Monthly Archives: April 2015

PEC, Autoguiding stars, and a test image of M81/82.

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It was clear last night, quite warm, although seeing and transparency were poor. More testing and calibration, and some real progress was achieved.

Better PEC

Now that I have focus perfect, and given my suspicion that I may be doing unguided imaging with this short refractor, I decided to re-do my PEC calibration. This time I used the main camera to get cooling and higher resolution. I captured about six worm cycles of data and recalculated the model.

Gathering data like this can be done before it is completely dark, so it was a good way to spend the hour of dusk while waiting for the sky to become completely dark. In the process, I noted two important things:

  1. Somehow, my TPoint model is off, even though it was near-perfect the other night. I needed to use a closed-loop slew to get to what should have been easy targets. I imagine I bumped something the other night while I was disconnecting and reconnecting the guide camera multiple times. Next testing evening, I will first try doing a re-synchronization and then, if necessary, a completely new TPoint model.
  2. SV80S-Field-CurvatureAlso, in the process of doing full-frame images with the large QSI camera, I can clearly see the field curvature. The extreme edges of the field are badly distorted, confirming this as the likely cause of the distorted stars I’m seeing in the guide camera, which is even farther from the center of the field.

So, I think I’m not going to worry about correcting those guide camera stars now, but, rather, see what I can do with unguided imaging. After all, this is a very stable mount, and I’m presently using a very short focal-length imaging scope.

Testing Unguided Imaging Limits

So, I slewed to a nice test image – a spot midway between M81 and M82 – and took test images at 10, 60, 100, 200, 300, 400, 500, and 600 seconds, with PEC on but no guiding. Those images, calibrated, are here:

Ignore the badly distorted stars at the upper left edge of the field, and look at the center. While I think I may have a minor collimation issue with the camera, I can see no difference between the stars in the 10-second image and those in the 10-minute image. With this mount and scope, I clearly don’t need Autoguiding for exposures in the 5-10 minute range. So, I’m going to shelf my calibration of the Autoguider until later in the season when I switch back to a longer focal-length imaging scope.

It also occurs to me that I have a field flattener for short scopes in a drawer somewhere.  If I come back to serious imaging with this scope later in the season (which I might want to do for a very wide field shot, such as M31) I’ll try that flattener.

Now, since I have a nice image framed anyway, I thought I would do some trial imaging of that M81 – M82 shot. I took 10 five-minute sub-exposures, unguided, and then a series of flat fields using the light panel mounted in the observatory. I already have a calibrated dark frame for use with this camera.

There sure are a lot of satellites in that part of the sky.  Lots of satellite trails to be removed.  Fortunately, the data rejection procedures in CCDStack make that fairly easy.

2015-04-17-M81-82-monoHere is the result, calibrated, combined, and cropped. (Still just a test – not a finished product, which would need better noise removal, scaling, etc.)

I would not have imagined I could get results like this unguided. This truly is a wonderful mount.

Static Electricity

A final note.  I’ve had several major static electricity shocks over the last couple of days, and I don’t remember this being a big issue in the past.  Twice tonight, a piece of equipment actually reset after a static spark.

What’s changed?  I think the answer is the new vinyl cover on the dome – I used to have a woven polyethylene cover, but switched to this vinyl one last year.  Maybe dragging the vinyl cover off the plastic dome is generating the charge.  Running a ground wire to the dome is now on the to-do list.

Hmm. Guide Collimation doesn’t look like the problem.

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Another clear night – that’s 3 in a row, possibly the only time I’ve had such a chance in a decade.

I spent it investigating my guider collimation problem.

Badly distorted, elongated stars in Lodestar guider in WSG guider port.Imaging with an SV80S (480mm f/6 triplet refractor) I’m getting very elongated stars in my guide camera, seeing through the off-axis guider port on the QSI583WSG camera. So elongated that I doubt guiding will be accurate. And considerably worse than I remember seeing a couple of years ago. What’s going on?

My assumption was that the pick-off prism or the guider camera mount was somehow out of collimation – that it wasn’t orthogonal to the incoming light path. That would seem consistent – the distortion in the guider stars is exactly vertical, as though the prism was out of alignment in the vertical direction, but fine horizontally.

However, before that it occurred to me that the entire camera/guider system might be out of alignment. Better check that first.

Using CCDInspector, I discovered a minor out-of-plane error in the main camera, which I was able to correct by snugging it better into the focuser drawtube.

Main camera field well centred, no collimation errorThen, a re-check shows a field with no skewing off orthogonal, although with significant field curvature (back to that in a moment). So, it’s not the overall alignment of the system.

Guide-Camera-CurvatureNext, I did a field analysis of an image from the guide camera, which confirms what I could see with my own eyes – there is major distortion, almost exactly vertical, toward the bottom of the image field.

Now we come to the problem. I think I need to tweak the alignment of my off-axis guider port’s “pick-off” prism. However, it doesn’t seem to have any such collimation adjustments. It’s solidly machined, as is the threaded guide camera mounting port. It’s not supposed to be out of alignment, and there doesn’t seem to be any way to adjust it if it is.

By now I have run out of evening time and patience, and I have to get up for work tomorrow, so I’m stopping, stumped, for now.

On the weekend I will use daylight to take the camera off and apart and look more closely for some way to adjust or shim the pick-off prism or guide camera mount.

