Strange star elongation issue

[angryowl]

1 hour ago, FLO said:

I received a communication from them 5th March saying they are watching your thread with interest and noted from your posts were planning to perform some additional tests to your mount and the RASA. They said they were looking forward to the results. 

Currently, neither they nor we can see anything here that confirms your RASA is faulty. I.e. it could be the mount. But when I say that I am aware it might appear we are being evasive. FLO doesn't do evasive so please be assured if your tests confirm your telescope is faulty we will collect it then forward it to Celestron-UK for testing. 

HTH, 

Steve 

Appreciate the reply Steve. Just though I’d check with you first as Celestron might have had some tests they wanted me to run.

I can understand that there’s not enough information and other potential causes eliminated at the moment to be able to say the OTA is to blame here. That’s not coming off as being evasive and I suppose I’ll have to go through more tests to eliminate as many things as I can and hopefully I’ll be able to do this some night this week.

Will post results once I get some clear skies.

[angryowl]

Sunday night was clear but just managed to find the time today to sort through the images and results. apologies in advance for the lengthy post.

With @Davey-T's refractor mounted on the mount, I monitored both servo motors when slewing and foung the average current draw to be 0.2A for both Ra and Dec. The values varied a bit when seen on the ServoConfig screen, with the minimum being 0.1A and manimum 0.5A. With the RASA mounted the current draw was pretty much identical with an average of 0.2A for both axes and never exceeding 0.5A. After slewing around for a bit I touched the servos and both were just really cold.

Next some guided and unguided shots with the RASA and guider.

The Atik was oriented as follows while on the East side of pier: South/up, East/Right

And on the West side of pier: North/Up, West/Right

The QHY5 was oriented as follows while on the East side of pier: North/up, East/Right

And on the West side of pier: South/Up, West/Right

 

Pointing South elevation 50 degrees, scope on E side of pier (600s exposures, first unguided, second guided) - forgot to lock the mirror during these shots

Guiding enabled and disabled graphs

 

Pointing South elevation 50 degrees, scope on E side of pier (5s exposures unguided 10 minutes interval) - forgot to lock the mirror during these shots

RASA

Guider

 

Pointing South elevation 52 degrees, scope on W side of pier (600s exposures, first unguided, second guided) - since a meridian flip was done, both guide and main image are now flipped 180 and follow the orientation listed above

Guiding enabled and disabled graphs

 

Pointing SE elevation 44 degrees, scope on W side of pier (600s exposures, first unguided, second guided)

Guiding enabled and disabled graphs

 

Pointing West elevation 40 degrees, scope on W side of pier (600s exposures, first unguided, second guided)

Guiding enabled and disabled graphs

Pointing West elevation 40 degrees, scope on W side of pier (5s exposures unguided 10 minutes interval)

RASA

Guider

Next with the scope pointing at the Zenith I racked the focuser up and down to try and measure the backlash in arc seconds. I did this from both directions, clockwise and anti clockwise several times. I measured an average movement of 64 pixels in Dec and around 7 pixels in Ra. Ignoring the Ra movement and using my image scale of 2.15 arc seconds per pixel this gives a total of 137 arc seconds. Looking at values others with SCT scopes have, I get conflicting results. Some sources say up to 30” is considered acceptable whilst others say up to 100” or even more is normal.

Dave, I tried to do what you suggested in your earlier post, but as I said before I never saw any elongation during exposures lower than 180 seconds so I ran the tests you suggested, but using 60, 300 and 600 second exposures.

Pointing South 20 degrees elevation (can’t go lower than this) exposures are 60s, 300s, 600s

Pointing North 32 degrees elevation exposures are 300s, 300s, 600s

Not sure what to make of the arc-shaped stars in the first 300s exposure near Polaris.

As a final test I proceeded to point the scope west and run 3, 4, 5 and 10 minute exposures consecutively with the mirror unlocked and then locked to see where elongation really starts showing. Looks like in both cases elongation becomes apparent around the 5 minute mark.

Edited March 21, 2019 by angryowl

[angryowl]

It just occured to me that I've not added any images of my setup so here are a few.

