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Equipment Discussions >> Mounts

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orlyandico
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Reged: 08/10/09

Loc: Singapore
Oblong Stars
      #5427420 - 09/18/12 01:26 PM

I'm guiding with a barlowed finder (about 320mm FL) and Meade DSI (9.6x7.5um pixels). The imager is an Orion 100ED with reducer (720mm FL) and QHY8 (7.8x7.8um pixels giving about 2.2"/pixel).

This is what my guiding graph looks like. Yeah it's no Mach1...



Based on this seems I can only maintain tracking to about 2" - but 2 out of 3 of my 10-minute subs still are oblate. Here's what oblate is (this isn't the worst oblong-ness, it's kind of in the middle)



I do have a bit of declination drift but nothing that the guider can't handle. What's going on? is it impossible to get perfect stars at 720mm with my setup?


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: orlyandico]
      #5427425 - 09/18/12 01:27 PM

And I already have anti-stiction turned on in DEC (so Maxim only guides in one direction) plus I re-calibrate the guider after every GoTo slew so no shenanigans with declination here.

My RA guide speed is 0.3X and DEC guide speed is 1X. My RA aggression is 3 and DEC is 7 (need that to keep up with the declination drift).


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tjugo
scholastic sledgehammer


Reged: 11/06/07

Re: Oblong Stars new [Re: orlyandico]
      #5427473 - 09/18/12 01:49 PM

Flexture? You can measure flexture like this:

-- Guide with your DSI normally, create a log
-- Run your main camera as autoguider, but don't send any corrections, just record the real tracking error. You can use PHD or another instance of maxim. Create another log.

Compare both logs, if both logs look the same you are chasing the seeing or you need to tweak the guider params. If you see a drift on the main camera log, there is flexture somewhere.

If flexture is the problem, check all your mechanical connections or use an OAG.

Cheers,

Jose


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: Oblong Stars new [Re: tjugo]
      #5427501 - 09/18/12 02:02 PM

Quote:

Compare both logs,



Compare logs by doing linear regression on the data. Pay attention to the slope values.


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freestar8n
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Reged: 10/12/07

Re: Oblong Stars new [Re: orlyandico]
      #5427503 - 09/18/12 02:03 PM

Hi-

Your image is rotated so the drift is in RA, not Dec. I don't know how that relates to the X,Y of your guider plot. Either way - since the oblongness is lined up with RA pretty well it is less likely to be flexure. You may just need to guide tighter in RA - which is usually easier than Dec.

Frank


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: freestar8n]
      #5428330 - 09/18/12 09:46 PM

hi Frank,

My RA actually is tighter than DEC - due to all the DEC backlash, stiction, etc. And I don't see how I can guide tighter in RA considering my RA guide speed is at 0.3X already.

Could the RA drift be due to altitude polar misalignment? I will admit my polar alignment is... indifferent at best (did not do a drift align).


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Oblong Stars new [Re: orlyandico]
      #5428605 - 09/19/12 01:22 AM

Orly

How good is the main optic aligned with your guide scope?
It is possible that you get field rotation because of this...

Flex between main optic and guide optic would be my 2nd guess, are you certain there is no flex in the connecting parts or focuser?

best regards
Chris


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: chboss]
      #5428666 - 09/19/12 02:31 AM

well yeah.. i am using one of those one-arm finder stalks

and the guide scope is not aligned with the main scope all that well. If a bright star is in the center of the main scope, it's about 2/3 to the edge in the guider (which has a much wider field of view). it's pretty hard to align the guider on the "one-arm" - you know, where there is only one set of 3-point screws, the other mount point is a rubber O-ring?

i'm getting better rings soon, so will try to address that.


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freestar8n
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Reged: 10/12/07

Re: Oblong Stars new [Re: orlyandico]
      #5428680 - 09/19/12 03:05 AM

Quote:

My RA actually is tighter than DEC - due to all the DEC backlash, stiction, etc. And I don't see how I can guide tighter in RA considering my RA guide speed is at 0.3X already.

Could the RA drift be due to altitude polar misalignment? I will admit my polar alignment is... indifferent at best (did not do a drift align).




It could be a lot of things - but it doesn't appear to be a problem in Dec. if the stars are oblong in RA. If the stars really are accurately stretched in RA, then it becomes more of a coincidence for it to be flexure or field rotation.

Using 0.3x for RA might be too small. It's hard for me to interpret the guide plots because they are in x,y and I don't know how they are oriented relative to the image. My guess is they are at 45 degrees to E/W since both show similar error. Anyway - you may just need to play with guide parameters to get it tighter.

If you are getting this all over the sky, is it always in RA? Do you have some imbalance in RA so it stays engaged with the gear?

To rule out field rotation - do the stars form long arcs when multiple exposures are stacked without alignment? Is the guide star N or S of the image? It would need to be for this to be field rotation.

So there are many things to do to help identify the problem. And since Dec. guiding and flexure are less likely to be the issue here, you have a better chance at a solution.

Frank


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Oblong Stars new [Re: freestar8n]
      #5428697 - 09/19/12 03:53 AM

Orly

If you use a normal finder stalk it is a big concern for stability. The normal three screw with O-Ring solution is not very stable especially when connecting a camera with a cable that will put strain on the mounting point...
These instabilities will cause erratic guiding, which is visible in your graph. There seem to be jumps on both axes.

Having not aligned the optical center of both optics will cause field rotation, I have been there myself.
Inspecting your image you will find that stars will be better in one corner of the image than on the other side...

Before trying anything else you need to mount the guide scope stable and align it properly to your main telescope.
Flexture in the focuser is another point you need to rule out.

best regards
Chris


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: chboss]
      #5428820 - 09/19/12 07:54 AM

Frank,

there are no long arcs. In fact succeeding subs have 0-2 pixel offsets according to DeepSkyStacker. And yes, the guide camera is at an angle from X and Y (Y is the declination by the way) by about 30 - 35 degrees. Yes I am pretty sloppy.

And I am using 0.3X in RA because

(1) my periodic error is pretty low (fundamental about 9" p-p)

(2) my GoTo controller and steppers seem to have a minimum "push" distance, if I use an RA guide speed of 0.7X or 1X, the guiding graph gets noticeably rougher.


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michael hester
professor emeritus


Reged: 11/28/08

Re: Oblong Stars new [Re: orlyandico]
      #5429378 - 09/19/12 02:53 PM

How good is your polar alignment? That is actually a very critical point. If you're more than 1 minute from the pole you can forget 10 minute subs, they'll streak even in a 700mm focal length APO. The reason is that you're getting field rotation about the guide star. I've noticed this in my exposures as they get longer. Between a set of 6 5 minute exposures the stars drifted in a direction not aligned with any axis.
It frustrated me too because I blamed it on periodic error.

I noticed in your graph though that your corrections are way too aggressive. You have a correction longer than 1 pixel. That will streak an image. This was the reason I went with PHD over Maxim DL. I could directly define how aggressive PHD was in keeping the star centered to keep PHD from destroying otherwise perfectly fine exposures. Even in a 250mm focal length guider the guide quality depends on the seeing I get. Maxim DL likes to chase seeing too much.

So check these things:
1. Make sure your polar alignment is tight. Drift align to get it as tight as possible.
2. Make sure the seeing is good for the current night of imaging. Maxim DL and even PHD sometimes will chase seeing and ruin otherwise good images. Measure how much of a correction the guider can make before you get streaking
3. Know that your maximum exposure is related to how well you polar aligned. You need to be closer than 2 minutes from the pole to get 10 minute or longer exposures.
4. My advice's mileage varies, but the concepts should be sound.


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: michael hester]
      #5429907 - 09/19/12 08:54 PM

hi Michael,

That's interesting. I'm pretty sure I am not 2 minutes from the pole. That is 1/30 degree!! it really has to be that accurate? I did an "iterative GoTo alignment" this time.

Might explain my issues over the last months..


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orion69
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
Re: Oblong Stars new [Re: michael hester]
      #5429929 - 09/19/12 09:10 PM

Quote:


3. Know that your maximum exposure is related to how well you polar aligned. You need to be closer than 2 minutes from the pole to get 10 minute or longer exposures.





That can't be true for 720mm FL since I use ASPA on CGEM which is for sure not that accurate and I'm doing 30min subs @765mm regularly with no or negligible field rotation.

Knez


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: orion69]
      #5435619 - 09/23/12 08:19 AM

ok, goody, i can now consistently (85% of the time) get round stars at 10 minutes and 720mm FL. i dug out my old Vixen OAG for this purpose

now for a further issue... even though the stars are round, the guider is really working like crazy. It's bouncing up and down around +/- 2" (RMS around 1.3"). this is with 2-second guide exposures.

And.. this bounciness is on the RA axis (!) surprisingly the DEC is relatively quiet (now that I did a better polar alignment).

Is this normal? I fear that the gears in my mount are causing this jumpiness, because the fundamental is quite smooth at 9" p-p - and I don't know where to get better motors and gearheads, since my motors are already $220 (SRP) Vexta steppers.


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Oblong Stars new [Re: orlyandico]
      #5435859 - 09/23/12 10:43 AM

Hi Orly

Good to hear you are making progress, the OAG removes any problem of flex (if the OAG is reasonably mounted). How long is the exposure time of your guide camera? I use 2s - 4s to make sure I am not chasing the seeing, this with PHD on LFE Photo.
That the DEC is not acting frequently is a good sign!

What is your exact motor, gear configuration? Any play in one of the components.
How is the gear mesh adjusted on your AP600 mount?

best regards
Chris

BTW Just finished watching the F1 in Singapore...


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orlyandico
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: chboss]
      #5435953 - 09/23/12 11:47 AM

i have been using 2 seconds. 4 seconds saturates the star so the centroid may be less accurate.

my motor is a vexta PK243SG18 with 18:1 gearbox, NEMA 17. The worm is 192-tooth. I have adjusted the gear mesh, it is tight but not overly so.

well for me the only benefit of F1 is that, because my office is within the circuit, few people drive to work during F1 week, which means parking is really easy to find...


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chboss
professor emeritus


Reged: 03/24/08

Loc: Zurich Switzerland
Re: Oblong Stars new [Re: orlyandico]
      #5439199 - 09/25/12 06:17 AM

Hi Orly

Sounds all good 2s should be OK for guiding.
At what focal length?

best regards
Chris

I only visited an F1 race once, just too many people... my experience is that I see more on TV.


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orlyandico
Post Laureate
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Reged: 08/10/09

Loc: Singapore
Re: Oblong Stars new [Re: chboss]
      #5439412 - 09/25/12 09:51 AM

900mm focal length, ha ha!

anyway I am PEC-training the thing right now with my trial PEMPro. Fundamental according to PEMPro is only about 6" p-p, and it's tough lowering it!

longer term, I have a 5000-ppr Baumer sine-cosine encoder coming in, and a Heidenhain IBV 660 interpolation box (which can do 400X interpolation). This will give me (5000 * 400 * 4 quadrature) = 8 million ticks. More conservatively if I only use 100X interpolation on the Heidenhain box that's still 2 million ticks which should be plenty.

Assuming both work, I will get to test Rajiva's Encoder-assisted PEC


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Jared
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Reged: 10/11/05

Loc: Piedmont, California, U.S.
Re: Oblong Stars new [Re: chboss]
      #5439597 - 09/25/12 11:35 AM

Quote:

Hi Orly

Good to hear you are making progress, the OAG removes any problem of flex (if the OAG is reasonably mounted). How long is the exposure time of your guide camera? I use 2s - 4s to make sure I am not chasing the seeing, this with PHD on LFE Photo.
That the DEC is not acting frequently is a good sign!

What is your exact motor, gear configuration? Any play in one of the components.
How is the gear mesh adjusted on your AP600 mount?

best regards
Chris

BTW Just finished watching the F1 in Singapore...




I ran into similar issues with a GM-8 some years ago. The cause of the problem was a balance issue (at least in my case). The RA worm needs to stay "loaded", so make sure that your mount is slightly East heavy for whichever side of the meridian you are on. It doesn't have to be much--just enough that the drive is always "pushing" the mount along rather than "pulling" it back.


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