EQ3 Pro sub troubleshooting

[mikey2000]

I was feeling bold and tried to get 80s exposures with my 150PDS on EQ3 Pro mount.  This is longer than I've tried and on M45 Pleiades, I had a mixed bag of results. (After spending an age getting the polar alignment correct with the drift method/reticule eyepiece)

 

Out of 37 pics, 8 were pretty good, 8 were 'ok' and 21 were total write offs.  Can anyone offer any suggestions here?  Here are some samples.  These are 1:1 crops from my Sony A6000 camera with 6000x4000 resolution.  The 'pretty good' ones don't show any problem when downscaled to fit my screen.  The really bad one's are obvious dustbin candidates....

Thanks in advance!

So, here are some samples:

[michael8554]

Hi mikey2000

Probably Periodic Error - cyclic variation of the RA rate.

Have you tried to Periodic Error Correct the mount?

Yes?

Analyse your images sequentially and you may discover a pattern of  good - marginal - poor - marginal - good - marginal  - poor etc.

If you can find your PE duration you could take exposures during the good periods - I agree, not an ideal solution,  guiding is the better solution.

Michael

[mikey2000]

Thanks for the reply:-).  There is a bit of a pattern with streaks of good pics interrupted by a few duffers.   Maybe PEC is the answer...

 

i was was also interested by the little mini stars in some duff exposures, almost as if the scope settled a few times in different places, as if it jumps in discrete steps sometimes

i did a bit of googling about this and landed on the idea of subtly unbalancing the scope so the drive cogs are meshing nicely, always "pushing" against the scope.   Amusingly, for the shots above, I had carefully balanced it as exactly as I could!

 

so, I have two tasks for the next starry night:  1) try the unbalance idea out  and 2) have a go at PEC.

 

I tried a trial run of PEC training with the synscan handset.   It measures one whole revolution of the RA control and takes just over 10 mins.  I think I can cope with that if it means I can get reliable 90s exposures :-)

 

hopefully, unbalancing will help the most as PEC training is lost very easily on this mount.

 

 

[wimvb]

Unbalancing will reduce backlash more than pe. You can determine how much pe you have by taking a single 10 min exposure. Star elongation = pe (peak to peak). As long as you only move the ra axis with the hand controller, pec should be maintained, theoretically. Realistically, it would be best to redo pec training every imaging session, but that's just too much work. The best remedy may be the one you already adopted: shoot away and select the best subs. There is no point in trying to time exposures with best pe phase. Just be prepared to take 4 times as many subs, if you only can use 25%.

[ollypenrice]

The golden rule of tracking troubleshooting is to have the camera orthogonal with RA ad Dec and to know which axis is which on the image. I'll guess that you did that and that RA is horizontal in your screen grabs. If so that's good news on the Dec axis front.

PE will generally manifest itself as a fairly featureless elongation. Backlash can produce something stars looking like 0-0  or a horizontal figure of eight because the mount spends most of its time on one side of backlash or the other, producing two slightly offset stellar images. You've seen a hint of this in places. Unbalancing to run a little east heavy is very effective.

Imaging and guiding go together like imaging and bankrupty!

Olly

PS Do you know the unethical but tolerably effective Photoshop fix for not too badly elongated stars? It might let you use a few more subs.

Stretch the image. Make a copy layer. Change the blend mode in Layers to Darken. Go to Filter-Other-Offset and put a tiny offset on the axis with the elongation (horizontal in your case.) By adusting the offset you'll be able to create fairly round stars. If a whole integer offset of 1 or 2 or whatever is too much, apply the one that is just too much and then go to Edit, Fade Offset and fade it to create the best star shapes. If the elongation is diagonal rotate the whole image till the trails are horizontal of vertical first.

 

[wimvb]

19 minutes ago, ollypenrice said:

The golden rule of tracking troubleshooting is to have the camera orthogonal with RA ad Dec and to know which axis is which on the image. I'll guess that you did that and that RA is horizontal in your screen grabs. If so that's good news on the Dec axis front.

...

Olly

PS Do you know the unethical but tolerably effective Photoshop fix for not too badly elongated stars? It might let you use a few more subs.

...

 

There's an equally unethical equivalent in PixInsight. It's in the deconvolution process.

[AKB]

42 minutes ago, ollypenrice said:

Imaging and guiding go together like imaging and bankrupty!

Sorry, off topic, but... Is there a compendium of Olly's 'Wise Sayings' ?  I mean, I could  trawl through all 26,000 posts - and it would undoubtedly be worth it - but I wondered if it was somewhere out there in more distilled form?

I've also noticed these recently, amongst others:

Quote

Autofocus is fine when it works. If you focus yourself it always works!

Quote

'fast' and 'cheap' don't belong in the same sentence. In fact 'fast, eye-wateringly expensive' and 'easy' dont belong in the same sentence either.

Quote

There's no spike like an Alnitak spike!

...

 

[mikey2000]

Olly  - thanks for the tips on making sure the camera is orthogonal.  I hadn't thought of that.  I had been pointing the telescope then rotating the camera to get my composition as I liked it.

At the start of imaging a particular object, I will take a few test shots with deliberate additional slow slewing in RA and DEC to get a 'direction frame' so I know which way is N/S and E/W (Or I could deduce it from the pics I take and cross reference with a star map.)

 

Thanks also for the photoshop tip.  I have been using AstroTools which has an automated Repair module that helps cure eggy stars to an extent.

[mikey2000]

Now, just so I am not misunderstanding something here, can I ask for some clarification on a few points.

 

I'll frame this as a bold set of statements.  Can someone criticize this to see if I've got the hang of things?

In a sub up to 120s, drift in RA can be mainly caused by poor polar alignment. With a decent alignment, Assuming my mount has an accurate RA speed, other errors are then mainly from RA backlash (0-0 looking star pairs in a line, cured by subtle unbalancing) or by periodic error.   Drift in DEC is mainly caused by poor polar alignment.

 

Discuss! 

Ps. I'm not honestly expecting reliable 2 minute subs on my EQ3Pro loaded up to within 500g of it's max payload....  But the information gleaned here will help me optimise and hopefully get more reliable 100s light frames...

[ollypenrice]

Polar misalignement means that the circle described by the driven side of the mount is not concentric with with the circles of Right Ascension on the sky. This will generate errors in both RA and Dec. 

An accurate RA speed means an accurate average RA speed across one rotation of the wheel and a consistent RA speed free of PE. You are more likely to get the former than the latter from any mount.

I'd have said that backlash and PE will, on most mounts, cause more error than polar mislagnment unless the misalignment is extreme. The easy way to look at you polar alignment is to star-align a first sub and a last sub to each other and then look at the edges of the frames. The offset in the frames shows you the extent to which your polar misalignment has introduced drift in the time between the capture of each sub.

(BTW, it might be worth saying at this point that, when you autoguide, a slight polar misalignment allows you to correct only in one direction in Dec. If you have Dec backlash then this is an excellent way to reduce it. There is no logic in this game!)

Olly

 

[mikey2000]

My head is truly spinning now!  Anyway, I think I've taken some of this on board.  I got a knockout polar align fairly quickly using the camera/drift technique (2 min exposures, 1min each way gave no visible < shape, even zoomed right into to 1:1 pixel view on my 24mpix camera, so no Dec drift visible at south meridian or east horizon))

I also unbalanced the scope (heavier on the east)

I also tried PEC training the mount - after a few false starts, it takes just over 10 mins. (the RA hand control stub moves one complete turn).

I got about 33% keepers on 60s and 50% keepers at 45s, aimed at the belt in Orion.  I understand that this might be the hardest DEC region as those stars are drifting so quickly there....

 

Looking at the individual subs, they were largely OK (some perfect, some with some occasional egging that seems to get rounded out easily in stacking/processing later).  Interestingly, the bad ones seems to be large jumps with pronounced star strails.  Here's an overlayed stack of 50 minutes approx, with no inter-frame alignment - these are fress from the camera.  I took out the totally poorly tracked subs (huge trails!)  I have time-coded the frames by starting with a green tone then shifting each frame closer to magenta.  So green blobs are at the start of the 50 mins then pinkish as you reach the last few (passing through white about half way through)

Here's the stack.  The red line up is nearly north, the red line going sideways is sort of west. (I think.  it might actually be more like N at the 11'clock west at 2 o'clock if you see what I mean)  the bright star is Alnitak.  It's curious - the overal trend is to drift left and up with side to side jumps, giving two clearly visible tracks.  I'm not sure what would close the ends of each potato shape.  Before I came up with the colour code idea, I had expected the loop to be a circular path, not a zigzag averaging along a certain direction.

Curious!

 

 

 

[mikey2000]

more news.....  I have spent some time looking at my subs.  Thanks to the comments above, I have new ways of looking at them and I've concluded my PA wasn't ever as great as I thought.

 

I haven't managed to make super-long exposures (due to clouds) but I've used Photoshop to stack together some subs to make extra-long exposures.  I'll show a few in a mo.  But, My Problem seems to be made of 3 parts....

1) 1 Arcsecond = 1 Pixel on camera.  This is fussiness (blush)  I should pixel-bin to screen size.

2) Periodic error - every zig zag you see below has the same period.  I can reduce (but not eliminate) this

3) Failed PA - over 40 mins, there is some Dec or RA drift

With a 150PDS on EQ3Pro with lilghtweight camera, I'm at the 'official weight limit' for the mount.  I hope to try a better attempt at PEC using the synscan handset but it KEEPS BEING CLOUDY!!!

 

I predict questions about mini-guidescopes and QHYII cams. ;-)  And EQ5 :-)

'you told me so'