Strange star elongation issue

[angryowl]

I’ve been struggling with elongated stars for the past few months now and have tried numerous things and read through similar threads here and on other forums with no success. This is really starting to get to me as I’ve lost track of the number of clear nights wasted so far.

The elongation direction is not consistent at all and varies over different parts of the sky.

The kit:

Mesu 200, Celestron RASA, 60mm StarGuider guidescope, QHY5L-II guidecam, Atik 414EX main imaging camera, Baader UFC, Baader F2 High Speed narrowband filters.

Things tried so far:

• Tested for cable drag and recently started routing all cables coming off the scope at the back of the dovetail.

• Both RA and DEC balance is spot on. This was performed several times with a digital fishing scale and the balance points marked.

• I’ve always used SharpCap for PA but also tried several runs of PHD static and drift alignment with the same results.

• Read the entire SiTech Operations manual for the Mesu as at one point I was getting elongation mostly in Ra and though the mount may not be tracking at a correct sidereal rate. There’s a method of calculating the proper track rate and have spent a couple of imaging sessions doing this with limited success. The method involves measuring the drift of a star in Ra in an interval of 10 minutes. Then after some clever math, one can change the Ra motor ticks in the ServoConfig configuration for the mount. This worked, but only for the position in the sky where the drift was measured and as soon as the mount was pointed to another part of the sky, the issue would immediately reappear. What’s worse is that the same tracking rate would no longer give round stars on a different night in the same position in the sky it did for the night before.

• Short exposures of a minute or shorter in any position in the sky give perfectly round stars. This eliminates collimation, issues with the camera or any other optical element in the light path as I see it.

• Differential flexure between the main scope and the guide scope is a non-factor as I get elongation during unguided exposures. Tried tweaking the settings in PHD2 thinking I might be able to guide it out. Results, again, vary depending on the night and position the scope is in. Sometimes guiding would give almost acceptable stars and some improvement over unguided and other times it would simply make matters worse.

• Additionally, rotating the camera 90 degrees results in the direction of elongation being rotated as well. The camera is coupled to the front corrector plate via screw adapters which I simply can’t see flexing.

The only time I can get round stars is with the scope pointing near the Zenith.

I’m at a point now that the only thing making sense is flexure within the RASA or imaging train. I know the camera is tightly fixed to the corrector plate so there can’t be any play there. The only place where flexure could occur would be at the main RASA Losmandy mounting plate. The plate is screwed in to the front and back aluminium holders by four M5 screws I believe. Thought these might have loosened over time but after checking, they’re as tight as they’d go.

Not sure how solid a RASA should be and I didn’t really test or look at this when I got the scope as there was no need, but when fully mounted, the scope can be easily rocked by applying light pressure to the top mounting plate. This is not an issue with the Mesu head as it’s simply rock solid and can’t move it at all regardless of the force applied. At this point it’s all pointing to the scope/mounting plate flexing due to gravity as the scope is not exactly light. This could explain the odd behaviour throughout different parts of the sky as the scope is flexing in different directions.

Trying to determine if it is indeed the scope flexing, I’ve done some testing yesterday. Setup everything as usual during the day and focused on a brick wall roughly 30 meters away on a day with no wind. Mount was off and both axes were locked with the mount hooks, so mount tracking errors or movement would be eliminated this way. Started with 5 short consecutive exposures, then at 10 minute intervals another 5 exposures were taken until 40 minutes elapsed. Each set of 5 exposures were integrated in PI using an average combination with no normalisation or any pixel rejection algorithms. This was done with the scope on both the East and West side of the pier pointing at the same brick wall and with the mirror both locked and unlocked.

Combining these integrated images into GIFs and plotting the Ra and Dec orientation clearly shows considerable drift over the course of 40 minutes. As can be seen the drift is not purely in Ra or Dec, but a combination of both.

Mirror locked. Scope on East side of pier.

Mirror unlocked. Scope on East side of pier.

Mirror locked. Scope on West side of pier.

Mirror unlocked. Scope on West side of pier.

These next four GIFs show the result of pushing on the scope’s top mounting bar. The direction of movement when applying pressure on the scope is very similar to the direction of drift in the first two GIFs. If I’m not mistaken this would indicate that the RASA is indeed flexing/sagging due to gravity. The scope was purchased from FLO about two years ago. Would this be considered normal behaviour or am I just barking up the wrong tree here?

Mirror locked. Scope on East side of pier. Before and after pressure applied on scope mounting bar.

Mirror unlocked. Scope on East side of pier. Before and after pressure applied on scope mounting bar.

Mirror locked. Scope on West side of pier. Before and after pressure applied on scope mounting bar.

Mirror unlocked. Scope on West side of pier. Before and after pressure applied on scope mounting bar.

 

I find it hard to believe that the image should drift this much over the course of 40 mins, but then again, I may be wrong and this isn’t out of the ordinary???

Any help with this from the awesome and knowledgeable SGL comunity would be greatly appreciated as I’m simply out of ideas and things to try.

[FaDG]

Maybe just a Simple test before passing to something more complicated: do you have another (less heavy) scope, or can you borrow one? 

If issue still there it would point to mount/saddle plate, otherwise definitely to the scope or scope/camera interface. 

Fabio 

[ollypenrice]

The RASA's weight will not trouble the Mesu in the slightest. I've piled far more weight than that onto mine.

If you get your best stars near the zenith then the obvious candidate is mirror flop (an exaggerated term for slight mirror movement.) Clearly, near the zenith the mirror is sitting symmetrically in its cell. It is always risky to guide a reflector with a guidescope because the guidescope cannot see the mirror flop. Neither can the mount, which might be tracking perfectly even unguided.

