Elongated stars

[paulf]

Hi all,

I've been having issues with elongated stars for a couple of months now and would appreciate any help anyone can offer as I'm running out of ideas

The following is a crop of a single 1200s Ha sub, 2x enlarged. The elongation always seem to be in same direction (I think - but need to re-test this). The guiding RMS values are typically 0.15 for RA, 0.10 for DEC, 0.2 total.

I've also attached an image of my set up and numbered all the things I can think of which would cause this:

1) Guide camera is securely attached to guide scope.

2) All three guide scope adjusting screws are tight.

3) The focus adjustment ring is tight.

4) The guide scope mount is securely attached to the dovetail bar. Mounted further forward than ideal but cannot get balance if further back on the dovetail bar.

5) The dovetail bar is securely attached to the scope tube rings.

6) The focuser assembly was loose, and has been tightened, and the retaining all bolt underneath is tight.

7) All cables are checked to be hanging freely during slewing and before commencing imaging.

8) Have adjusted focus draw tube tension. How tight should this be? It will move slightly if you hold the camera/filter wheel and push at right angle to focuser.

9) Old flattener did rock up and down with compression screw fully tightened. So have bought a Hotech SCA flattener which uses expanding rubber O-rings within the draw tube to lock very firmly in place.

10) Flattener and SX filterwheel screw together via 10mm T2 spacer. Screwed tight such that the filter wheel cannot unscrew without some force.

11) Filter wheel directly attached to Atik 314L using T2 screw thread. Again screwed as tight as I think it can be.

12) Tube ring screws as tight as possible.

13) Dovetail bar is further forward than I would like but cannot balance otherwise.

14) Could there be any issue with RA gears? If they needed adjusting, how would I know? Mount seems to turn smoothly.

Other ideas:

15) Guiding - I use PHD2. It was overcorrecting which I thought may be the issue, but have now reduced maximum pulse widths (from 2000ms to 250ms), and used the EQMOD pulse guide monitor to reduce 'width gain'. This didn't solve the issue though.

16) I did try to go 'back to basics' e.g. just camera, but I couldn't achieve focus without the flattener, and then couldn't put a filter on the camera! Tried a 1.25" extension tube and screwed in filter but the extension tube was wobbly...

17) Try turning off dithering, then doing a number of shorter subs and see if any 'creep' continues across subs. This would eliminate guiding as the issue?

18 ) Do a 'really good' drift align (don't normally do this) and try a 5-10min sub without guiding.

So I hope someone can help, I can't take many more wasted clear nights!!

I'm looking for any suggestions or procedures which can decisively eliminate items from my hit list and narrow down a single item as being the cause.

Thanks in advance,

Paul

[red dwalf]

is the distance from the rear of the flatner to the ccd chip correct ? i ask this as it is important with my william optics gt81, my distance is something like 71mm, 13.5mm from chip to front of ccd camera, 29mm starlight xpress filter wheel, i use one of this also, 20mm t2 adapter from front of wheel to reducer then about 6.5 from rear of reducer to the lens.

not sure about your setup but that might help.

secondly in eqmod driver setup, make sure that the ascom pulse guiding settings for the ra and dec rates are set to x0.50 and are ticked, then set up the brain in phd 2, important things i found where to make sure the tracking rate is also set to 0.50 rate, if not phd will change it in eqmod once started up, then the calibration settings needs the focal length of the guide camera, it will detect the pixel size of the camera and make it`s own calibration settings.

i also reduce the pulse rate monitor in eqmod but only after it has been running a few seconds, it need both to be set to 100 when phd does it`s calibrations then reduce afterwards.

also might be work googling phd snapshots just to make sure you have the latest version of phd2.

also is the scope balanced well in ra and dec ?

[Freddie]

Is the elongation in DEC or RA or a bit of both?

[steppenwolf]

Your guiding figures indicate that your guide telescope is tracking pretty accurately, The star elongation is in the RA axis - does a graph of the tracking show a lot of see-sawing?

I suspect that the issue is differential flexure - imaging and guiding systems are not coupled closely enough.

7) All cables are checked to be hanging freely during slewing and before commencing imaging.

I would prefer to see that cable from the autoguider 'locked' down to the rear of the top mounting bar so that it cannot apply any movement to the autoguider 'scope.

[Zakalwe]

I dislike those guidescope mountings with a passion. The front of the guidescope is held in a rubber ring which is just asking for flexure.

[johnrt]

I'm going to add my vote to flexure being the culprit.

[Freddie]

From the info. provided , how do we know elongation is in RA?

[steppenwolf]

From the info. provided , how do we know elongation is in RA?

I carried out a plate-solve on the image to determine sky position and camera orientation.

[Freddie]

Thanks Steve. Good job to put the effort in to going that extra step.

[paulf]

Thanks all!

is the distance from the rear of the flatner to the ccd chip correct ? 

I bought new flattener so still fine-tuning but yes - should be 55mm and I'm close to that. The elongation is across the whole field so I've been able to factor out the field flatness as an issue.

also is the scope balanced well in ra and dec ?

Yes. Although I did try unbalancing slightly in RA which I think is the thing to do? 

Is the elongation in DEC or RA or a bit of both?

Thanks to Steppenwolf it appears it is in just RA

Your guiding figures indicate that your guide telescope is tracking pretty accurately, The star elongation is in the RA axis - does a graph of the tracking show a lot of see-sawing?

I suspect that the issue is differential flexure - imaging and guiding systems are not coupled closely enough.

I would prefer to see that cable from the autoguider 'locked' down to the rear of the top mounting bar so that it cannot apply any movement to the autoguider 'scope.

I was getting see-sawing and I think the guiding was over-correcting. But last time out I reduced all the RA related fields in PHD to the point of under-correction and it didn't seem to make any difference (I was convinced that would be the solution that evening...).  Does the fact the elongation is just in RA point to the guiding though? If I was to rotate the scope 90 deg and still had elongation in RA does that rule out movement in focuser/flattener?

Will look at that cable!

I dislike those guidescope mountings with a passion. The front of the guidescope is held in a rubber ring which is just asking for flexure.

Would you recommend guide scope rings?

[ollypenrice]

I really don't like any aspect of the guide scope attachment. A guide scope does not need to be in adjustable rings, it just needs to be bolted down hard, fast and solid more or less parallel with the imaging scope. This one is perched a mile away from the sope at the end of a long lever and has any number of flexure possibilities. The need for adjustable guide rings disappeared ten years ago.

Olly

[paulf]

I really don't like any aspect of the guide scope attachment. A guide scope does not need to be in adjustable rings, it just needs to be bolted down hard, fast and solid more or less parallel with the imaging scope. This one is perched a mile away from the sope at the end of a long lever and has any number of flexure possibilities. The need for adjustable guide rings disappeared ten years ago.

Olly

Thanks Olly - can you suggest a commercially available bracket which would be an improvement? And do you think that the guider attachment is the specific reason for my issues? Ideally I'd like to get the best I can out of what I have got before investing any more money

[steppenwolf]

The need for adjustable guide rings disappeared ten years ago.

+1

[Starflyer]

I chased flexure for many many months before giving in and moving to an OAG, it was the only solution that worked and continues to work for me.

[red dwalf]

there isn`t any slop in the axis of the mount is there ? just reminded me of my friends problem he had a while back, proper service of the mount sorted it out. 

[paulf]

there isn`t any slop in the axis of the mount is there ? just reminded me of my friends problem he had a while back, proper service of the mount sorted it out. 

How would I test that? Try to move the scope back and forth around RA while mount is tracking?

[iksose7]

Your guide scope looks like it is in a very strange position. On your diagram next to the number 6 there is an area where you can attach the guide scope with ease. Thats what i use on my scope and dont have any issues.

Callum

[paulf]

Hi Callum, problem is I can't balance the scope if I move the guidescope back. Could add some sort of counterweight I suppose

[red dwalf]

How would I test that? Try to move the scope back and forth around RA while mount is tracking?

you can leave the scope in the parked position with clutches on and see if you can move the mount in ra and dec at all to see if there is any play in them, should be virtually no movement or rocking.