My fall-back plan if I can’t adjust that will be not to worry about it. I’m pretty sure the problem is greatly exaggerated by the rather tight field curvature in this scope – evident in the curvature map shown above. The pick-off prism is very close to the edge of the field, where curvature is strongest. Maybe that’s why I don’t remember having this problem before – because I was using a different scope, with a famously flat field.

With this short focal-length scope I should be able to take 5- to 10- minute images unguided and, if guiding is needed, at this short focal length a separate guide scope should work well.

So, I’m going to have a look at the mechanics on the weekend. If I can find a way to adjust the prism alignment, I will try that. Otherwise I’m going to declare “don’t care” and use this short scope without the off-axis guiding. Its season is nearly over anyway – I’ll be returning to a longer focal-length, and flatter-field, scope for the summer. If I have a guide-star problem with that scope I’ll pick up this problem again, but I’m guessing I won’t.

So, assuming I don’t find an adjustment, next clear night will be spent refining my PEC training now that I have perfect focus, and then I’ll do some tests of unguided exposure length to see what the Paramount can do at a short focal length. I expect to be impressed.

Upgraded to FocusMax v4, Guider Collimation

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Since switching to TheSkyX as my primary control application, I have tried its built-in @focus2 autofocus routine a couple of times, but I missed FocusMax, which I used to use and really liked. Last night it was clear enough for focus testing, and I reinstalled FocusMax.

To my surprise, the software has been acquired by CCDWare, and is now a commercial offering. It’s no longer free, but I have found CCDWare’s other products to be excellent and well-supported, and I don’t mind paying a bit for reliable, low-hassle software.

focusmax-vcurve-5As hoped, the commercial V4 is a major improvement. The original free package worked well when it worked at all, but it was always a challenge to install, integrate with various versions of the OS, configure drivers, etc. With the new version, after figuring out the initially complex user interface, it just worked, no hassles. Using the convenient “first light wizard” I did several V-curve calibration runs, and it’s giving me great focus easily.

focusmax-focus-5

Next problem.

Now that I have excellent focus it’s clear that I have a Collimation problem with my guide camera. When I get pinpoint stars off the main camera, an image through the attached guider is giving very elongated stars. Calling them “ovals” would be an undeserved compliment – they are long streaks. I assume this indicates that the pick-off mirror in the QSI camera’s WSG guide port is not properly collimated to be exactly 90 degrees to the light path.

While stars don’t have to be pinpoint for good auto guiding, since the software calculates the centre of each circle, I am pretty sure they should be round, or at least ovals. It must challenge the software to guide on little line segments. Next chance, that will be my next maintenance project.

Finished Polar Alignment, First PEC Training

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We finally had two clear nights of excellent conditions this past weekend. The sky was clear, the temperature was above freezing for the first time in months, there was no moon, and, being very early spring, there were no bugs. I have been waiting several months for an opportunity like this to finish installation and calibration of the mount.

2015-04-12-Polar-ResultsFirst, I finished refining the polar alignment. This ended up taking four passes, when I thought only two or three would be needed. After the first polar alignment modeling session over a month ago, substantial polar error remained and a rather large correction was recommended by the TPoint software. Last night, I applied that large correction and ran a new modeling run. I didn’t expect the results to be perfect, just closer, and, sure enough, TPoint reported substantial improvement to the polar alignment but with a minor correction still needed. I must have misread the decimal point in the correction instructions because, on pass three, the polar alignment was still off by about the same amount and another minor correction was recommended. I made that correction, paying more attention to getting the decimal point in the right place, then ran a final TPoint model just to check the alignment.

The results were very good. TPoint is reporting my polar alignment to be off by 1/2 arcminute in azimuth and 3/4 arcminutes in altitude. That is certainly good enough for now.

2015-04-12-pec-captureNext, I wanted to do an initial, first cut, of the mount’s periodic error correction. I collected about four worm rotations’ worth of data using the autoguider camera. Even without PEC turned on, the Paramount’s impressive performance showed up. Uncorrected, I recorded a periodic error of ± 1.1 pixel, or about ± 3.8 arcseconds. That is better than the best I ever achieved, with PEC turned on, on my Losmandy G11 amount.

2015-04-12-pec-curveThis generated a periodic error correction curve to be uploaded to the mount which applies a correction of ± 0.4 arcseconds.

2015-04-12-corrected-computedI verified this by doing another PEC data capture, this time with PEC turned on. Sure enough, the corrected PEC is now showing at about ± 0.5 arc seconds. That is, frankly, incredible; especially since I’m not particularly impressed with my PE data capture yet. Seeing was quite poor tonight, probably in the range of 3-4 arcseconds, so my PE data capture was probably seeing the target star scintillating due to bad seeing more than it was seeing mount drift.

2015-04-12-LeoTriplet-2x2-600-singleJust for fun, and as a more practical test, I decided to end the evening with a 10-minute guided exposure of the Leo triplet. I used five-second guide camera exposure intervals, with a low guiding aggressiveness to avoid “chasing seeing”. Although I forgot to take a screenshot of the guiding graph, guiding during this 10-minute exposure showed that it was making very small corrections, oscillating around a 1.5 arc second error. The single resulting image is shown here, without dark or flat fields. Nice pinpoint stars.

 Update: PE worse than I thought

Updated added later:  I have realized I made an error in recording the PE data above – I didn’t have the camera aligned “top-north”, so some of the error was masked as Dec motion.  Error is worse than recorded – in fact, it seems out-of-spec at about +/- 9 arc-seconds uncorrected.  Other posts will record the story of tracking this down and correcting it.