I've removed the motorised focuser seen on the back of the RASA a few months ago as I thought that may have been causing the elongated stars due to radial imbalance. Also the 3d printed parts on the guider are there as I wanted the flip mirror functionality, but a few months ago I took them off and replaced them with the metal parts that came with the guidescope as I thought I couldn't guide properly due to flexure in the PLA parts. This wasn't the case and I got exactly the same results and graphs with the same amount of elongation in the main imaging camera.

[michael8554]

Too much data for my tiny brain to process, so what are your conclusions ?

Guidescopes in adjustable rings with soft-tipped screws are often considered to be an instant source of flex - remember we're talking microns.

Even with a basic ZWO ASI 120M guidecam I've never needed adjustable rings, there's always been suitable stars available.

Michael

[angryowl]

This is how I see the drift moving:

Pointing South elevation 50 degrees E side of pier – both RASA and guider drift east similarly

Pointing South elevation 52 degrees W side of pier – both RASA and guider drift west (although RASA drifts up in Declination a bit too)

Pointing SE elevation 44 degrees W side of pier – RASA drifts mainly north and guider drifts NE

Pointing West elevation 40 degrees W side of pier – RASA drifts north and guider drifts NW

Not sure what to make of this as it could point to differential flexure or flexure in the OTA when guiding, but still doesn’t explain the drift during unguided exposures. What’s interesting is that the guider most times drifts roughly in the same direction as the main scope.

Don’t know what else I could do to eliminate the mount as the test with Dave’s refractor proves that the mount can definitely produce good results unguided for 10 minute exposures. At this point I honestly don’t see the mount being the culprit here. Also, the arc shaped stars seen in one of my earlier tests and this time in the 5 minute shot pointing near Polaris are what concern me the most.

@michael8554 – Thing is I too never had to search for a guide star and never needed to adjust them so the guider screws are screwed in as far as they would go effectively squishing the Teflon at the tips. But you’re right, even so flexure could still be introduced by these and I'm not ruling it out.

[SamAndrew]

When pointing south, both the guide scope and main scope are experiencing the same movement, which makes me think the whole scope is moving relative to the mount. In other orientations I would guess you're seeing a combination of a bit of differential flexure between the guider and main scope, and the main scope moving relative to the mount. 

I wonder if there is a way to attach the guide scope directly to the mount and not to the main scope? then run the guide assistant to see if the guide scope has stopped moving? that way you can definitely eliminate the mount + extra weight of the rasa (I know the refractor test should have resolved this, but it's a fair bit lighter)

[angryowl]

Some more progress on this.

Piggy backed the refractor on top of the RASA using a very solid adapter plate made of steel profiles welded together. I’m pretty sure the plate does not introduce any flexure in the mix. Started by guiding the RASA using the refractor and QHY camera. I also guided without sending corrections to the mount and just monitoring the guide star in PHD and this showed the star drifting in the exact same direction as it did in the main OTA. I wasn’t able to get a single sub to guide properly using the refractor as all subs showed substantial drift.

I also ran the other test of taking short exposures with both refractor and RASA at 10 minute intervals to see the direction of elongation and amount in both scopes. The direction was always similar in both scopes and the drift amount in arc seconds was always lower in the refractor.

At the same time I also had the 60mm guide scope mounted on the bottom dovetail bar of the OTA. This is as close I could get it mounted to the mount plate and still have the full weight of the RASA on the mount. Again I measured the drift which was the same in direction and amount and tried guiding through it with no success.

I then moved the Atik camera to the refractor and took some unguided exposures and these also show elongation even at 5 minutes.

It was clear that the issue was now pointing at either the mount or tripod. I started looking at the tripod. As I have the two plates on top, separated by six M10 bolts (not the best design I know) I thought I’d eliminate potential flexure coming from the bolts by sandwiching some washers between the two plates and tightening everything up. Although I tried this some months ago, I also placed some large wooden boards underneath the 70mm diameter tripod legs as I was thinking the legs may be sinking into the ground due to the additional weight of the OTA. Neither of these made any difference and the drift was very much the same.

I took things inside and securely attached a small laser module on the mount head plate with the RASA mounted and balanced. Then I pointed in some positions I know the drift occurs and marked the laser position accurately as it was projected on a wall. After about 30 minutes, in almost all positions I tried, there was a relatively small but visible movement in the laser position. I started pressing and tugging on different parts of the mount and tripod and to my surprise the only thing that moved the laser point on the wall considerably was me squeezing the bottom plate of the mount to the top plate of the tripod.

The only thing that would allow this compression is the 0.5mm thick PTFE self adhesive pads I had on. Upon inspection of the mount bottom plate and PTFE pads it was clear that the contact area of the mount plate is actually rather small only around the edge as can be seen in the images. Plus, when inspecting the PTFE pads, the self adhesive glue was almost completely dry and I was able to remove them easily with a thin blade. Additionally, it looks like when the adhesive dried the pads started lifting up slightly in places.

After taking the pads off, I remounted the RASA and pointed the laser in the same positions only to find that even after an hour the position of the laser didn’t move at all. It was clear for a few hours last night so thought I’d test again and I only had time to take a few subs before the clouds rolled in. I still observed some elongation in unguided subs, but mainly when pointing West and the good news is I was able to easily guide it out using the 60mm guide scope mounted on top of the OTA. Also my PHD RMS numbers are higher now at about 0.7” compared to the 0.3” I was getting constantly when guiding before and still getting the drift.

I  sort of rushed things last night getting out so didn’t have time to completely remove the residual adhesive on the tripod head and I think I’ll also look at sanding the mount bottom plate so it has more contact area with the tripod plate.

As it stands now, it seems like the issue could have been the PTFE pads all along. I added them when I built the tripod to allow for easy azimuth adjustment when polar aligning, but during PA last night I didn’t observe any difficulties or significant stiction when making adjustments.

I’ll do more testing after cleaning the glue and levelling the mount plate but hoping that the ageing adhesive combined with the low contact area were the root of my problems.

The PTFE pads

Mount bottom plate

Adhesive residue

Unguided/guided 600s exoposures pointing West

Unguided/guided 600s exoposures pointing SE

[angryowl]

On 28/03/2019 at 18:14, SamAndrew said:

When pointing south, both the guide scope and main scope are experiencing the same movement, which makes me think the whole scope is moving relative to the mount. In other orientations I would guess you're seeing a combination of a bit of differential flexure between the guider and main scope, and the main scope moving relative to the mount. 

I wonder if there is a way to attach the guide scope directly to the mount and not to the main scope? then run the guide assistant to see if the guide scope has stopped moving? that way you can definitely eliminate the mount + extra weight of the rasa (I know the refractor test should have resolved this, but it's a fair bit lighter)

Thanks for the suggestion.I mounted the guidescope as close to the mount as I could during one of my latest sessions and it drifted in the same direction and the same amount as the main OTA. Turns out this may have been caused by the PTFE pads between the tripod and mount, but further testing is needed to confirm this.

[angryowl]

Forgot to mention the subs above were taken with the mirror unlocked.

[souls33k3r]

Remind me to give you a slap on the head and to tell you never to use PTFE EVER AGAIN Good God man. I'm so relived and happy for you mate.

[angryowl]

1 minute ago, souls33k3r said:

Remind me to give you a slap on the head and to tell you never to use PTFE EVER AGAIN Good God man. I'm so relived and happy for you mate.

Hehehe, thanks mate. Don't want to get too excited as I said it could be only part of the issue, but further testing should confirm.

[michael8554]

PTFE is very low friction material but unfortunately "creeps" under load, in other words your washers will get thinner,  and the ones with more load on because of the scope position will creep most.   Again, microns here will translate to arcsecs in your imaging.

So PTFE is fine for Dob bearings, no precision there ? but not between your wedge and tripod etc.

Michael 

[angryowl]

6 minutes ago, michael8554 said:

PTFE is very low friction material but unfortunately "creeps" under load, in other words your washers will get thinner,  and the ones with more load on because of the scope position will creep most.   Again, microns here will translate to arcsecs in your imaging.

So PTFE is fine for Dob bearings, no precision there ? but not between your wedge and tripod etc.