[perfrej]

The trails are evenly thick, which means that there is no sudden jump as in slack. It could be a constant oscillation back and forth in Ra or it could be a constant drift. I too say it reeks of flexure.

As I work with equipment that requires absolute rigidity I get a very bad feeling from the look at that rig. The guide scope, as Mr Penrice and others pointed out, is a very likely source of flex, as are both Vixen bars and the 2" compression things.

/per

[Zakalwe]

Are you sure that that setup is balanced correctly? I had an Equinox 80 and it didn't need to be pushed that far forward to achieve balance

Once I added a filterwheel and OAG, I started to use a Baader counterweight on top to keep the scope near the mount centreline:

It can be hard to find decent rings for a guidescope. I had some adjustable rings kicking around that I used and they were screwed down TIGHT. There's never a need to move a finderguider as the FOV is pretty huge. Someone on here used pipeclamps like these to hold a finderguider.....it didn't look fancy, but I bet that it was a LOT stiffer than your setup!

There's lots of clamps out there if you do some searching:

This is from a karting supply:

Or from a stage equipment supplier:

http://www.djanddiscostuff.com/stands-storage-c139/clamps-brackets-c146/oslim-2-p2053/s2122?utm_source=google&utm_medium=cpc&utm_term=phase-one-oslim-2&utm_campaign=product%2Blisting%2Bads&gclid=CjwKEAiAjsunBRCy3LSlz_PJqCgSJACJY7yK_U6Rc-kifX40IYmbbKk0x3J656bTV3pRvhZZU-Q57hoCCKzw_wcB

I'd also recommend getting a set of Williams Optics CNC rings (the 90mm ones fit the Equinox perfectly) and moving to a Losmandy dovetail. Much more beefy and stable than Vixen dovetails.

[paulf]

Thanks again all, I've read all your posts and made some notes!

Are you sure that that setup is balanced correctly? I had an Equinox 80 and it didn't need to be pushed that far forward to achieve balance

Once I added a filterwheel and OAG, I started to use a Baader counterweight on top to keep the scope near the mount centreline:

It was when I added the USB filter wheel that I had to move the guider forwarder to get balance. I see you had a counterweight at that point, and I will definitely try moving guider back and adding some weight as a first step.

Or from a stage equipment supplier:

http://www.djanddiscostuff.com/stands-storage-c139/clamps-brackets-c146/oslim-2-p2053/s2122?utm_source=google&utm_medium=cpc&utm_term=phase-one-oslim-2&utm_campaign=product%2Blisting%2Bads&gclid=CjwKEAiAjsunBRCy3LSlz_PJqCgSJACJY7yK_U6Rc-kifX40IYmbbKk0x3J656bTV3pRvhZZU-Q57hoCCKzw_wcB

I'd also recommend getting a set of Williams Optics CNC rings (the 90mm ones fit the Equinox perfectly) and moving to a Losmandy dovetail. Much more beefy and stable than Vixen dovetails.

Those stage equipment clamps look like they are in my budget, will give something like that a go.

I'd told my wife that I had got everything I needed so the CNC rings and losmandy dovetails will have to wait for a while - unless stiffening up the guider isn't "enough" and they become essential

[paulf]

Looking clear for part of tonight for a test run, so have made some initial changes based on feedback in this topic.

Waiting on a couple of clamps for guidescope (should be delivered tomorrow), but in the meantime have moved the guidescope back on the dovetail bar. To balance the scope have added a 'comedy' counterweight made from a couple of plumbing bits (!) and put a DSLR quick release plate at top for a bit more weight.  These add up to just short of 500g, once I have tried new clamps then I think I will get a 1kg Baader counterweight which will then allow me to move the main scope dovetail bar forward.

[paulf]

Well that made a difference I think. The RA graph is still jumpy but the results have certainly improved. Looking forward to getting guidescope clamps tomorrow then will order a more long term counterweight solution and hopefully I will be back on the right track again...  Thanks for your help everyone, been struggling with this for a couple of months.

[johnrt]

Whoa, your guiding graph should look a lot flatter than that with the short focal length of a finder guider. How good is your polar alignment?