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what's this,PE? Polar alignment? (from DSLR forum)

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#1 pubquiz

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Posted 21 April 2009 - 10:17 AM

Hi All

I started this in the DSLR forum the other day (please see there) but since I think it is more mount related I will continue here.

I am getting egg shaped stars.I am guiding with PHD with a Megrez 72 and imaging with a Celestron 8" SCT.

Mount is a Losmandy G11 pier mount in observatory.

The guiding stays locked on at all times and I THINK I have it pretty well polar aligned ...no drift after 5 mins when drift aligning..

I was asked in the previous post if I had any data on the guiding...so have attached a pic of the log from PHD.don't know whether it's of any help..is this bad or normal?

As mentioned I havn't done PEC so wondering if this will help?

In the last post and pics I had the scope perfectly balanced but was advised to adjust it so it was slightly east heavy and can see the logic of this so last night I tried it and all the following pics are with it balanced east heavy.

it was also suggested it could be flexure...I checked all fit of components and all seemed tight. The scopes are mounted side by side on a thick alloy plate...the SCT direct to the plate via it's dovetail and the Megrez with an ADM mini dovetail rail and rings...again all seem rock solid.

The 'Star elongation' doesn't really seem much different from 5 mins to 20 mins?

All pics are a single unprocessed frame of 5mins, 15 mins and 20 mins.

Cheers

Tom

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#2 pubquiz

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Posted 21 April 2009 - 10:18 AM

M3 One frame 5 mins

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#3 pubquiz

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Posted 21 April 2009 - 10:19 AM

M3 one frame 15 mins

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#4 pubquiz

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Posted 21 April 2009 - 10:20 AM

M3 one frame 20 mins

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#5 ozpapafox

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Posted 21 April 2009 - 11:31 AM

Hi Tom

FWIW, I had a similar problem with my EQ6 Pro mount recently with elongated stars like your shots and after a while I decide to re-do the PA by drift method again over a 10-15min period instead and found that it did have a slight misalignment which was not evident in a 5-min run. Also I discovered that my RA axis was slightly wobbly - ie I could feel a bit more of free play than normal even when I locked the clutch!

Attached is a pix taken under light-polluted condition in my suburban backyard - M42 shot was done with Vixen VC200L, Baader SkyGlow/IR 2" filter, mount EQ6Pro, unmodded EOS 5D @ ISO800, 6 subs x 2.5mins unguided, no flats/darks processed in DSS and CS2 - the problem was solved!!

HTH

Cheers
Bill - (From Down Under - Perth)

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#6 MikeML

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Posted 21 April 2009 - 11:56 AM

Hello Tom,
From what I can see from the pics ( I rotated the image so that North is up) is that it looks like your elongation is mostly in RA. Off hand I'd say that you may be to aggressive in your RA correction and overshooting.
Mike

#7 pubquiz

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Posted 21 April 2009 - 01:00 PM

Bill: Thanks for that (my mate Carl moved to Perth Last year and I have releatives there) :)Yes those stars are pretty round for 2.5 mins unguided :cool:...will certainly give the alignment another check.

Mike:I have attached the settings screen from PHD ...does anything look set wrong? guiding scope is Megrez 72 F6 and guide cam is a QHY5 (Orion Starshoot Autoguider)

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#8 Joe Cipriano

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Posted 21 April 2009 - 01:06 PM

Tom:

I'll third RA. You may want to increase the "East-heavy" imbalance slightly. My personal "rule-of-thumb" (with my CGE, YMMV...): When released from horizontal, the RA takes around 5 seconds to rotate to vertical (CWs down). Also, redo your drift alignment; I generally try for double my anticipated exposure time with no movement. As I'm under heavy LP, exp time is pretty much limited to 3 minutes or so per sub; I try for 6-10 minutes with no drift. As you're in an OB, I'd seriously consider setting aside one night just for alignment - spend a couple of hours getting it nailed, then don't worry about it for the rest of the season (generally, you should check your alignment every few months - more often if you regularly switch out OTAs).

As Mike stated, also check your RA corrections - maybe tweak aggressiveness down a little.

HTH

#9 pubquiz

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Posted 21 April 2009 - 01:11 PM

PS Bill (or anyone) Is there a good free download to do drift aligning using the guide cam (or DSLR) or webcam (I have a Meade LPI) and the PC screen?

Or is it a matter of using the reticule eyepiece and checking after 15 mins?

Tom

#10 pubquiz

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Posted 21 April 2009 - 01:24 PM

Hi Joe

Just checked the time as you describe and it is taking 4 secs to go fom balance weight arm horizontal to vertical...that's with the weights as far as they will go.

Think I will do as you suggest and spend a night getting the alignment spot on.

Tom

P.S. Off to the Pub quiz now for a few pints will keep thread updated with results.

#11 Tapio

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Posted 21 April 2009 - 01:31 PM

I had similar problems when trying to achieve longer than 1-2min guided shots.
since I asked the question in forums I've rechecked my balancing (and made it east heavy).
funny enough this made the difference.
Couple of things you could try changing in PHD:
aggressiveness lower (maybe 50-70%) and exposure times (I usually have 1.5 - 3 secs).
When I log my guiding graph, I can trust that it's sufficient when it stays between the first lines.

Oh, and software called WCS is good for polar aligning. You can try it for free but it isn't too expensive if you like it.

#12 freestar8n

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Posted 21 April 2009 - 01:32 PM

This looks like classic flexure when guiding an SCT with a guidescope. May be due to flexure in the guidescope mounting, or within the SCT.

Key indicator is - if you take a sequence of 5m shots, and you stack the frames, do the stars stay fixed and oblong, or do they 'march along' from one frame to the next. If they march, it is flexure and has nothing to do with guiding or balance.

I guide a C11 with off-axis guiding because I could never adequately solve flexure issues - except with shorter exposure.

Frank

#13 MikeML

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Posted 21 April 2009 - 02:24 PM

I'd definitely cut down RA aggressiveness. As others have stated, make sure you're a bit off balance in your RA. If your mount has a setting for guide correction rate you probably want to have that under 100% as well. I would think that if you were chasing seeing that would show up as a declination error.
Mike

#14 Poochpa

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Posted 21 April 2009 - 03:00 PM

Since your oscillation index in PHD shows .28, which is good, I would leave RA and Hysteresis settings where they are. The only things I would change at this time in PHD is the minimum star movement back to the default .25 and increase your guide exposures to 2 seconds, which is what I use for the same guide camera. Always have the mount slightly East heavy which you are now doing. One clue to the problem may be that you are getting about the same amount of star elongation regardless of whether your exposure is 5 or 20 mins. Such would indicate that the problem is not your polar alignment. I would try using PEC. Unfortunately, you will have to do this each time you image, as your mount can't store the PEC corrections permanently unless you are using the Gemini system.
Mike

#15 freestar8n

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Posted 21 April 2009 - 03:14 PM

you are getting about the same amount of star elongation regardless of whether your exposure is 5 or 20 mins



I don't see this at all. The 5m is pretty sharp, the 15m is quite elongated, and the 20m is a bit longer than 15m. Looks like flexure to me. In fact - I would expect this level of flexure with the given configuration.

Frank

#16 pubquiz

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Posted 22 April 2009 - 08:12 AM

Thanks All for your replies.

Will have a mess with the agressiveness settings once I get my PA spot on ...see what various settings do.

It is a shame the PEC isn't permanent on the G11...don't fancy having to do it every time I image :( even it is only 4 mins or so.

Looks like the consensus think it may be flexure...which is bad news as like I say everything feels solid, where is a likely place for this flexure?...can post a pic of the setup if it helps.

I do have a Celestron Radial Guider that I got with the scope/mount that I have never used so I suppose I could use that if all else fails....it's a pity though as I bought the Megrez to use as a guide scope as well as a visual/photo widefield tool.

Tom

#17 freestar8n

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Posted 22 April 2009 - 09:26 AM

Looks like the consensus think it may be flexure...which is bad news as like I say everything feels solid, where is a likely place for this flexure?...can post a pic of the setup if it helps.



If you have a series of images, one after the other, just blink through them and see if the streaked stars are moving along from one frame to the other. If so - it is not a guiding problem.

If one side of the frame is less stretched than the other, then it may be field rotation around the guide star.

Either way - it is hard to remove flexure from the internal components of an SCT. Some people go nuts trying to fix it for long exposures, and the solution is either shorter exposures or switch to imaging with a refractor. Or switch to OAG as I did.

Frank

#18 ozpapafox

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Posted 22 April 2009 - 11:02 AM

Hi Tom

I have no reason to use autoguiding yet since I resolved the "elongated stars" issue by doing drift aligning seriously - but do have the Orion Starshoot Autoguider with PHD Guide (latest version off the Stark Labs website which gives more control options than the one supplied by Orion in the box) and have both StarTarg ( http://www.andysshot...troduction.html - doesn't cost much and Andy is a great bloke) as well as this freebie from http://users2.titani..._Crosshair.html to use for polar alignment and with the Orion SSAG both works a treat!! :waytogo:
I do use the 9mm illuminated reticle EP and do a final check with it - at least 10 mins is what I'd go for

Cheers
Bill - (From Down Under - Perth)

#19 John Carruthers

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Posted 22 April 2009 - 01:56 PM

PS Bill (or anyone) Is there a good free download to do drift aligning using the guide cam (or DSLR) or webcam (I have a Meade LPI) and the PC screen?

Or is it a matter of using the reticule eyepiece and checking after 15 mins?

Tom


Yes, there's EQAlign 2 (free), I find it much quicker than eyeball drifting, 5 minutes / axis, job done :-)

#20 pubquiz

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Posted 22 April 2009 - 02:08 PM

Thanks John will download that.

Just checked a sequence of shots as suggested.

I opened them in Nebulosity then clicked Preview and watched them one by one. The didn't seem to 'march' just stayed on top of each other will prob be at least weekend before i get a chance to do a proper PA so will do that first.

Tom






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