Michael 

That makes a lot of sense now, but didn't know any better two years ago when I built the tripod. Looked at the EQ8 and saw that the tripod they sell had PTFE pads and the mount is rated at 50kg, I thought adding 0.5mm thick pads on mine would be a safe bet. However, I'm thinking the EQ8 might have more contact area with those pads than my Mesu did which would make a difference.

[SamAndrew]

Great news, look forward to seeing some results from the RASA now  

[Jakub Szyma]

First post here, hello 

Do yuo maybe have the final verdict?

I have identical issue like you do, also bought my RASA in FLO last November.

Away from zenith the stars are elongated, regardless if the mirror locks are in use or not. In the zenith the stars are perfectly round.

After meridian flip the situation changes, east side it's bad, west side is better but not good.

 

angryowl, can you please do the very short exposures to verify that? I do 1s, 5s , 10s and 15s to check if the efeect is there, and it is always there - stars are elongated.

I use ASI 178 so I thought it might be the pixel size (much smaller than the scope spot size) but with ASI 1600 it is just the same.

Here example of 5s shot (at Leo Triplet, 200%):

 

and here after meridian flip, better but not perfect

Edited April 13, 2019 by Jakub Szyma

[angryowl]

13 hours ago, Jakub Szyma said:

First post here, hello 

Do yuo maybe have the final verdict?

I have identical issue like you do, also bought my RASA in FLO last November.

Away from zenith the stars are elongated, regardless if the mirror locks are in use or not. In the zenith the stars are perfectly round.

After meridian flip the situation changes, east side it's bad, west side is better but not good.

 

angryowl, can you please do the very short exposures to verify that? I do 1s, 5s , 10s and 15s to check if the efeect is there, and it is always there - stars are elongated.

I use ASI 178 so I thought it might be the pixel size (much smaller than the scope spot size) but with ASI 1600 it is just the same.

Here example of 5s shot (at Leo Triplet, 200%):

 

and here after meridian flip, better but not perfect

Hi Jakub

Welcome to SGL, you’ll find the very helpful and experienced people here will provide invaluable assistance and guidance when dealing with issues such as yours.

Don’t yet have a final word on this, but so far it seems like the PTFE pads were only part of the issue and although I can now guide most of the elongation out, it’s definitely still present in most guided subs.

In response to your question, I’ve tried anything from 1 second exposures all the way to a minute in most directions in the sky and no elongation whosoever was present in these short exposures. I find that in subs up to 3-5 minutes I usually see no sign of elongation wherever I’m pointed. That being said, the first thing I’d suggest is rotating your camera 90 degrees to see if the direction of elongation changes by 90 degrees. If it doesn’t, that could point to the camera not being completely orthogonal. This would explain seeing elongation during such short exposures and also remember when pointing at the zenith the gravity would no longer be pulling the camera/imaging train to one side to such an extent and would explain why stars come out fine.

Apart from this, we’d need to know a lot more before anyone on here can offer any meaningful advice on troubleshooting this.

Please let us know how you get on with this as I’m very much interested in what this’ll turn out to be.

[Jakub Szyma]

Hello, thanks for feedback. I do not want to make here mine issue tyhread, was justy curious if you found  the soluition as it seems bery similar to mine issue.

The issue is across the frame, and when I rotate the camera it remains there, just has different direction (so it is optics, not camera)

I just did few tests, and I have some very obvious observastions:

1) When I point at zenith on western side I have perfect stars, when I point at zenith on eastern side (after flip) my stars become elongated.

2) The farther from zenith I get, the worse elongation is.

3) Mirror lock does minimal improvement, but does not solve the issue

4) When I unscrew one or two screws of the feather touch focuser, the stars change shape (get better) - I did that as I've read in the net about somobody having focuser shifting the mirror by improper assembly. But mine is permanent, I cannot improve it.

5) I tried screwing camera tighter to the corrector but it did not improve anything

Thereofre my RASA is simply only usable at zenith on western side.

 

All along this is really strnge as RASA has spherical mirror and should be resistant to such mirror flipping.

 

 

[angryowl]

Managed to get out during a clear night last week and tried guiding with the refractor piggy backed on top of the RASA and also with the 60mm guider clamped onto the bottom dovetail of the OTA. I also cleaned up the residual adhesive on the tripod head and sanded the bottom plate of the Mesu down which resulted in a much larger contact area with the tripod head.

