Jump to content


Photo

AstroTrac tracking error

  • Please log in to reply
15 replies to this topic

#1 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 18 July 2014 - 04:46 PM

I'm trying to get trailless 5min shot at 180mm on a APS camera with unguided AstroTrac and not have much success.

Polar scope calibration is one issue. But when I look at the star drift, it's more complex than I had thought.

This is stacking of 50 subframes 2 mins each without star alignment. The first image is plate solved with Unimap to visualize the grid.

I could not get my heads around the diagonal drift in 100min since any mis-alignment should be in DEC direction unless the polar axis is way off the NCP. Right now I'm forced to use 2 minutes sub due to this issue.

Horizontal direction is RA. The small bump in the trail happens when I checked the exposure.

Attached Files



#2 gdd

gdd

    Surveyor 1

  • -----
  • Posts: 1610
  • Joined: 23 Nov 2005
  • Loc: Lynnwood, WA (N/O Seattle)

Posted 18 July 2014 - 05:44 PM

Are you sure the polar scope is aligned to the polar axis of your setup? You should be able to do that in the daytime by tilting the polar axis horizontally so you can view a distant terrestrial object through the polar scope and verifying the NCP mark does not move when you rotate the polar scope or when you rotate the Astrotrac about the polar axis.

Gale

#3 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 18 July 2014 - 07:56 PM

The polar scope is aligned to its axis already. The error of reticle moving against the background is roughly the precession gap size of Polaris between 2000 and 2030. I don't know if error in this along would cause such great trailing in 200mm. But I really doubt the trailing in RA direction is from my inaccurate polar alignment.

#4 gdd

gdd

    Surveyor 1

  • -----
  • Posts: 1610
  • Joined: 23 Nov 2005
  • Loc: Lynnwood, WA (N/O Seattle)

Posted 18 July 2014 - 08:07 PM

Strange, and because you mapped the grid lines based on the plate solve it can't be caused by the orientation of the camera.

What is the scale of your grid lines. I checked a couple of my images for drift.

Edit: For my M76 image where the camera orientation was 3.386 degrees off, the vertical (roughly RA) drift was 1.21 arcsec/minute and the horizontal (roughly DEC) drift was 0.3 arcsec/minute. The horizontal drift due to camera orientation is 0.07 arcsec/minute. My polar alignment was done by polar scope, no drift alignment.

Edit: For my Altair image where the camera orientation was 7 degrees off, the vertical (roughly RA) drift was 1.35 arcsec/minute and the horizontal (roughly DEC) drift was 0.3 arcsec/minute. The horizontal drift due to camera orientation is 0.16 arcsec/minute. My polar alignment was done by polar scope, no drift alignment.

The camera orientation drift needs to be added to or subtracted from the horizontal drift to obtain the DEC drift, I'm not sure which without check very closely.

Edit: I recall reading about polar alignment errors in the Michael Covington book. The drift can be in the RA direction, the DEC direction, or combination of both. If you draw a line between a star at the center of your image and the true NCP, the false NCP can be on that line or off to the side. If it is on the line the entire drift will be in the RA direction. If it is off to the side the maximum amount possible the entire drift will be in the DEC direction.


Gale

#5 rainycityastro

rainycityastro

    Viking 1

  • *****
  • Posts: 741
  • Joined: 29 Mar 2010
  • Loc: Seattle, WA

Posted 19 July 2014 - 02:22 AM

This might be a dumb question.
Are you absolutely certain that there is no slippage anywhere, i.e if you are using a ballhead, it is rock solid?

#6 Hesiod

Hesiod

    Viking 1

  • -----
  • Posts: 535
  • Joined: 13 Jan 2013

Posted 19 July 2014 - 07:38 AM

I use a rather similar rig. I had the following issue
1 misaligned polarscope (my sample has a losmandy like reticle, so was actualy easy to adjust)
2 misaligned polarscope arm: i need to put a small piece of paper under th3 polarscope collar to obtain the proper inclination (otherwise the polar alignement was right only at certain angle of the arm)
3sinking. It is the biggest issue, and more vexing the heavier is the photographic load. With the 200mm lens is really nasty because the lens is not balanced and it can happen several times, basically everytime I change target, while with scopes the tripod sinks only forward due the counterweights. Obviously it does not happen on concrete-like surfaces. To go past it I shot a test pic, then chek the wedge "bubble-level" and do the required corrections. For any given target 1 or 2 checks are enough, unless on sand or other really soft ground

#7 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 19 July 2014 - 10:57 AM

I'll check 2 and 3. Last time I checked No.2 is when I got this rig. I don't know if the arm will drift throughout the 3 year period.

#8 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 19 July 2014 - 11:05 AM

Edit: I recall reading about polar alignment errors in the Michael Covington book. The drift can be in the RA direction, the DEC direction, or combination of both. If you draw a line between a star at the center of your image and the true NCP, the false NCP can be on that line or off to the side. If it is on the line the entire drift will be in the RA direction. If it is off to the side the maximum amount possible the entire drift will be in the DEC direction.


