Jump to content


Photo

Sky View Pro RA tracking error

  • Please log in to reply
36 replies to this topic

#1 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 12 February 2011 - 06:53 AM

I am having an issue with my Sky View Pro mount (2003 era) and unguided imaging. I am using an AT6RC and DSLR (7.2 um pixels) and hoping to get exposures in the 45-60s length. However, I am experiencing significant drift in the right ascension axis.

What concerns me is that the tracking error is not uniform PE…the star trails often have little jumps in it (see the attached image from a 100% crop). It almost appears as if it tracks for several seconds, then slips to a new position, then tracks for a little while longer before slipping again. What's worse is that the drift usually continues in a single direction, effectively tracking at less than 1x sidereal.

I have ensured that everything is properly balanced (slightly east heavy...I've also tried balanced and west heavy for the heck of it). I've torn down the mount, relubed, and adjusted the gears, etc., but nothing is working.

I am wondering if possibly there is a problem with the tracking motor that would cause it to skip like this, either with the electronics or possibly the internal motor gearbox. Have you any of you had similar problems before?

Part of what confuses me is that the tracking performance varies depending on where the scope is pointed. If I shoot at declination ~0 deg (M42), maybe 5% of the images are well tracked at 30 sec, but at ~60 deg (M81) I can get around 50% good images at 45 sec. Obviously, the linear movement of the stars is less closer to the pole, but I'm also wondering if there's something mechanical with the mount going on. This isn't just periodic error because each frame drifts farther in the same direction and never reverses direction. Longer exposures just create longer streaks.

I've struggled with this for far too long and I'm about ready to kick it to the curb and buy an Atlas/EQ-6 or CGEM. I like the SVP, and it's quite stable with the AT6RC, but it can't track beans so it has to go unless I can fix it. Do I have unrealistic expectations for this mount to give me 30-45 sec exposures with the AT6RC?

I should also mention that when I've used my 300/2.8 camera lens on the mount, I can only manage around 45s, maybe 60s exposures before seeing similar issues, so it seems to be somewhat independent of weight and focal length.

Help please! :confused: :help:

Attached Files



#2 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 12 February 2011 - 07:03 AM

As an example, here is typical of what I was getting with 30 sec on M42 (0% usable frames) :bawling:,

Attached Files



#3 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 12 February 2011 - 07:04 AM

yet I was able to get 45x45sec on M81/82 (about 50-60% usable frames) :crazy:.

Attached Files



#4 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 12 February 2011 - 12:09 PM

Since you are not guiding it is important to drift align the mount. By doing a good drift alignment you should only see RA errors, but as with any physical set up other factors can also be problematic.
The reason the problem looks worse with the at6rc is that your image will cover about 1.1 arc second per pixel and with the 300mm lens the image scale will be about 4.5 arcseconds per pixel.


#5 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 12 February 2011 - 02:43 PM

Yes, you are absolutely correct. I've actually had good polar alignment with no detectable declination drift (any drift is completely overcome by the RA error). My apologies if the orientation of the images I posted is not clear.

I realize that the image scale with the AT6RC is much more demanding of good tracking, but even at 300 mm I am unable to get 60s exposures on a regular basis.

In any case, the mount should not be jumping/slipping which you can see in the 1st 2 images I posted with the "double stars". Any ideas? :question:

#6 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 12 February 2011 - 02:52 PM

How are you balancing your mount? Something that can make a large difference in RA tracking on my CG-5 (very similar to your Skyview Pro) is to always make sure the mount is balanced slightly Easy heavy.

If the mount unbalanced slightly Easy Heavy then the RA drive will be constantly pushing the scope "uphill" such that the worm gear teeth always stay in contact with the ring gear. That way the tracking should be as smooth as possible given the mechanics of your gears.

If however the mount is *perfect* balanced or slightly West heavy what will happen is that the RA drive will give the RA axis a bit of a nudge and then there will be a bit of a gap before the worm teeth "catch up" and make contact again, at that point it gets another nudge, etc. You end up with a sharp back and forth motion of the stars that span distance in of your gear backlash (the slop in the gears).