The results are not really what I expected, but a lot better than before and guiding is more consistent. That being said, guiding with either the 60mm guider or the refractor still doesn’t produce round stars. The elongation is much smaller than before and looks like guiding is now actually correcting for most of it, but still not completely. Not sure why this is as the elongation direction and amount are now very similar in both guide and main images.

The guided subs are now almost usable, but I’d still very much like to get as close to perfectly round stars and just don’t see this happening with the current guider. Therefore I’ve started designing an OAG using a right angle prism and will be using the bare CMOS board from a QHY5L-II-C camera to avoid obstructing the light path with either the camera body or the USB cable. I have most parts and it’s just a matter of finding a way to mount them.

I won’t be troubleshooting this any longer as I’ve spent far too many clear nights on this and I now know that an OAG will certainly solve the problem. I really wanted to get to the bottom of this but it just seems like it’s not practical at the moment.

Will update this thread once the OAG is complete

Some examples of what elongation looks like now

Edited April 26, 2019 by angryowl

[Davey-T]

I still think you should try it on another pier when you get the chance just to eliminate the tripod.

Dave

[angryowl]

Just now, Davey-T said:

I still think you should try it on another pier when you get the chance just to eliminate the tripod.

Dave

Oh yeah, that I will do given the opportunity. Perhaps when Ahmed gets his one finished he'll let me throw it on top for a quick test ?

[souls33k3r]

2 minutes ago, angryowl said:

Oh yeah, that I will do given the opportunity. Perhaps when Ahmed gets his one finished he'll let me throw it on top for a quick test ?

Grab a popcorn ?

[kirkster501]

Sorry I might have missed something...  are you using an OAG yes or no, it is not clear ??  You tried it and didn't have the spacers is what I deduced?   I have encountered all sorts of weird issues  with guide-scopes when coupled with imaging instruments with a mirror.  I move over to OAG and nearly every problem goes away.  

I think you have to persevere and get OAG working with your set up.  Guidescopes are fine for refractors but not SCT or RC type scopes.  That's my opinion and experience.  Others may think differently of course.

I also echo the point about the PTFE. 

Can you try a different guide-cam?  I also had the issue where the chip  was moving inside the guidecam in different sky positions by the weight of the cable (even though it was cable tied) and affecting the guiding. Slight movement of the mount moved the chip minutely and threw the guiding out in weird ways.  It took me months to figure this out.

Edited May 14, 2019 by kirkster501

[angryowl]

18 hours ago, kirkster501 said:

Sorry I might have missed something...  are you using an OAG yes or no, it is not clear ??  You tried it and didn't have the spacers is what I deduced?   I have encountered all sorts of weird issues  with guide-scopes when coupled with imaging instruments with a mirror.  I move over to OAG and nearly every problem goes away.  

I think you have to persevere and get OAG working with your set up.  Guidescopes are fine for refractors but not SCT or RC type scopes.  That's my opinion and experience.  Others may think differently of course.

I also echo the point about the PTFE. 

Can you try a different guide-cam?  I also had the issue where the chip  was moving inside the guidecam in different sky positions by the weight of the cable (even though it was cable tied) and affecting the guiding. Slight movement of the mount moved the chip minutely and threw the guiding out in weird ways.  It took me months to figure this out.

When I tried the OAG lent to me by @souls33k3r I did manage to get the spacing right and guiding gave me pinpoint stars in all exposures. So far the OAG is the only thing I tried that actually works and currently I’m in the process of building a DIY one. The reason I’m not going with a commercial one is that the guide camera body and USB cable get in the light path of the scope and cause weird diffraction spikes whereas with a DIY one, I’m hoping to avoid that.

Chip moving inside the camera…wow that’s scary! Glad you got that one sorted as I can see it being hard to solve. Seeing as the images in my OTA and guidecam drift the same direction and amount I’m inclined to believe the guidecam/chip is securely attached and not moving relative to the main OTA. At least not by any significant amount anyway. But a really good suggestion, thanks.