Strange star elongation issue

[Davey-T]

Be worth sending Lucas a link to your thread here to get his opinion as it may be a mechanical fault.

Dave

[angryowl]

4 minutes ago, Davey-T said:

Be worth sending Lucas a link to your thread here to get his opinion as it may be a mechanical fault.

Dave

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.

 

[steppenwolf]

I have been reading this thread and sharing your frustration but I have no specific answer for you although I do feel that although it would not be usable in the real world for imaging, an OAG test run would settle some of the debate for you so I would urge you to try and borrow one from somewhere. Failing that, a test with a refractor as the imaging 'scope would partially answer some of the unknowns if you could have it close coupled to the guide scope.

The mount could, of course, be at fault but I'd be looking elsewhere first and mechanical stability of what goes on the mount head would be my first port of call.

[angryowl]

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.

[michael8554]

Your recent unguided Guide Assistant runs show your RA PE heading off towards a peak of 8arcsec and more ?

That might explain why the stars "jump" between long exposures.

But your guided graphs look great, PHD2 is handling things well.

PHD2 doesn't get fooled by large excursions, it will continue chasing them for as long as it takes, and that will show in the GA graphs.

Michael 

 

 

[angryowl]

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.

[souls33k3r]

33 minutes ago, angryowl said:

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.

Call it a C8 one more time ? it's an EdgeHD ? 

Nee bother matey, living close to one another should have some perks and i'm just happy to have what you need to test. Keep it as long as you like, no rush. Hope it works out for you matey.

Edited February 20, 2019 by souls33k3r

[angryowl]

11 hours ago, souls33k3r said:

Call it a C8 one more time ? it's an EdgeHD ? 

Nee bother matey, living close to one another should have some perks and i'm just happy to have what you need to test. Keep it as long as you like, no rush. Hope it works out for you matey.

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.

[souls33k3r]

22 minutes ago, angryowl said:

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.

 

Good man. Hope it works out for you. 

[angryowl]

13 hours ago, michael8554 said:

PHD2 doesn't get fooled by large excursions, it will continue chasing them for as long as it takes, and that will show in the GA graphs.

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?

[michael8554]

7 hours ago, angryowl said:

PHD2 should be able to correct for such long excursion/drift and keep the guide star centered? Or

It may not. It will keep trying, but since such large excursions are often caused by backlash or stiction, it may take a long time to correct, unless you have high Aggression.  So the star could leave a small guide star box - but perversely too high Aggression could cause overshoots and yoyo-ing.

Michael 

[FaDG]

17 hours ago, michael8554 said:

It may not. It will keep trying,

But then you will know, as PHD will display increasingly larger corrections in its graph, until finally formally complaining that "Due to the max RA/DEC duration setting it can't compensate the Drift".

Yet, in the images the OP posted in the previous page, when guiding the graph remained nicely flat, and the drift was only in the unguided ones. Again, this rather points to flexure.

Fabio

Edited February 22, 2019 by FaDG

[angryowl]

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.

[angryowl]

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.

[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.

[StuartJPP]

If your OAG and main camera are now rigidly mounted then I am sure your issue will disappear. 

I used to have similar star trailing with my refractor and it turned out to be flexure of the lens (OTA) foot. The flex was undetectable by feel/naked eye. My trailing would happen if I guided or not and was eventually eliminated by securing my OTA in a much more secure manner at both the front and back of the dovetail.

Your flexing will still happen of course but the guiding using the OAG will correct for it.

See this thread for animation showing the star trailing I used to have before the issue was solved.

 

 

Hope your new setup works out for you and you are able to find the source of your problem.

[angryowl]

@StuartJPP - Thanks so much for this and the thread you linked was a great read. Also, absolutely fantastic final image you got there.

The issue you had looks very similar to mine indeed and glad you managed to get yours fixed by further securing the OTA to the dovetail. As I said I wouldn't mind investing in a set of Parallax rings and some proper dovetail plates, but when you pay £3000 for a scope you shouldn't really have to in order to get acceptable stars in 300s or 600s exposures. That is, if the issue is indeed with the scope flexing.

I agree, using an OAG would guide the flexure out and hoping it does the next time I'm out. If it does and it turns out something is flexing, I can't be using an OAG to image with the RASA so my only option would be to return it as I got it from FLO with two months of warranty left.