Gale


Hi Gale, it seems if the misaligned NCP is still on that line, the drift should purely be DEC. If it's off to the sides than there will be RA component. But given my misalignment is under 1 degree, I doubt that will be diagonal drift unless I'm pointing the camera to the celestial pole? The region I'm photographing is Rho-Oph so roughly -30 DEC.

#9 Hesiod

Hesiod

    Viking 1

  • -----
  • Posts: 535
  • Joined: 13 Jan 2013

Posted 19 July 2014 - 03:28 PM

I'll check 2 and 3. Last time I checked No.2 is when I got this rig. I don't know if the arm will drift throughout the 3 year period.

Not very sure about the meaning of your words, sorry. My sample's reticle has an independent crosshair to verify the right "collimation" regardless of the actual precessional shift. The scope (and arm-scope) is ready when a star, or any other target, stay exactly in the middle of the cross, regardless the polarscope turning and switching

#10 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 19 July 2014 - 05:15 PM

I'll check 2 and 3. Last time I checked No.2 is when I got this rig. I don't know if the arm will drift throughout the 3 year period.

Not very sure about the meaning of your words, sorry. My sample's reticle has an independent crosshair to verify the right "collimation" regardless of the actual precessional shift. The scope (and arm-scope) is ready when a star, or any other target, stay exactly in the middle of the cross, regardless the polarscope turning and switching


I'm trying to say that my arm is collimated when I first bought this. But I haven't check the arm for years. As for the polar scope, I always had the issue to make the reticle dead center. I've attached an animated gif for illustration.

GIF_IMAGE

This is by far the most accurate I could go. The problem is the reticle center moves along a line instead of a circle when I rotate it. If I re-adjust to bring the 2 distant points to their mid point, then the reticle will move in a line perpendicular to the original one.

#11 Hesiod

Hesiod

    Viking 1

  • -----
  • Posts: 535
  • Joined: 13 Jan 2013

Posted 20 July 2014 - 12:16 PM

Thank you for the clarification (i am not an english-speaker and my skills are quite rough).
To improve the alignement you may try to correct halfway each time. In any case, the arm own error could be larger, so maybe it is worth a check. Its adjustment is far easier, a small stripe of paper placed between the arm and the polarscope magnetic "collar" could be very effective

#12 Plane

Plane

    Explorer 1

  • -----
  • Posts: 51
  • Joined: 09 Dec 2009

Posted 20 July 2014 - 09:02 PM

Hi,

For polar scope alignment,I placed the center on the corner of a street lamp and then rotate 180 degree to see the offset. I always makes adjustment to the 3 screws so that the new center is half way and I repeat the above process again. The problem is, no matter what I do, the 180 degree will be centered but the other 90 degree direction will be off. The movement of the reticle center does not seem to follow a circle.

I cannot check the arm at home since I need a distant object, at least 1 mile away, to account for the parallex of the arm diameter under 1 arcminute.

#13 Tim

Tim

    Messenger

  • *****
  • Posts: 415
  • Joined: 04 Aug 2004
  • Loc: North of Toronto

Posted 22 July 2014 - 12:00 AM

Thank you for the clarification (i am not an english-speaker and my skills are quite rough).
To improve the alignement you may try to correct halfway each time. In any case, the arm own error could be larger, so maybe it is worth a check. Its adjustment is far easier, a small stripe of paper placed between the arm and the polarscope magnetic "collar" could be very effective


how do you tell if the arm needs adjusting? should we always use it in the same position?

#14 Hesiod

Hesiod

    Viking 1

  • -----
  • Posts: 535
  • Joined: 13 Jan 2013

Posted 22 July 2014 - 05:53 AM

Well, that will not always possible because sometimes the scope/lens will cover the polarscope. The check itself is really easy, you perform the polar alignement, then move the arm and look at the reticle to see if it seems misaligned: if it is so, you have to slightly tilt the polarscope, maybe placing strips of paper, until the alignement looks properly done for each of the arm angle. A more precise method employ the crosshair: you put Polaris, or some other target, in the middle of the crosshair and make it stay there even after switching the arm tilting the polarscope. It is important to remember that the starting position could be "un-collimated" itself, so more angles you check, more accurate and reliable would be the final result

#15 Tim

Tim

    Messenger

  • *****
  • Posts: 415
  • Joined: 04 Aug 2004
  • Loc: North of Toronto

Posted 22 July 2014 - 10:11 PM

Hi Hesiod I always polar align before attaching a scope or camera so haven't had an issue but maybe I should be polar aligning afterward? how do you attach the strips of paper and what kind of paper do you use?

#16 Hesiod

Hesiod

    Viking 1

  • -----
  • Posts: 535
  • Joined: 13 Jan 2013

Posted 23 July 2014 - 05:42 AM

Hi! Usually i do the real alignement only before the shot.
I think it is only a matter of personal preference
I employ pieces of a standard a4 copy paper, and place them between the polarscope collar and the arm. Really, a single strip can be enough. Sometimes have thought about a piece of scotch, as a permanent solution, to be glued onto the arm, but until now it is only an idea






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics