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Alignment ?? - Sirius mount, elongated stars, etc.

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#1 jsines

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Posted 12 March 2013 - 01:59 PM

Quick question - I searched through the forum and couldn't find an answer, maybe someone knows the obvious answer to this question, maybe I could post a youtube video of my problem later when I get home if it doesn't make sense.

Sirius mount, Orion ED80T, viewfinder, and Canon 450D are attached, nothing else. I level the mount, I polar align, then drift align with an Orion 12.5" illuminated reticle on the south meridian and on the east for several minutes. I live in a white zone so I'm not expecting 10 minute unguided subs. Everything looks good, no movement after 3-5 minutes, so I 3 star align the mount, and then get alignment succesful. I then rotate to an object and start imaging - the intervolameter is set for 120 second exposures, 15 second break, then another. Get about 40-50 exposures.

I look at results in PixInsight's Blink - focusing in on the stars. They look slightly elongated, and they go in a pattern - elongated to the right, curved up, then elongated to the right, then curved up. The object, which was in the center of the frame in the beginning, has moved slowly over exposures to the upper right of the frame.

Any idea how to fix this? I'm wondering if the 12.5" reticle is too big? Maybe I'm not seeing the drift? I level in Dec and RA, maybe that's not right? I'm not sure what's happening, but my results are elongated stars.

Thanks!

#2 jrcrilly

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Posted 12 March 2013 - 02:07 PM

I'm not sure what's happening, but my results are elongated stars.

Thanks!


It would be helpful to know the direction of elongation. If the bottom of the camera is square to the dovetail plate, then RA is up and down and dec is left-right. If the camera is "sideways" (bottom of camera perpendicular to dovetail plate) then left-right is RA (East-West) and up/down is dec (North/South). If the camera is at some other orientation then neither axis is square to the frame and it's hard to tell unless you nudge the mount in one axis or the other during an exposure to see which way things move.

#3 D_talley

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Posted 12 March 2013 - 02:08 PM

What are you guiding with to remove the mount PE?

#4 jsines

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Posted 12 March 2013 - 03:08 PM

It would be helpful to know the direction of elongation. If the bottom of the camera is square to the dovetail plate, then RA is up and down and dec is left-right. If the camera is "sideways" (bottom of camera perpendicular to dovetail plate) then left-right is RA (East-West) and up/down is dec (North/South). If the camera is at some other orientation then neither axis is square to the frame and it's hard to tell unless you nudge the mount in one axis or the other during an exposure to see which way things move.


The bottom of the camera is square to the dovetail plate. The direction of the elongation in the images is dec (left to right), then either a right-angle squiggle (left, 90 degree angle, up) or an RA (up), then another dec elongation (left to right). The image appears to drift slowly from the middle of the frame to the upper right of the frame. I see the elongation when I zoom in, but it's not noticeable when I look at the whole image. However, stacking them makes it more pronounced.

I'm not touching anything once I press the intervolameter button. I basically press the button, then go inside and periodically check that it's still got electricity. :)

#5 jsines

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Posted 12 March 2013 - 03:19 PM

What are you guiding with to remove the mount PE?


I'm unguided. Nothing is going into the autoguider jack right now. I figured since I'm imaging from a white zone I'm not going to get 10 minute exposures, so a decent alignment would produce decent results. Maybe that's not the case, or maybe I'm doing something wrong. I've never done PEC training.

I just read through the manual again, I may be doing something wrong - I usually do the Pointing Accuracy Enhancement. For example, if I'm imaging the Rosette Nebula, I'll recenter on Betelgeuse. However, I go to Betelgeuse, then hit ESC twice, it says "recenter", then I recenter and hit ESC. The manual says I should go to Betelguese, recenter, then hit ESC twice, then ESC again when it blinks "recenter Betelguese".

It would make sense that after doing recentering incorrectly, it thinks an object is somewhere else. Not sure.

#6 wolfman_4_ever

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Posted 12 March 2013 - 04:24 PM

You never said where you slewed to after your polar alignment....

Polar alignment is different for different parts of the sky.. diffraction can becomes a major issue below 45degrees.. Seeing is another factor. I can get .5 arcmin polar alignment using various methods.. as the star tracks across the sky from the meridian to the horizon, the polar alignment error increases. Maybe this is your issue?

This is one reason many use pointing models..

A write up about this is HERE ..

A direct Quote from AP:

The King rate is .028% slow vs. the true sidereal rate. This is the best
compromise over the largest sky area. If you are at 40 degrees latitude and
pointing within 1 hour of the meridian, the driving rate for zero RA drift
would vary from .02% faster than true sidereal at a declination of 80 degrees
to .13% slower than true sidereal at a declination of - 30 degrees.

If you are 3 hours from the meridian, either east or west of the meridian,
then the tracking rate would vary from 0.2% slow at +80 degrees declination
to 0.28% slow at -30 degrees declination. This latter number is almost equal
to the solar rate.

As you can see from the above numbers, there is no one rate that will result
in zero RA error over the whole sky. The King rate is the closest for
tracking within 3 hours of the meridian for declination angles +- 20 degrees
of the celestial equator. The one saving grace as you go further north, the
RA drift amplitude in arc seconds per second will get smaller and smaller,
and will approach zero as you approach the pole.

