Mystified by TILT keyword value
Stacking program, astrometric (plate) solver, fits file viewer
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han59
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On further inspection, forget the feature request, it's not clear at all to me how the value for this keyword is being determined (see attached screenshot). It'd also be nice if you included units (e.g. pixels or arcseconds) in the keyword comment space. Thanks, Jesse
3.92, yes the is weird. I will have a look if i can reproduce it.
The unit of 8.62-7.90= 0.72 is pixels. But it will probably confusing to report as pixels. Delta HFD is a side effect of tilt.
I ran the ASTAP batch process on eight files generated via ACP with PinPoint WCS information already in the header. None of the TILT keywords matched what ASTAP window displayed. The first six files differed in the hundredths and the last two started to differ significantly. The last one was in the screenshot, and I put it on Dropbox
ASTAP start to pick hotpixels. A single pixel is ignored but they tend to cluster a little. To fix that set in tab alignment (ctrl+A) the minimum HFD to 3. Then it should ignore hot pixels and produce more consistent values. See screenshot.
In your case you can also influence the result by selecting extra stars in viewer menu "Image inspection F5". The star are pretty spread out and the majority are faint stars where some flux at the foot of the star is lost by the detection. So faint stars tend to get a lower HFD value. Best is to keep the extra stars option unchecked. Then only bright stars are measured giving a more consistent values.
Does this work for you?
Han
I had an other look to your image. The strange thing is unequal HFD values in the image. For bright stars the HFD is pretty stable at around 8.5 but fainter it drops more then normal.
My first idea was your system is suffering from infrared light cause out of focus effects but your using a Johnson-V filter and excellent equipment. The hot pixels are strange since the sensor temp is minus 20 degrees Celsius. The sensor Kodak KAF-16803 should be good. So I can't explain it unless the image is stretched by Pix insight?
Han
The main issue I'm having is that the TILT keyword in the header doesn't match what shows in the ASTAP -> Image inspection -> Tilt window when the settings ASTAP haven't been touched (viz. they're the same). I'm not clear that this has been resolved.
I'm a little confused that your more recent post states:
But your previous post seems to explain this strangeness:
Which works for me.
Regarding your last post, the example FITS was an uncorrected light from an observatory I'm peripherally associated with. The KAF-16803 CCD is somewhat dated and is clearly showing it's age. I'm a partner in an observatory with a KAF-16803 that has similar 'warm' pixel issues not to mention some bad columns. I've been lobbying to replace both (although I'd have to chip in $$ for ours :)
Thanks,
Jesse
I did check it here again. The difference is in the option extra stars. The pulldown menu "Image inspection F5" allows an option extra stars but it will also be used in the batch process.
The option extra stars will include fainter stars. Since the HFD is a little smaller for faint stars it will cause a little shift in the values.
I have just added a small remark to the TILT keyword comments indicating the extra stars option is used or not. SNR>10 is extra stars
The strangeness for me it the large differences between the HFD of bright and faint stars. I think is is just because the stars in pixels are pretty large. The HFD expressed in arcseconds is 6.7" which is large but I assume the long focal length & opening makes it more sensitive to seeing.
Han
I've put another image on Dropbox from a different telescope that includes the TILT keyword inserted by ASTAP. I was unable to duplicate this keyword value in Image Inspection regardless of option settings. The keyword value is 0.57. Octagon with extra stars is 0.32 and Octagon without extra stars is 0.61. I can't match switching to Triangle and changing the Ignore stars with HFD seemed to have no effect.
What settings were used to obtain the 0.57 value?
Thanks,
I checked here with the latest ASTAP version from yesterday. I get also 0.61 for normal and 0.32 for extra stars.
Maybe this was measured with an older ASTAP? Send me the astap.cfg (config ) file so I can have a look. In ASTAP go to viewer menu FILE, SETTINGS, SAVE SETTINGS AS... and save the settings to the desktop or somewhere else and send the astap.cfg by mail to me.
Cheers, Han
Sorry to get back late. I just now downloaded 12/05 (I'd been using 11/30?) ran on another set of files and get matches with the GUI and keyword. Thanks! Jesse