I took 64 new science images of eps Cep last night - these look much better than the previous ones, with little blurring or streakiness and no saturation in the target star, and ASTAP easily solved all of them. My problem is getting them aligned in AIJ. As before, aligning with manually selected stars fails after a number of plates that varies depending on which stars are selected (typically no more than 20). Aligning using WCS headers succeeds... but the result is imperfect, with a small but visible jump between plates 45 and 46. The displayed RA and DEC of at least the target star do not differ appreciably between these two plates. Why might this happen? Can anything be done about it?
Going through the original plates, it's clear that I must have accidentally moved the camera slightly between those two exposures, as there is a very noticeable jump going from 45 to 46 in the originals. But I thought that with solved plates, AIJ should have been able to align them even so.
ASTAP can calibrate and align the images without stacking then. It will yield a set of images with the ending ".aligned.fit". Perhaps you can feed those into AIJ already aligned.
Unfortunately this doesn't work - all I get is a series of errors "Error decoding site longitude or latitude!"... which is not surprising as that information would have to be encoded somewhere in the light files (I assume in the FITS headers) and it's certainly not - the camera I'm using doesn't have a GPS nor is there any way to set current latitude and longitude to encode it into the RAW image file. I know the site coordinates but have no idea how or where to put them into the FITS header.
This time I let it continue, and it turns out the errors were "benign" or at least non-fatal. But the result was a single file with a filename beginning with "no object", not a set of images. I assume I did something wrong. I used "astrometric alignment", perhaps I should have used a different alignment method?
The documentation recommends "Star alignment", but this yields the same thing, a single file (apparently a stack) not a series of images.
Not sure how to align without stacking, and the documentation gives me no clue...
Sorry, I wasn't clear. You need to go into ASTAP's "Stack Method" tab and chose "Calibration and alignment only" as the stack method. With this setting you will end up with a set of aligned images and not a single stacked one.
Thanks, I just found that by pulling down the Stack Method menu. The result is *perfect* as far as alignment goes, but the calibration seems to be doing very strange things to the images, causing the counts to vary wildly from pixel to pixel, and introduces a large count in the background (~2000), especially of images beyond the 45th. Not sure why this is, or if it is normal.
(In the process it also emitted a warning for each image that "Alignment will ruin the Bayer pattern!!")
Would be nice if it could just align *without* calibration, as I think I'd rather led AIJ do the calibration.
Not sure what is happening but ASTAP's calibration has been pretty well tested by use. Maybe there is something funny about it when doing it in batch as that might not be the common case.
Are you working with color images (OSC)? The message about the Bayer Pattern seems it suggest that ASTAP thinks these are colored images.
I've sort of forgotten what your original problem is, but if you are doing photometry and these are color, the recommendation is to separate the channels and do photometry on the green channel only (to match visual). There are commands in the "Batch Processing" submenu for separating channels.
If you really want to work with the full color image, you might ask on the ASTAP forum to see what this message means.
Yes, they are color - I'm not sure it's even possible to take grayscale images with this camera. Is the channel separation in ASTAP the correct algorithm for a DSLR with its two green channels? The AAVSO DSLR manual says that this is best done by extracting both green channels and averaging them, but the ASTAP documentation doesn't explain what "Tools -> Batch processing -> Raw colour separation -> Extract raw GREEN pixels and combine" actually does.
Also, does this need to be done with the calibration frames as well?
Yes, my understanding is that they are averaged. Feel free to confirm this on the ASTAP forum. Han knows what he is doing and this feature exists just for this purpose. You do need to make sure you are choosing the correct bayer pattern for this to work correctly.
I worked with the AIJ folks to add this feature to AIJ as well. Han helped them implement it correctly. If you want, you can do the separation in AIJ and ASTAP and compare the results.
Thanks, I will trust that answer... but still, must this be done for the calibration frames also? Or do the calibration algorithms know to only subtract channel by channel and do nothing with light frame channels that aren't present (i.e. R, B)?
My understanding is that you need to separate the green channels on the calibration images (darks, flats and bias) as well before using them in the process. I think you would have to do the same in AIJ.
I’m somewhat new to photometry but I did start out using a DSLR and my AAVSO mentor said that is how you have to handle color images.
Thanks, that is what I'll do then!
Just as a FYI: even with the images containing only the green channels, ASTAP warned about alignment ruining the Bayer pattern. But I was able to do aperture photometry on the resulting images and could even (though just barely, as there is considerable scatter) detect the variability of eps Cep.