You can never get rid of that backlash entirely so keeping the mount ever so slightly east heavy solves the problem. :)

#7 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 12 February 2011 - 02:54 PM

BTW, to do that intentional imbalance with my mount I first find the perfect balance (making sure all my imaging gear is in place before finding it) and then move the counterweight up or down the shaft by about an inch depending on if I am imaging to the East or West of the meridian. That seems to be enough of an imbalance for my mount to do the trick.

#8 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 12 February 2011 - 03:15 PM

from what I have seen with some of my own mounts the double star effect is not uncommon effect of periodic error. If we assume that your worm gear diameter is 2.5 inches that is a radius of 1.25 inches. If we further assume that the precision of your worm to gear is 1/10000 inch .0001 inches which is a high precision meshing you still have an angular error of 16 to 17 arcseconds. Since the periodic error is not necessarily smooth as we would like the manisfestation of a double star effect isn't unusual. To smooth out the humps in the periodic error you could try to lap the worm gear/worm with some cerium oxide and light oil, clean well then re lube. Without guiding the frustration will be there, and even with guiding things happen that will be frustrating. By the way your m81 m82 is pretty good considering the conditions you are working with.
Dale

#9 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 12 February 2011 - 03:19 PM

I have been balancing slightly east heavy just as you suggested (one of the tips I've picked up thanks in part to the wealth of knowledge on the CN forum!)

Like you, I find perfect balance first with all the gear, and then shift the counterweight a little to make it slightly east heavy.

I too had been thinking about the gear slop and have tweaked gear meshing, tension in the worm shaft, preload on the RA axis thrust bearing...just about everything I could think of but nothing will make this thing track straight. :bangbangbang:

#10 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 12 February 2011 - 03:25 PM

Your image of the core of M42 shows obvious double-star lobes.... reminds me of issues I have had with the DEC axis on my mount sticking. How stiff is your RA axis? For my DEC axis a regrease and slightly looser reassembly improved things tremendously. I *suspect* a slightly lose assembly with the east-heavy balance would be better then a perfect-gear-mesh but tight assembly.

Edit: I see you already did re-lube the mount.

#11 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 12 February 2011 - 03:48 PM

Matthew, I thought I would mention a method to see just what the periodic error of your mount looks like. If you shift the azmuth of your mount by 40 degrees or so and take an exposure equal to longer than one worm cycle the stars will trace a diagonal trace. all of the humps and bumps along that line will show the periodic error along the way. It is sort of a pain to perform that test but it will show the periodic error of the drive. no periodic error would show a straight line.
Dale

#12 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 13 February 2011 - 03:34 AM

Thank you both for your comments and suggestions.

Before I relubed it, the RA axis was somewhat stiff and I suspected that might be causing some binding. When I reassembled everything, I was careful not to tighten up too much on the RA thrust bearing and I also left just a little backlash in the worm so that it's relatively easy to turn by hand. I don't have a good before/after comparison, but it seems to have improved a little. I temporarily loosened up the meshing even more, but that made it very sloppy and didn't help noticeably, so there's definitely a limit to how loose it can be.

That's a good idea to offset the polar alignment and let a long exposure drift in declination to see the PE. I'll upload the results when I get a chance.

#13 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 13 February 2011 - 05:08 AM

That's a good idea to offset the polar alignment and let a long exposure drift in declination to see the PE. I'll upload the results when I get a chance.


I did the drift test tonight, 3 exposures 20 minutes long each. It appears that my worm period is ~10 minutes based on how long it took for an ink dot on the worm to make 1 revolution. The images agree.

I carefully aligned the camera so that declination corresponded to X axis in the image and RA is Y axis. North is to the left and West is up.

The image is a 50% crop from all 3 exposures. The bright star in the first 2 images is Regulus, so declination was about +10 deg for this test. The third image was taken on a nearby star because the mount was interfering with itself after the first 2 exposures.

EDIT: another difference between the 3 exposures is that the first one started as what I would consider slightly east-heavy, and in each of the other exposures I moved closer to perfect balance. I think the 3rd one is pretty close to perfect balance, but it's hard to tell for sure with the stiction in the RA axis.

I have defined linear drift here as the number of arc seconds per minute of exposure time that the star moves in the RA direction that is not periodic error.

I calculated PE, peak to peak, by first rotating the image until the "axis" of the sinusoid was level, and then measuring the peak to peak in pixels and converted to arc seconds (1.08" per pixel).

What I'm not sure about in this test is if the linear drift portion is due to the fact that I'm not polar aligned, or if there is a tracking rate error. I've previously suspected that I'm getting some tracking rate error, but I'm not sure if these images prove that.

In any case, the periodic error is HUGE and is plenty sufficient to convince me that this mount is not up to imaging with the AT6RC, at least not in its current condition. :tonofbricks:

Attached Files



#14 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 13 February 2011 - 11:48 AM

Matthew, What you are calling linier drift is due to the offset alignment. If the tracking rate was off then your double star problem would be streaks instead. I think if you compare the movement of the little spikes in your curves you will find a correlation to the seperation of your double stars.
Guiding would allow you to take longer exposures with shorter focal length objectives. The longest focal length that would be useable would depend on the guide system mount combinations ability to guide out the short spikes.
Dale

#15 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 14 February 2011 - 02:43 AM

Thanks Dale (and Falcon) for all your advice. I think I see an Atlas and autoguiding setup in my future...

#16 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 14 February 2011 - 10:34 AM

Matthew, May you have dark skies and an enjoyable future. Astrophotography can be rewarding and somewhat frustrating at times...

Dale

#17 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 14 February 2011 - 01:26 PM

Matthew: does your Sky View Pro have a single axis drive, a dual axis drive, or a goto controller right now?

If it is the dual-axis or goto versions you should be able to do auto guiding with your current mount, could make a large difference. (To autoguide with the dual-axis dive requires a bit of modification to the controller, but not too difficult from what I have read)

Not that I would diswade you from an Atlas - I certainly wish I had an Atlas/EQ6 myself! If budget is constrained though you could get autoguide going first to improve what you already have, then get the atlas second and use autoguide with that for even better results. :)

#18 gdd

gdd

    Apollo

  • -----
  • Posts: 1392
  • Joined: 23 Nov 2005
  • Loc: N Seattle suburb, WA

Posted 14 February 2011 - 03:22 PM

Hi Matthew,

While you are waiting for your Atlas and/or autoguider, you might try pretending your AT6RC is your 300mm lens. You will will still have the resolution of your 300mm lens, but when the images are viewed at a 1/4 scale you will have much smaller pinpoint stars.

Is the tracking rate of your SVP adjustable? If not, try purposely mis-aligning PA slightly. If the true celestial pole, the false celestial pole, and the target are approximately in a straight line you will track slightly slow or fast for an hour or so. That would take care of the drift error. But it is really the PE that is the big problem. It is varying back and forth between 10 and 30 arcsec/min. Every once and a while it may cancel out your drift error.

Gale

#19 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 14 February 2011 - 05:42 PM

Thanks Dale! The occasional lucky shot that works out has definitely got me hooked!

Falcon, the mount I have is only with the single axis drive. Also, I've seen a note on Orion's website saying that SVP mounts before 2004 cannot be retrofitted with their goto system. I bought mine in 2003, so I'm thinking there must have been some mechanical changes. I guess that doesn't really matter if I was going to try modifying the controller box, but mine is only single axis.

Gale, your idea wouldn't work for a perfectionist like myself...the temptation to look at the images at 100% would just be too great! :roflmao: I think I would actually be better off just using the 300 mm since it's F/2.8 vs F/9, and it's a ridiculously sharp piece of glass.

Nope, the tracking rate isn't adjustable. I guess if I ever get really desperate I could try the mis-align trick, but that sounds like it takes far too much patience. :cool:

Fortunately, I'm single so that means I am my own CFO! I should be able to get an Atlas sometime soon, but even contemplating this has caused the clouds to roll in and the rain to start falling. :(

Does anyone here have any experience with Atlas vs CGEM? I've read the comparison threads but still haven't been able to make up my mind. This seems silly, but I'm leaning toward the Atlas if for nothing else it uses the Vixen dovetails like all of my stuff. I know there are adapters, but they're pretty $$$ which I'd rather put toward an autoguiding setup. :grin:

#20 Falcon-

Falcon-

    Aurora

  • *****
  • Posts: 4869
  • Joined: 11 Sep 2009
  • Loc: Gambier Island, BC, Canada

Posted 14 February 2011 - 05:56 PM

Too bad about not having the dual axis drive... If that 300 f/2.8 is as good or better then the 300L f/4 I have been able to use I would indeed recommend you keep using it, including after you get an Atlas or CGEM! There are quite a few objects much to large to fit in the FOV of the AT6RC so it is very good to have a "wide field astrograph refractor" like that 300mm lens ;)

That being said though I can think of a way to use the AT6IN on your existing mount - try some planetary photography! There is some software called EOS_movrec that will record the live-view video output of DIGIC III and 4 cameras, you can use that to capture some short video of the Saturn or Jupiter or the Moon and stack the results with RegiStax. Since video is going to be on the order of 1/15th of a second or faster exposures the tracking error will not be a problem at all.

I should note that I have not seen anyone use eos_movrec with a 1D MK III yet, but it is DIGIC III based so I would *think* it should work. :)

#21 gdd

gdd

    Apollo

  • -----
  • Posts: 1392
  • Joined: 23 Nov 2005
  • Loc: N Seattle suburb, WA

Posted 14 February 2011 - 06:26 PM

Hi Matthew,

You can get a G11, or maybe even a MACH1GTO for no more cost than your camera and lens. Or maybe that is your camera OR your lens!

Gale

#22 dale c

dale c

    Sputnik

  • *****
  • Posts: 45
  • Joined: 30 Mar 2009
  • Loc: Sedro-Woolley, Wa.

Posted 14 February 2011 - 06:38 PM

Matthew, if you don't mind push to you might consider a used or even a new Losmandy G11. I have used the GM8 with good results but the G11 is better. Presently using a 10 year old MI-250 for photography and the GM 8 for visual. Both mounts will last longer than I have left.... :p

#23 gdd

gdd

    Apollo

  • -----
  • Posts: 1392
  • Joined: 23 Nov 2005
  • Loc: N Seattle suburb, WA

Posted 14 February 2011 - 07:03 PM

Hi Matthew,

I'm getting a G11 with the OPW upgrade for better unguided tracking. Will add autoguiding as I get to longer FL.

Gale

#24 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 15 February 2011 - 01:09 AM

Too bad about not having the dual axis drive... If that 300 f/2.8 is as good or better then the 300L f/4 I have been able to use I would indeed recommend you keep using it, including after you get an Atlas or CGEM! There are quite a few objects much to large to fit in the FOV of the AT6RC so it is very good to have a "wide field astrograph refractor" like that 300mm lens ;)


Oh yes, the 300/2.8 is a gem with amazing resolution and pinpoint stars all the way to the edge...I definitely want to keep using it, especially once I can get exposures over 60s...:jump:

That being said though I can think of a way to use the AT6IN on your existing mount - try some planetary photography! There is some software called EOS_movrec that will record the live-view video output of DIGIC III and 4 cameras, you can use that to capture some short video of the Saturn or Jupiter or the Moon and stack the results with RegiStax. Since video is going to be on the order of 1/15th of a second or faster exposures the tracking error will not be a problem at all.

I should note that I have not seen anyone use eos_movrec with a 1D MK III yet, but it is DIGIC III based so I would *think* it should work. :)


oooh, that's a good idea! I didn't know that was possible to pull video from the older cameras but I should give it a try. I've gotten some decent single shot images of the moon, but I haven't even tried getting the planets.

#25 mfarrell

mfarrell

    Explorer 1

  • *****
  • Posts: 81
  • Joined: 14 Jun 2007
  • Loc: Northern California

Posted 15 February 2011 - 01:12 AM

You can get a G11, or maybe even a MACH1GTO for no more cost than your camera and lens. Or maybe that is your camera OR your lens!


Haha, that may be true, but a new mount isn't going to help pay the bills, unfortunately. :( I have the Canon gear for the freelance sports photography I do, but it takes nice astro photos too!






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics