Jump to content


Photo

G11 Star Elongation: ONLY NEAR THE MERIDIAN

  • Please log in to reply
89 replies to this topic

#76 blueman

blueman

    Photon Catcher

  • *****
  • Posts: 5485
  • Joined: 20 Jul 2007
  • Loc: California

Posted 26 October 2012 - 12:30 PM

I can image for 9 huors and other than the movement caused by dithering, there is no movement between the subs, even after the meridian flip.
So, you can track for many hours without drift.
Blueman

Last night I imaged The Jellyfish for 5 hours straight.



Isn't 5 hours a long time to expect the mount to track perfectly? Even if you drift align you are ensuring no star trails for only 5-20 minutes or so. Michael Covington in "Astrophotograpy for the Amateur" explains that a misalignment can result in an fast/slow tracking rate and/or field rotation depending on the direction of the misalignment. However guiding should have corrected the fast/slow tracking rate problem.

Gale



#77 gdd

gdd

    Surveyor 1

  • -----
  • Posts: 1711
  • Joined: 23 Nov 2005
  • Loc: Lynnwood, WA (N/O Seattle)

Posted 26 October 2012 - 12:43 PM

I can image for 9 huors and other than the movement caused by dithering, there is no movement between the subs, even after the meridian flip.
So, you can track for many hours without drift.



What is "dithering"?

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.

Gale

#78 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 12:49 PM

Please look at this for dithering:

http://www.hiddenlof...s/dithering.htm

Peter

#79 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 01:03 PM

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.


I am not sure if this is possible without modeling. The stars move at different rates at different parts of the sky due to refraction. Stars track at sideral rate at the Meridian. So without modeling the sky with modeling software like The Sky, I am not sure it is possible to track perfectly even with perfect polar alignment.

For example in my case. One night before the sky got dark, I polar aligned my mount and slew the mount to a bright star near target DSO for imaging. I centered the star with my imaging camera and the sky was still not dark. I went inside the house to watch TV until the sky got dark. 30 minutes later I went outside to check if the star is still centered of my camera. It drifted in RA direction. I wondered why it drifted in RA and later I found out about refraction.

I believe autoguiding is necessary even with perfect polar alignment and without modeling.

Peter

#80 dawziecat

dawziecat

    Vanguard

  • *****
  • Posts: 2425
  • Joined: 20 Oct 2010
  • Loc: Rural Nova Scotia

Posted 26 October 2012 - 01:06 PM

Can you stack all subs WITHOUT alignment?

Peter


Attached, stacked without alignment. No dithering.

Attached Files



#81 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 01:24 PM

Thanks for creating a new image. The star trails look straight and it looks like flexure to me.

This setup might be more stable:

http://www.teleskop-...hototripod.html

Peter

#82 SMigol

SMigol

    Ranger 4

  • -----
  • Posts: 318
  • Joined: 30 Jul 2010
  • Loc: California, USA

Posted 26 October 2012 - 02:08 PM

Very interesting that the lower left has nearly no movement but the rest of the frame features arcs around that lower point. Was your guide star in the lower left? If so, I'm still thinking field rotation which is polar alignment.

#83 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 02:56 PM

Yeah, you are correct. I had a hard time seeing the image since it's highly compressed. Now you mention it, I do notice stars are longer and slightly curved relative to lower left. It does look like field rotation.

Try align guide scope with main scope so the same guide star is at the center in both guide and main scopes. Assuming your mount is well polar aligned, choosing a guide star away from target DSO can artificially mimic bad polar alignment. Once both guide and main scopes are aligned, pick a guide star right at the center.

Peter

#84 dawziecat

dawziecat

    Vanguard

  • *****
  • Posts: 2425
  • Joined: 20 Oct 2010
  • Loc: Rural Nova Scotia

Posted 26 October 2012 - 04:08 PM

I have mounted the guider onto the Canon taking lens. I hope rigidly enough to preclude flex but it's hard to feel certain about that. If I get good 15 minute subs over a wide swathe of sky, it will be absolute proof of flex to my mind.

It it doesn't, it might just mean I was not able to attach the guider to the lens with sufficient rigidity, in which case, nothing will be proved.

I will also check the alignment by turning off dec guiding and seeing what PHD indicates. I have done that in the past and there was no dec deviation in a 10 minute interval. That was some time ago though and the mount head has been removed and replaced since although the tripod was not moved.

#85 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 04:21 PM

If you are responding to my latest post, I was not implying flexure. I was incorrect the first time I saw your latest image without alignment. Your polar alignment may be fine but if the guide star is quite a bit away from the center of your main camera, it could artificially mimic bad polar alignment. I mentioned to align (or sync) your guide scope with main scope and try to find a guide star at dead center of guide scope. This same guide star should also be dead center of main scope as well.

Peter

#86 blueman

blueman

    Photon Catcher

  • *****
  • Posts: 5485
  • Joined: 20 Jul 2007
  • Loc: California

Posted 26 October 2012 - 04:54 PM

Oh, maybe I misunderstood the question?
Yes, I guide, but I thought it was being stated that with guiding you will still get drift.
Blueman

So 9 hours of tracking (without guiding)can be done with a careful drift alignment.


I am not sure if this is possible without modeling. The stars move at different rates at different parts of the sky due to refraction. Stars track at sideral rate at the Meridian. So without modeling the sky with modeling software like The Sky, I am not sure it is possible to track perfectly even with perfect polar alignment.

For example in my case. One night before the sky got dark, I polar aligned my mount and slew the mount to a bright star near target DSO for imaging. I centered the star with my imaging camera and the sky was still not dark. I went inside the house to watch TV until the sky got dark. 30 minutes later I went outside to check if the star is still centered of my camera. It drifted in RA direction. I wondered why it drifted in RA and later I found out about refraction.

I believe autoguiding is necessary even with perfect polar alignment and without modeling.

Peter



#87 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 05:10 PM

Yes, but it's not yet clear what caused the drift during guiding. That's what this thread is about.

Other stated that with perfect polar alignment you can image for 9 hours unguided and I assumed that modeling of the sky was not included so I don't think it's possible due to refraction in the sky.

Peter

#88 blueman

blueman

    Photon Catcher

  • *****
  • Posts: 5485
  • Joined: 20 Jul 2007
  • Loc: California

Posted 26 October 2012 - 08:33 PM

I do not use a model, I polar align and synch to one star and the AP-900 is ready to take 9 hours of images with no decernable drift.
Blueman

Yes, but it's not yet clear what caused the drift during guiding. That's what this thread is about.

Other stated that with perfect polar alignment you can image for 9 hours unguided and I assumed that modeling of the sky was not included so I don't think it's possible due to refraction in the sky.

Peter



#89 Peter in Reno

Peter in Reno

    Fly Me to the Moon

  • *****
  • Posts: 6195
  • Joined: 15 Jul 2008
  • Loc: Reno, NV

Posted 26 October 2012 - 09:03 PM

That's probably because you always use autoguiding to correct any refraction. I also have A-P mount and do the same thing you do. Without modeling and perfect polar alignment, I don't think I can trust unguided imaging for 9 hours unsupervised.

Peter

#90 blueman

blueman

    Photon Catcher

  • *****
  • Posts: 5485
  • Joined: 20 Jul 2007
  • Loc: California

Posted 26 October 2012 - 09:51 PM

As I stated twice, I do not do unguided 9 hour images. I was under the impression that the question was, can you guide for 9 hours without drift.
Blueman

That's probably because you always use autoguiding to correct any refraction. I also have A-P mount and do the same thing you do. Without modeling and perfect polar alignment, I don't think I can trust unguided imaging for 9 hours unsupervised.

Peter








Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics