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Messages - james7

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General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 March 20 02:40:12 »
Mike, I'm using an APS-C camera with a Bayer pattern so I can't really compare my resolution directly to yours, but I typically image at anywhere from 1.5"/px to 2.4"/px (mostly 1.5 to 2.0"/px) and I often have a FWHM between 4 and 5 arc seconds. However, my eccentricity values are seldom better than 0.5 which is something that I'm trying to improve. I don't think my non-round stars are solely related to guiding/tracking, because even when I do unguided subs in the one second to ten second range it's difficult for me to crack 0.45 even in the best parts of my field. This happens with three different scopes where the only commonality is the camera (I have two Celestron GEMs, an AVX and a CGEM). Frankly, I'm beginning to question my technique (or lack thereof) and that's why I was curious about what other PixInsight users were measuring.

General / Re: Star Alignment Summary?
« on: 2014 March 19 23:36:13 »
I've already got access to the log, it's just that I was hoping that someone might have already written a script or tool to parse the log and output either a summary or better yet a plot of the registration offsets. The data that would be useful would be a time stamp of when the image was captured, the exposure, and then the dx and dy offsets, rotation, and possibly the RMS and peak errors. Maybe two sets of files in a comma separated list, one very simple -- dx, dy, rotation, timestamp -- and the other with all of the above and a summary (mean dx and dy, mean rotation, range of dx, dy, and rotation, count of subframes, time range over which the subframes were captured). Of course a tool that would provide a nice summary and maybe plot the dx, dy values would be really great.

I'm looking to see if anything already exists before I consider writing something of my own.

General / Star Alignment Summary?
« on: 2014 March 19 22:09:58 »
Is there any tool or script that can be run in PixInsight that will perform a star alignment on a stack of subframes and output a summary of the registration offsets? It would be great if there was a way to plot the +/- offsets for all of the subframes with a statistical summary. In fact, it would be nice just to get a comma separated list of the values that could then be processed in a spreadsheet or with some other tool. Having such a tool in PixInsight would make it possible to analyze your guiding or even measure things like periodic error. The data is being written to the console, there just needs to be a way to log the results in a convenient format so that it can be processed.

General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 March 19 21:42:54 »
If you are interested, I have a new Takahashi 180ED that I have been collimating.  The data are out here:

I am going to get some more as soon as the clouds clear here.
Jerry, did you finish your work on the collimation? I note that the link you provided showed eccentricity values around 0.5 which is above (i.e. worse) that the 0.42 that I've seen recommended for round-looking stars. It's very rare that I get much below 0.5 with any of my scopes (all fairly fast refractors). I've never been able to determine whether that last little bit to get to 0.42 and below has to happen through guiding or better optics (or focus, etc.).

Since we're now imaging at 5 micron pixels and below I'm beginning to wonder whether it is even possible to routinely achieve eccentricity values at or below 0.42 (uniformly across a wide field). Geoff posted some really good numbers, but I suspect that is for a fairly narrow field of view (i.e. probably not much more than one degree on a side).

General / Re: PI Getting really slow start
« on: 2014 March 05 13:59:34 »
Have you tried to clear or purge the file cache that is maintained by ImageIntegration? I'm not certain whether that would affect what you are seeing but this cache does have a significant impact on the performance of PixInsight. It does speed up repeat integrations but in my hands in can also slow down some operations in PixInsight.

The preference to clear or enable this cache is in the ImageIntegration module, under the arrow/pointer icon at the bottom of the ImageIntegration panel.

General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 28 02:32:46 »
Okay, then your median FWHM seems to be about 6.6 arc seconds and your (uncropped) field coverage is about 3 degrees 11 minutes by 2 degrees 8 minutes.

Announcements / Re: New Website
« on: 2014 February 28 02:14:25 »
...The problem is Safari. The latest versions seem quite buggy. Since we don't have any Apple mobile device, it is very hard for us to look for a solution. I strongly recommend Mozilla Firefox or Google Chrome. Both work without any issue.
I don't think the new website works with any iOS devices, which I think comprise the majority of all mobile devices. Safari has bugs, no doubt, but it may be one of the most standards compliant web browsers that exists (when excluding Flash, which I trust you are not attempting to use).

General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 28 01:24:59 »
Phil thanks. What kind of camera (DSLR or dedicated CCD) and what is your image scale in arc secs per pixel? Based upon your eccentricity values it looks like the TRF2008 works fairly well on the Skywatcher (although when not knowing the size of your sensor I can't calculate your field of coverage, but it looks fairly wide).

General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 27 14:51:22 »
Geoff, given your telescope's aperture it seems like your FWHM is probably limited only by your seeing conditions while with my AT72ED it's probably more of a limit from my scope (i.e. at 72mm of aperture the Airy disk is about 3.6 arc seconds to begin with and I'm getting a FWHM that is about one arc second larger than that). At prime focus and with my chosen camera I'm also working no where near the critical sampling that would be required to see the full resolving power of my scope (but I'd think that the same would be true for you since you're working with a sub-arc second Airy disk).

Thanks for your notes. Would it be possible for you to post one of the contour maps showing your performance. It would be interesting just to see something that was that good.

General / Re: FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 27 13:36:25 »
rob, thanks for the feedback. If your median eccentricity is running around 0.54 what do you see on your contour plots near the center of the field? I recently did some untracked and very short exposures (less than one second, ranging down to 1/30 second) and while I still have some analysis to complete it looks like those shots still have eccentricity values above the 0.42 limit that is supposed to indicate "round" stars. It could be astigmatism or some other optical trail of the telescope(s) or it might even be the camera (non-square pixels) but I'm beginning to wonder whether eccentricity values below 0.42 are even possible with my equipment.

Short, untracked exposures are kind of interesting because you can calculate the precise angular movement of the star based upon its declination. That then becomes the "error" and you can evaluate the images to see what is detectable (visually) and how that is measured by PixInsight. Given my camera's pixel size my general rule of thumb for untracked exposures is to use shutter speeds that are equal to or less than 100 / focal length in mm. Thus, for a 430mm focal length you get 100/430 or about 1/4 second. When I look at the images I find that to be a very stringent limit as I can usually go to twice that before I begin to see obvious elongation in the stars.

General / FWHM and Eccentricity, What Are Typical Values?
« on: 2014 February 26 18:08:19 »
I've been using the SubframeSelector and FWHMEccentricity scripts to sort my images and I'm wondering what other users are seeing when they run these scripts. It's all very well to select frames based upon the capabilities of your own equipment and technique, but I'm curious about what values other users are getting. To make any comparison at all we'll need to know what your capture scale is (arc seconds per pixel), whether you are using a monochrome or full-color sensor (assumed Bayer pattern), and the aperture, field coverage (focal length and sensor size), and type of your telescope. Reports should also include the stage in image development where the images were measured. Typically, that might be immediately after calibration and debayering (if required), but before stretching or any other digital development.

My greatest concern (or interest) is in eccentricity values as I very seldom see results that are better than the 0.42 limit that is supposed to indicate round, undistorted stars. My FWHM values seem reasonable, but my typical median eccentricity values are always above 0.45 (values in the 0.5 to 0.6 range being quite common). When viewed visually at 1:1 scale, the center-field star shapes run from slightly elliptical to what I'd call very close to perfectly round. The odd thing is that even on very short exposures (say, one second) my center-field eccentricity values still don't reach that 0.42 limit. I recognize that some of the poorer median values may be caused by field curvature and edge aberrations, but I struggle to get values below 0.5 even in the center of my field (on multiple telescopes, not just from one sample).

As an example, here is the summary from my Astro-Tech AT72ED (a non-flat-field, 72mm aperture, f/6 doublet ED telescope mated to an APS-C Sony NEX-5N camera). The capture scale is 2.3 arc seconds per pixel with a 4.8 micron pixel size and the diagonal field of coverage was 3.7 degrees. This data shows a lot of field curvature (as would be expected), but even in the center of the field the eccentricity is between 0.6 and 0.55. Over this same center the FWHM ranges from 4 to 6 arc seconds. Here is the data from the full frame (includes the "bad" edges):

FWHM 4.76 arc seconds
Eccentricity 0.725
StarSupport 4168
FWHMMeanDev 1.54
EccentricityMeanDev 0.121

The sample was taken from a stack of ten images that were exposed for 10 seconds each, there was no image calibration, the ten images were debayered (VNG), registered, and integrated with no normalization or pixel rejection. The measurements were then taken on the linear image using the SubframeSelector script (with only one image, the stacked result as input).

Is anyone willing to put forth there own measurements or add some comments on their own use of these scripts.

General / Re: Experience with IOptron?
« on: 2014 February 17 21:14:34 »
just saw a guy on flickr that appears to have a bad oneā€¦ or is this v1 vs v2?

That was me and it was version 2 of the iOptron SkyTracker. I ended up returning the unit since with about 90 arc seconds of periodic error it was only useful for lenses below 100mm (unless you wanted to reject the majority of you subs). With a 50mm lens (APS-C) you'd probably be able to do some captures between 30 seconds and one minute, but for anything longer you'd be limited to wide-angle lenses in the range of 18 to 35mm. Basically, I found that it wasn't capable of doing DSO work. It would be okay for wide-angle captures of constellations and such, but not much else.

I actually tried two different units, the first one had less periodic error (but still not great) but it stopped working after one day. The second one I used on several different nights and the tracking performance was consistently bad. If you search on the internet you'll find several test reports that all show periodic errors in the 100 arc second range (+/- 50 arcs), so it seems that a good number of these are really that bad. However, there are a few users that have shown results that suggest that some of the SkyTrackers can be used with lenses over 100mm.

The polar scope is really nice and mechanically the entire unit seems sufficiently sturdy, but there seems to be a pretty wide range in the quality of the tracking.

Bug Reports / Re: SubframeSelector
« on: 2014 January 02 21:59:11 »
I also see this under Mac OS X (can't resize panel in the horizontal). Otherwise, SubframeSelector seems to work but it would be nice if you could view a larger part of the tables and plots without scrolling. I hope progress can be made on this issue before the next major upgrade to PixInsight and thanks to Juan and company for looking into this problem.

Yes, I had this same problem under Mac OS X 10.7.5 but the latest release (PI-macosx10.6-x86_64- seems to have fixed the crash. Haven't had much of a chance to use this release as yet, but so far things seem to be working. Thanks to Juan and crew for the quick fix for this rather serious issue.

General / Re: Help With My Image
« on: 2013 December 09 14:29:53 »
Also, before you do the DBE make certain that you've cropped out any areas around the edges of the image that contain incomplete data (usually caused by registration shifts between the subframes in your stack). On my images these generally show up as darker or multicolored, straight boundaries near to the edges of the master integration.

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