How's my driving? Dial 1-800......

Yeah, I knew the scope was a big first step, even compounded the risk by giving up on a 3 months and counting back order and bought a used scope on eBay instead… But so far, things have actually gone smoother than I expected. For sure, not always smooth....; ). And the months of backorder were probably a good thing as it gave me time to do a lot of reading and to get started learning the software using data I got on itelescope. Anyhow, after sending my post I mostly worked my way to the gist of your explanation regarding RA and Dec tracking but I really appreciate the detail you provided. In essence, star alignments get the scope there, and polar alignment keeps it there, right? FWIW, I noticed that the direction of the star distortions was generally not in line with the overall drift, which I think is consistent with what you said. You're correct that the mount's periodic error correction function is a record and playback process. I'll play with it next time out, but at the moment I don't a have a cross-hair eyepiece and using the center ring on my camera screen as a reference won't be very exact.

Regarding focus over the 6 hours, to be honest I just got it going and went to bed (other than a groggy 3am battery change in my bathrobe...). At the moment it's still more about working through everything than the data. In any case, the sample images I chose bracket the session to show the overall draft; the first (best) image was taken about two thirds of the way through, the second image near the beginning of the session, and the third towards the end. I don't think I see much of a deterioration in focus, but maybe more experienced eyes.... Also note that at the start the moon was out at a little more than 40% and less than 60 deg**** from my target and then set a couple hours into the session.

It was a calm night and my only cables are the power and handset cords, so probably not much drag to cause the double stars, though I'm curious if the worst, where there's clear separation between them (not shown in my samples), correspond to the 3am battery run.... Until the snow melts, I use the scope on a wooden deck so vibrations can be an issue.

Anyhow Lynn your reply really helps my understanding and is very appreciated!!

Cheers,
Scott

****Side note: Where can I find the angle from the moon for a particular object? I know telescopius lets you filter viewable objects with an minimum angle setting, but if it also gives the angle for an object, I'm not finding it.

Scott, where the scope is pointing in the sky is not a factor for focus.  However, the atmosphere can effect focus.  As the target moves through the sky it can go trough thicker or thinner atmosphere.  Target closer to the horizon are in thicker atmosphere.   This is why imagers, like myself, like to image near zenith if possible.  The largest factor in change of focus is the scope itself.  During warmer temps physical elements of the scope will expand and as temp cool they will contract. Different scope designs can be effected more or less by temp changes.  The smaller the F ratio the more the scope is effected by temp change.  Lets say that over the night a scope contracts 1/10mm (I have no ideal how much a scope contracts).  That's like you moved you focuser 1/10mm.  A F5 scope will have a short light cone and be effected more than a longer F10 light cone.  The different seasons and weather effect this a lot. A warm humid summer night may see little temp change but a Spring/Fall night may see several degrees change.  This is also effected by climate.  The western desserts will see a big difference.  Different focal ratios also have different focus zones.  Your C9.25 at F10 has a long focus Zone ( the focus distance a scope can achieve good focus).  Where the new RASA scope have a very short focus zone and the in-focus distance is very small and critical.  I did  F2 Hyperstar imaging with a C11 for years.

However, where the scope is pointing does effect tracking greatly.  Picture the latitude lines around a globe.  Imagine moving your finger around the equator and covering the entire circle in 30 sec.  Now do it around the most northern latitude.  Notice the difference in speeds needed to cover the entire circle.  Your mount has to do the same thing.  The equatorial design of the mount adjust for this, but it can cause problems with auto guiding.  M81 & 82 that you were imaging is close to the pole and very forgiving of RA/DEC error.  Imagine imaging the pole.  The mount doesn't have to do a thing, It just sets there.  If you image a galaxy in Leo you may fine the mount is having a lot more tracking error and loosing a lot more sub frames.

Periotic error correction is not really necessary unless your mount worm gear has a bad spot/spots that causes sharp erratic jumps in tracking.  If the was the case, you would have seen a periotic lost of every other or every third sub to streaked stars.   If the the periotic error is smooth, it can be auto guided out.  It can be necessary for imager doing long guide exposures (10sec) to combat seeing.

I did unguided 1 min sub frames a couple of years or so.  When I finally move to auto guiding it was a game changer.  A lot of beginning imagers seem to be moving to auto guiding right away.  Based on their experience, that can be a larger learning curve.  Like you, I needed to learn a lot of the basic concepts and programing first.  I also started with mono black&white.

Lynn

A focal reducer will decrease the focal length and therefore shorten the light cone.  Actually, I think that is how a reducer works.  Its optics change the angle of the light cone.  Some reducers will vary the amount of reduction based on distance from reducer to chip.  The Astro Physic 27TVPH I use on my AP130GTX does that.  It is not a field flattener.  There CCDT67 for SCT is the same.  The Celestron 6.3 reducer/flattener is different and the camera chip needs to be at a certain distance from the chip to achieve best flattening.  That's because flatteners actually have a focal length just like a scope.  But the Celestron non-Edge only flattens a small area and not really aliquant for a DSLR.  I prefer the Starizona brand.  I use the older F7.5 and it will flatten an area large enough for a DSLR.  The newer one is f6.3.  Of course more expensive than Celestron's F6.3 , but not much more that there Edge reducer/flatteners.

Lynn

Thanks yet again! My reducer is the Edge 0.7x  and with it the image scale is 0.672 arcsec/pix. Without it, 0.47 arcsec/pix (pixel size of the camera is 5.36 -- http://celestialwonders.com/tools/imageScaleCalc.html)

Cheers,
Scott

The over all drift is not important regarding resolution.  The drift in each particular sub frame is.  The overall drift will result in some of the image becoming cropped after stacking.  Only the area where all sub frames overlap will be shown/usable in the final stacked image.
As for drift in one single sub frame, the signal as not remained centered on a select group of pixels, but as shifted across a wider range of pixels.  The result is the pixels got lest light because the light was spread out.  As in a traditional photo were the camera moved, you end up with a blurred image.  This is called motion blur as opposed to blur caused by optics or focus.  

However it is desirable to NOT have a star/signal on the same pixel in every sub frame.  This is due to pixel bias.  Not all pixels are created equal.  Some are more sensitive than others.  Some or dead/black.  Some or hot/white.  This is increased by shot noise ( the random nature of light hitting the chip due to atmosphere.   Imaging a number of buckets set out in the rain.  Some have big holes in them and all the rain leaks through. Some have small holes and hold most but not all the rain, others or good and catch all the rain and some are covered and catch no rain.  Now the rain is rather random and does not accumulate in all the buckets equally. Light in the same way due to the atmospheric  turbulence.  Imagine that a large sheet of plywood, with holes in it, is put over the buckets with the desire to fill some buckets more that others.  And one of those plywood holes is right over the bucket with a hole in it, and it not gathering any of the rain coming through.  SO, the random quality of the rain and the poor consistency of the buckets render pretty poor signal or a poor measure of the rain fall into particular buckets.  The same kind of things are going on the the way pixels are able to collect light.  So your drift is kind of like the plywood keeps sliding off the buckets.   The smaller the holes and the larger the buckets the less that madders.  That is why large pixels or easier on tracking than smaller ones.

Imagers will combat pixel bias with Dithering. That is a process of deliberately moving the mount/camera a couple of pixels between each sub frame.  That way a light pixel or a dark pixel doesn't always end up gathering the same signal.   The signal is then deposited in a number of different pixels over the session.  Some more sensitive that others.  The signal, NOT THE PIXELS, is then aligned and stacked averaging out the pixel bias.  A stacking program aligns on the stars.  It could care less which pixel the star is centered on.

The irony is, the better tracking the mount, the more likely dark/bright pixels will stack on top of each other.  With your image drift, you got natural dithering.  That will greatly improve background noise.  As your tracking improves, dithering will become more necessary.  You will need Auto Guiding to do that.

Lynn

Also, if in 6 hours my drift was 7.5 arcmin, does that mean my polar alignment was off by something like double that i.e 15 arcmin?

Scott

Assuming I've got it straight, here's my visual….. If polar alignment is off, then the angular distance between the mount's pole and the target is going to be different than it is between the celestial pole and target. This results in two nested but non-concentric orbits** around the two poles that have different radii but share a single point (the point when the telescope was slewed to the target initally). As the object traces its path along the two different orbits, its actual position (celestial orbit) starts to drift away from the position along the mount's orbit and reaches a maximum drift at 12 hours and that difference is equal to the polar misalignment of the mount. The two points then start to drift back towards each other until they're lined up again at 24 hours. So, long story long, the amount of drift at 12 hours is going to be greater than at 6 hours. I'm not sure it's linear difference though (i.e. at 12 hours, double what it was at 6 hours) since we're dealing with curves, but then I'd have to dust off my own very old math skills…..

Don't have a polar scope, but I have a clear view of Polaris from my location and use the finder scope for a rough initial alignment. After star alignments, I then use Celestron's All-Star Polar Alignment process. Next clear night I'll try the drift method too. Is that in addition to, or in place of? Wasn't sure if the mount being synched to the first polar alignment means a second would make things worse, not better….

Regarding drift and dither, it sounded like, lacking proper dithering, drift can be beneficial to some degree as a stand in. So maybe a better way to ask my question is, is that benefit worth any amount of resolution loss?

Enjoying the conversation and lessons! And it'll be a while before I get the image of rain buckets and plywood out of my head….but in a good way!

Cheers,
Scott

**edit: changed ‘concentric orbits’ to ‘nested orbits that are not concentric’