It seems to me that your experiment on the brick wall may not be able to distinguish between various possible sources of flexure. You can put pressure on the scope mounting bar but where, exactly, does that pressure distort the system? I wouldn't like to say.

Olly

[angryowl]

25 minutes ago, FaDG said:

Maybe just a Simple test before passing to something more complicated: do you have another (less heavy) scope, or can you borrow one? 

If issue still there it would point to mount/saddle plate, otherwise definitely to the scope or scope/camera interface.  

Fabio 

Thanks for the reply Fabio, unfortunately I don't have another scope at the moment as that would indeed have helped in eliminate the mount as the source of the issue. I might be able to borrow a C8 though so that may well be my next step in troubleshooting this.

On my last imaging session, I tried using the 60mm guider as a standalone scope mounted to the Mesu with a improvised metal plate, but couldn’t get balancing right and the clouds rolled in.

[souls33k3r]

1 minute ago, angryowl said:

 I might be able to borrow a C8 though so that may well be my next step in troubleshooting this.

It's here if you need it mate

[angryowl]

11 minutes ago, ollypenrice said:

The RASA's weight will not trouble the Mesu in the slightest. I've piled far more weight than that onto mine.

If you get your best stars near the zenith then the obvious candidate is mirror flop (an exaggerated term for slight mirror movement.) Clearly, near the zenith the mirror is sitting symmetrically in its cell. It is always risky to guide a reflector with a guidescope because the guidescope cannot see the mirror flop. Neither can the mount, which might be tracking perfectly even unguided.

It seems to me that your experiment on the brick wall may not be able to distinguish between various possible sources of flexure. You can put pressure on the scope mounting bar but where, exactly, does that pressure distort the system? I wouldn't like to say.

Olly

Thanks for replying Olly. You're absolutely right with regards to the Mesu's abilities and I’m sure my relatively ‘light’ scope poses no problem.

Yeah, the brick wall test was more or less done to confirm something in the scope or the scope itself is flexing when sitting perfectly still. As you say, what is flexing or why it’s flexing so much still needs figuring out.

[angryowl]

7 minutes ago, souls33k3r said:

It's here if you need it mate

Much appreciated mate ?. Might actually have to take you up on that it seems.

[angryowl]

Here’s two 10 minute consecutive exposures with the mirror unlocked and locked. Very odd to say the least, baffled as to why the direction in elongation, after locking the mirror down, changed direction like that. Again Ra is up-down and Dec left-right.

[angryowl]

To add two more things I’ve considered so far.

I doubt it is polar mis alignment as the drift direction and amount is consistent throughout the entire frame. From what I know, PA issues should show up in the image with varying amounts of drift throughout the stars in the field with the corners being most affected. Although the FOV with the Atik 414EX is rather small, I imagine bad PA would still look different.

Another possibility is that one of the four elements in the secondary may have somehow gotten loose? I doubt this as the aberrations would have been visible in the stars in the image by now.

[michael8554]

I would suggest that perfect balance is not best practise.

Even the best mount will have mechanical flex, so balancing heavy in one direction can take up the slack.

Also does your mount have zero PE ?

Michael 

[angryowl]

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.

Edited February 18, 2019 by angryowl

[FaDG]

25 minutes ago, angryowl said:

From what I know, PA issues should show up in the image with varying amounts of drift throughout the stars in the field with the corners being most affected

You're basically right: if guiding, huge PA shows up as field rotation around your guide star, and if it's quite far from the centre of the FOV it could seem drift. 

If unguided, PA misalignment induces mostly Dec drift and a slight RA drift. 

BUT, misalignment needs to be quite hefty to show up like that, a small amount is often kept to ensure consistent dec guiding without backlash. 

Well, for us mortals, anyway... I never used a Mesu! ? ? ? 

 

[souls33k3r]

58 minutes ago, angryowl said:

Much appreciated mate ?. Might actually have to take you up on that it seems.

Bring your car around and grab it off me mate because there's no way I can carry it all the way to your home

[michael8554]

Not necessarily East heavy.

In which direction do you get movement when you rock the top of the ota ?

Michael 

[angryowl]

5 minutes ago, FaDG said:

You're basically right: if guiding, huge PA shows up as field rotation around your guide star, and if it's quite far from the centre of the FOV it could seem drift. 

If unguided, PA misalignment induces mostly Dec drift and a slight RA drift. 

BUT, misalignment needs to be quite hefty to show up like that, a small amount is often kept to ensure consistent dec guiding without backlash. 

Well, for us mortals, anyway... I never used a Mesu! ? ? ? 

 

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.

[FaDG]

Then, no way it can show up in images, even extremely long ones. 

You'll have to look elsewhere, i fear. 

[angryowl]

9 minutes ago, michael8554 said:

Not necessarily East heavy.

In which direction do you get movement when you rock the top of the ota ? 

Michael 

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

[angryowl]

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.

Edited February 18, 2019 by angryowl

[Davey-T]

Looks like the guiding not correcting enough or even not at all, have you got a guide log ?

Dave

[vlaiv]

I had something similar when I was using guide scope. 

Now that I'm using OAG - I don't have it any more. Problem with RASA is use of OAG, right? You can't use OAG with front mounted sensor?

[vlaiv]

Well, actually - there is idea - if there is enough backfocus can you try using OAG just to check guiding and exposures.

It will certainly create diffraction havoc in subs, but you'll at least know your mount is guiding good.

[michael8554]

Worrying that you have had to correct the tracking rate.

Maybe worth looking at the actual, rather than the quoted PE ?

Run the PHD2 Guiding Assistant for 10 minutes and look at the RA curve in the Log Viewer.

There are also earlier Mesu posts on SGL that show an uncorrectable by PHD2 fast PE component that gives elongated stars.

[angryowl]

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.

[angryowl]

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

[angryowl]

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.

Edited February 20, 2019 by angryowl