Elongated stars on one side of the meridian

[gregbradley]

I am using an AP1600GTO mount with some machined counter weights with locking knobs and some gym weights with the oversized centre hole filled in with other material (but its still not tightly fitting).

I am finding recently I am getting perfect round stars when the scope (AP RHA) is on the west side of the mount but when the scope is on the east side of the mount the stars are slightly elongated. Not terribly but annoyingly.

I am using a MMOAG for guiding and have a bolt on adapter with screw fittings to a FLI Atlas which accepts an adapter to the MMOAG and then a FLI 5/7 filter wheel (quite large and a bit heavy) then the camera which in this case is a tiny and light SX Trius 694.

I recently tightened all connections (they were already snug) and I recalibrated the autoguider when the scope was on the east side.

I need to check the autoguider graphs more closely but from what I have seen it looks like the guiding is pretty amazing. On the west side the guide graph was .48 of an arc sec which is crazy good. When it flipped I got lesser star elongation but it tended to be there.

What I also did was tie off the cables to the scope handle in case it was cable drag. I also turned off Protrack corrections in case it was pushing it off.

I was running 4 second exposures and I made them 3 second. I use the guide assistant in PHD2 to set the parameters on the night (no rote settings).

I did notice that the mount can make a slightly different noise in some points of a slew. You can reseat the worm by simply loosening 2 screws and the retightening them. I think I will do that next.

Any suggestions? I am thinking its most likely something flexing.
I could go to 5 minute subs instead of 10 minute with little consequence as 5 minutes is probably longer than ideal for this camera on this scope anyway.

I am also wondering if the slab moved slightly and my Polar Alignment is slightly off. I guess I should check all the screws that hold the mount down for snugness while I am at it.

I might do a quick recalibration TPoint run and see if it comes up with any significant Polar alignment error but the west side guiding would indicate PA must be pretty spot on.

Greg.

[KenGee]

Hi Greg, You checking the things i would. the only thing i'd suggest is, does yur rig have another scope on it? if yes can you attach a camera to see if you see it there. That would allow you to isolate it to the optical train if it doesn't.

[Atmos]

If the star elongation is dependant upon exposure time it sounds like a possible balancing issue (no idea why it would just crop up). I suppose if the PA is off it could work better on one side of the meridian than the other.

If it isn't dependant upon time I would go to the extreme and say that it could be mirror shift on one side of the meridian.

[ericwbenson]

Hi Greg,
I would take 10sec unguided exposures on either side of the meridian and diagnose those first. Any difference will be caused by flex in the OTA. Flex in the imaging train just causes movement of the focal plane (i.e. pointing offset), not star elongation.

If there is no difference and on longer guided subs (1-10 min) you see a difference E vs W, and all the stars are affected the same, then it would be the mount. In that case the axis of the elongation is important to isolate the DEC vs RA axis.

I think the RHA being such a fast instrument it will be susceptible to even the tiniest tube flexure.

Makes me wonder how they will ever get the LSST working, alt-az mount and deformable optics help I suppose.

Best,
EB

[Peter Ward]

I can't say I ever saw this with my RHA....and seriously doubt anything is shifting in the OTA as it is all locked down at the factory and is non user adjustable.

That said, check the secondary retaining ring (it screws off...might be a bit loose)

More likely the assembly is not squared on, with the tilt in the focal plane being
compensated for one side of the pier via flexure, but exaggerated on the other.

Just my 2 cents worth....

[gregbradley]

When you say assembly do you mean the scope or the scope and its mounting plates etc or the mount?

I agree its unlikely the mirror or tube of the scope is flexing. I reseated the worm gear on both axes tonight (it takes just a minute thanks to clever AP engineering). The mount's slewing sound did sound a little better. I did manage a 10minute sub with scope on the eastern side and round stars. But
cloud suddenly appeared stopping proceedings. I'll check the secondary ring as well.

I have gotten round stars on either side plenty of times in the past and other times its was polar alignment off etc, usual stuff.

Thanks for all the replies. Sometimes these little flexures are hard to track down.

Greg.

[Peter Ward]

Sorry I wasn't being very clear. Camera assembly.

[ericwbenson]

Yes, but tilt in the focal plane results in out of focus stars somewhere in the FOV, but they should still be round if only a bit bigger, unless astigmatism is present. Greg, post a small crop around one of your 'elongated' stars.
EB

[gregbradley]

Thanks for that clarification. I thought that was what you meant but wasn't 100% sure.

Well that certainly is a possibility. I extensively worked on tilt with my cameras and I am sure you did as well. It took me a while to work out a procedure to correct it fairly quickly. At first I had the image and the camera orientation wrong much like a newbie doing Polar Drift alignment for the first time. So I was correcting the wrong corners all the time.

I got a replacement tilt tip adapter from SX and adjusted it to what I had found with shims. First off it seemed pretty perfect which surprised the hell out of me. So I think tilt is OK and CCD inspector seemed to agree (more or less). But I will keep that in mind as it was problematic. I checked the secondary ring and its tight.

I was wondering about the FLI filter wheel as its quite large and heavy and offcentre. I do have a lightweight SX filter wheel but no 36mm filters so I was thinking as a last resort I could get some filters and see if that corrects it.



I'll dig one up. Stars seem in focus over the whole image though. I know what they look like when tilt is there as I've literally spent hours sorting out tilt. I think I got on top of it but definitely something to check if nothing else handles it. My M78 image in the deep space section shows some minor star elongation at the tops of the stars. The subs when the scope was on the west were perfectly round. The ones when the scope was on the east were slightly elongated at the top (mostly the dimmer stars as brighter stars sometimes absorb a small amount of elongation).

Greg.

[gregbradley]

I just checked CCDinspector on 2 subs one on the east and one on the west.

There is a fairly large difference in tilt. So that looks like something is flexing on one side compared to the other causing the tilt.

Greg.