#7 jrcrilly

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Posted 12 March 2013 - 05:58 PM

The bottom of the camera is square to the dovetail plate. The direction of the elongation in the images is dec (left to right), then either a right-angle squiggle (left, 90 degree angle, up) or an RA (up), then another dec elongation (left to right).


OK; two different things are going on. The dec drift pretty much only comes from polar misalignment so you may need to watch it more closely during drift alignment. Maybe watch it with the camera? The RA squiggle is mostly periodic error. A good PEC training should reduce that (at least, I THINK the Sirius has PEC).

Careful alignment of the frames should keep it from growing when you stack.

#8 jsines

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Posted 13 March 2013 - 02:30 PM

You never said where you slewed to after your polar alignment....

Polar alignment is different for different parts of the sky.. diffraction can becomes a major issue below 45degrees.. Seeing is another factor. I can get .5 arcmin polar alignment using various methods.. as the star tracks across the sky from the meridian to the horizon, the polar alignment error increases. Maybe this is your issue?

This is one reason many use pointing models..


thanks for that link. that was a very helpful article, and made a lot of sense.

For the past several sessions I've been polar aligning and then imaging the southern sky, the Orion Nebula, the Horsehead Nebula, etc. The one that prompted this post was the night I imaged the Rosette Nebula. I started imaging it very close to the meridian, a little to the west maybe, and then imaged it for almost 2 hours, until it was about 25-30 degrees from the horizon. So, I think your suggestion may be correct.

I think it may be one of a few issues that may be going on.

#9 jsines

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Posted 13 March 2013 - 02:35 PM

OK; two different things are going on. The dec drift pretty much only comes from polar misalignment so you may need to watch it more closely during drift alignment. Maybe watch it with the camera? The RA squiggle is mostly periodic error. A good PEC training should reduce that (at least, I THINK the Sirius has PEC).

Careful alignment of the frames should keep it from growing when you stack.


Yes, the Sirius has PEC. I've never done the PEC training or had tracking set to sidereal+PEC.

I'm using the 12.5mm Orion illuminated reticle, but I'm wondering if that's not accurate enough for drift alignment or PEC training. Maybe I'm not seeing the drift when it occurs.

I've got a lot of things to work on with this. Now I need to wait until the next clear night. *sigh*

#10 Phil Sherman

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Posted 13 March 2013 - 03:45 PM

You won't get an accurate polar alignment using the reticle eyepiece without spending a lot more time doing polar alignment. You'll do better with a photographic alignment. This is done using the same concept as the reticle but the reticle is replaced with a combined tracked/slewed exposure. For the azimuth adjustment, start with the scope pointed just W of the meridian at the celestial equator. The following steps will get you started:
1. Set your exposure time to 70 seconds
2. Set your slew rate to 1x sidereal
3. Set tracking rate to sidereal
4. Start the exposure and do the following for the indicated exposure times
00-05 track normally
05-35 Slew E
35-70 Slew W

Examine your image. The star trails will form a V with a bright star at the beginning of the trail. The end of the trail will be a little W of the starting point. The size of the opening of the V is an indication of the amount of alignment error, the relationship of the returning trail (W) relative to the starting one (E) will show you which direction to adjust the mount. If the adjustment makes the opening larger, it's the wrong direction. When the V collapses to a single line, you are polar aligned.

Repeat this process pointing E or W for the altitude adjustment. If you want more accuracy, you can increase the slew times to 60 seconds each. I found that the adjustment knobs didn't have enough resolution to support the longer exposure times for a portable imaging mount. If I was on a fixed pier, I'd try for the longer exposures.

The more level you get the tripod before attaching the mount, the less interaction there will be between the two polar alignment adjustments. You may want to repeat the polar alignments a couple of times to improve accuracy.

A Sirius mount will not give good results for imaging with exposures longer than 15-20 seconds unless you've implemented PEC. The easiest way to do this is to run the mount from your computer using CdC, the ASCOM platform, and EQASCOM (aka EQMOD). If you have a camera that can be connected to PhD for guiding, you can just run PhD while the EQMOD software records the periodic error then generates a PEC curve which it will then start using. Once you have the PEC curve, you can go back to imaging and not guide.

If you've balanced your mount to be a bit E heavy, have good polar alignment, and limit your exposure to 30-45 seconds, you should be able to use at least 80% of the images. I was able to do this with my Sirius before upgrading it to an Atlas a couple of years ago. Sky fog at my house limited me to 20sec exposures with my 8" f/5 Newtonian when using a Canon camera. My DSI Pro camera required even shorter exposure times until I added a light pollution filter to the mix.

Phil

#11 jsines

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Posted 14 March 2013 - 02:16 PM

Thanks, Phil. That makes total sense, and it sounds much more accurate than using a 12.5mm illuminated reticle which won't get me to the recommended 300x mag that the Orion manual suggests for drift alignment.

I haven't upgraded to guiding with a computer (yet), plus I have a mac and the sky pollution where I live is probably vmag 4.0, so I think getting a LPF and going for 1-2 minutes is my best bet right now. Ideally I can get the 1-2 minute exposures to be as accurate as possible unguided.






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