively round, the drift between the exposures in evident. However, not sure what to think of this as the EdgeHD’s focal length is 2032mm and I’m not particularly happy with the adapters I had to use to get proper focus. I had to use a cheap 1.25” adapter which had two screws to hold the camera nosepiece in place. I’m not sure if this could be a source of flexure, but I’m sure it’s not nearly as sturdy as having all thread adapters in the image train. Also mounted the guidescope on the Mesu, but using an MDF board rather than metal as the one I had would not fit properly. I screwed both the guidescope and Mesu saddle screws very tightly and there was the slightest flex in the MDF piece itself. For this reason, I’m not entirely convinced of the validity of this test, but I was frustrated and out of things to try so did it anyway.

Michael, just so I understand. Do you mean PHD should be able to correct for such long excursion/drift and keep the guide star centered? Or it thinks all is fine during slow drifts like these and it just guides out the smaller excursions resulting in somewhat round stars, but still elongated and still allowing the larger drift to take place?

Hehe, EdgeHD 8 it is then. On a side note, I can't believe how small and light the scope is compared to my RASA. Think I've got the spacing right by adding some adapters I had laying around, so ready to test tonight. Still looking at getting the OAG to come into proper focus but I think I'll manage.

Thanks for the incredible generosity of @souls33k3r I now have an OAG and a C8 on borrow which I'm hoping to put to the test tomorow night if the forecast holds true. Planning on testing the C8 on the mount guided and unguided and then fit the OAG to the RASA to see if it makes any sort of improvement. Will report back once I have some results.

Hi Steve Would love to get my hands on an OAG or another scope as that would at least eliminate the mount as the issue. In absence of an OAG or scope I’ll try to find a way to mount the 60mm guider with the QHY5L-II directly on the mount and see how that performs. The image scale of the Atik 414EX/RASA is 2.15” and the guider/QHY5L-II is 2.97” so results could be comparable.

A couple of months ago I did reach out to Bernard from Modern Astronomy in the hope he would liaise with Lucas with regards to what I thought at the time was a mount issue. But then I played around with the Ra ticks and managed to get acceptable stars around my object at the time so just thought that was the problem and told Bernard to put this on hold for now. If I get nowhere with this, I'll definitely try and get Lucas to have a look at this as it may be something he’s seen before.

Vlaiv - I had considered that, and it would perhaps be the best way to get to the bottom of this as it could correct for mount tracking errors (if there are any) and any mirror or scope flexure. Unfortunately I don't know anyone with an OAG so sadly not a possibility at them moment. Michael - I remember running the PHD Assistant a few times but at the time didn't look at the Ra curve in particular. Will definitely be doing this then next time I'm out imaging.

Another test I've done when looking at differential flexure was the same as above with the only difference that none of the cameras were guiding, just monitoring the guide star in each image. 50 degrees elevation with the scope pointing South. First image is through the guide scope, second is through the main scope

Dave - I have both guide and debug logs from the last imaging session whilst imaging NGC 281. I have gone through guide logs both in PHD and the PHD Log viewer to see if I may be missing something, but the graphs all look flat and usually my RMS never exceeds 0.5”. A quick peak at the guide graph in PHD Log Viewer Guided Unguided Here's the guide log for this night PHD2_GuideLog_2019-02-15_182355.txt Could I be right in assuming that this is due to the fact that PHD can guide small deviations just fine, but struggles with larger ones and simply can’t see the guide star drift out that much over the course of a longer exposure. So PHD thinks it’s doing a great job and correcting small movements while not being able to cope with the large drift. Hence the reason of flat guide graphs. I may well be wrong on this but this is my theory so far. A few months ago, I tried an experiment to see if I find differential flex between the RASA and guidescope by running two PHD sessions in parallel. One session for each guidescope and main scope and only the guide camera was issuing guiding commands to the mount, the main camera was just monitoring the star over the course of 10 minutes. Tried to orient both cameras as closely as possible but managed to get them close enough. As can be seen the guide camera happily guided away with a relatively flat graph, but the monitored star in the main camera was drifting considerably.

To illustrate the issue better, these are four 300 second exposures guided. Guiding was helpful in this case, but up to a point after which it couldn't keep up with the drift. I would have to stop guiding and exposing and re-centre the image every five subs otherwise it would slowly drift quite a ways.

When pressure's applied on the top bar, the movement seems to be in both Ra and Dec.

I always get down to or less than 30" PA with SharpCap. Since having this issue, I always do another iteration of PA to confirm and it's always spot on. This is also true of the PHD PA routines.

Thanks for the suggestions Michael. The Mesu shouldn't need to be East-heavy from what I understand as it's friction drive and has no backlash. But I did balance East-heavy on two occasions and it made no difference. The PE is meant to be 4" peak to peak, but not had a chance to either measure or do a PEC run.