[angryowl]

This is what guiding with the OAG last night got me. Tried different parts of the sky especially the ones where the drift was the worst before. I wasn’t able to include a filter in the light path so couldn’t take 5 or 10 minute exposures to show the elongation, but I tried something different.

I polar aligned as usual and started tracking.

Then pointing near Procyon with the scope on the West side of the pier with the mirror unlocked. I took 5 second exposures every 10 minutes whilst the mount was purely tracking, not guiding. This is the result of the stacked exposures.

 

I then calibrated the OAG and started guiding then did the exact same thing. This is the result.

 

The same unguided, mirror unlocked, scope on east side of pier near Capella sequence was done. Only difference was I used two exposures as clouds were rolling in.

 

Now with the OAG guiding.

 

Same procedure with the scope on the east side of pier pointing SE with the mirror locked down and unguided

 

And this is with the OAG guiding

It's clear that mirror flop or mirror movement between exposures (as long as the mirror locks are doing their job of securing the mirror tightly) is not the culprit here. Does this look something like the main OTA flexing then???

Edited February 24, 2019 by angryowl

[vlaiv]

6 minutes ago, angryowl said:

It's clear that mirror flop or movement between exposures is not the culprit here. Does this look something like the main OTA flexing then???

How did you conclude this?

From above images, I think that equally valid conclusion would be - it's down to mirror flop and locking it down is not working properly (at all)?

[angryowl]

3 minutes ago, vlaiv said:

How did you conclude this?

From above images, I think that equally valid conclusion would be - it's down to mirror flop and locking it down is not working properly (at all)?

Sorry, that may not have come across as well as I had intended. Provided that the two mirror locking mechanisms are doing exactly what they should be doing (completely locking the mirror down) then there is no reason whatsoever as to why the mirror should be moving in relation to the OTA assembly. As you say it may be that the two locking mechanisms may be faulty and might not secure the mirror down well enough.

[angryowl]

This is the PHD graph whilst guiding with the OAG near Procyon

And this is the graph with the OAG simply monitoring the guide star drift in the image

[vlaiv]

That second graph is curiously flat - as if RA rate is set to something else than sidereal.

DEC drift is close to what would one expect from average PA error.

Any chance RA rate is set to something else than sidereal?

How about camera / corrector assembly? Any chance there is play there, like less than perfect fitting extension rings / tubes or something, coupled with cable snag?

[angryowl]

43 minutes ago, vlaiv said:

That second graph is curiously flat - as if RA rate is set to something else than sidereal. 

DEC drift is close to what would one expect from average PA error.

Any chance RA rate is set to something else than sidereal? 

How about camera / corrector assembly? Any chance there is play there, like less than perfect fitting extension rings / tubes or something, coupled with cable snag?

The RA tracking rate used for this session was set to the original one the Mesu came with. Lucas tweaks these at his factory to sidereal rate before sending them out so I suspect it's very accurate. As I've mentioned before I did try tweaking the RA rate of the mount by following the drift in RA in a star and calculating the correct tracking rate to counteract with no success.

The camera/corrector and any spacers or adapters were tightly screwed in place and there's no way they could be flexing.

As to the cable snagging, I have triple, even quadruple checked them before every exposure and they are routed by the bottom dovetail anyway.

Really appreciate all the help and insight provided on this thus far vlaiv

Edited February 24, 2019 by angryowl

[angryowl]

What I really like to know is if the scope, without and OAG, can give me acceptable exposures longer than 2 or 3 minutes without any expensive or complicated modifications to it. I mean I could go the route of buying Parallax rings and some proper hefty dovetails, but that’s money I could be spending someplace else, and might still not guarantee me drift-less images.

I’d have no issue guiding the RASA with an OAG or even an ONAG if it didn’t cause awful diffraction spikes and artefacts in the image. If anyone here has any other suggestions as to how I could effectively guide these drifts out I’d happily try them. Longer FL guidescope or the guidescope mounted on bottom dovetail or any other variation I suspect would produce more or less the same results.

The scope was purchased from First Light Optics about two years ago and still have two months of warranty on it. I’d really like to know now if the scope is at fault here as if it is, I have no alternative but to return it as I cannot and will not accept such performance out of a £3000 piece of equipment.

[Davey-T]

Have you tried with Ahmeds SCT yet ?

Dave