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International Year of Astronomy | American Association of Variable Star Observers (AAVSO)
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International Year of Astronomy | American Association of Variable Star Observers (AAVSO)
Hello!!
Another one question about the sample_data package. I can see a folder name flat-dark. What are these images used for?
I have used the images in the folders dark, flat, image (i created my own bias from an "image" image) and i have the right mag for eps aur. But i wonder about this folder which i did not used.
Regards,
Grigoris
Hi Grigoris,
The "flat-darks" are the dark images that are subtracted from the flats. Typically, the flats are taken at a different time and location than your star images. For example, I take my flats inside by aiming the camera at a blank wall. I also do this after the camera has come to room temperature. I, therefore need a separate set of "darks" taken at that same temperature.
The "standard" calibration procedure in AIP4WIN has steps for entering all these images.
I hope this helps.
Tom
Interesting, in this respect AIP4WIN and IRIS differ: in IRIS , the default workflow suggested by the "Preprocessing" dialog suggests you don't use dark flats at all (assuming that the flats use a rather bright target and therefore very short exposure time. This means that the "dark current" accumulating during the exposure time can be neglected, therefore no need for a flat darks.
BUT IRIS assumes you DO have BIAS frames.
Which makes me think .....I believe we should expand the beginner tutorial to include more info about making bias and flat frames.
CS
Heinz
Yes, IRIS does need a bias (or offset) image to start working with. But it is really very easy (if you don't have bias images) to create a fake one.
You just open an image (a real image that contains real data, ie not a flat or dark image) and at command line you type:
> fill 0
> save offset
(or anything else , its just a name). Then you place this as an offset and you can continue.
(hint by: http://petersastro.50webs.com/tutorials_preprocessing.html )
Regards
Grigoris
Hello !!
Could we have the date of the observations? I am trying IRIS with them but i need to know either the date or a rough estimation of eps aur's mag so as to determine the procedure.
Regards,
Grigoris
Hello Grigoris,
The images were taken on 26 Jan 2010 at 0115 UTC.
Tom
So by comparison with datat submitted for around that day the V mag should be ... see Teaser.
Which is exactly what I got with Brian's Excel Sheet and following the IRIS tutorial as it is now.
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3.70
That's great!! Only that ... I cannot reach to that conclusion ..yet!
By performing the whole procedure at the tutorial I end up with an image in green. Since you say that the procedure is correct then I should be more or less correct so far and then I do something wrong in the photometry (now..how can I do this ??).
In the spreadsheets you day that we put the instrumental values...What are these values in the case of IRIS ? I tried the ones that gives me but are negative and the whole calculation is not performing well. On the other hand when I try to do it manually (to give an mag constant so as to have for example eta Aurigae = 3.172) I take 3.772 for eps without color correction ! If I do color correction then I go to ~ 3.88.
Where could I be wrong ??
Regards,
Grigoris
Hi!
>By performing the whole procedure at the tutorial I end up with an image in green.
jJust to make sure: what you should get by following the tutorial is a B&W image representing the green channel (named "final-g" in the tutorial). If you get a picture that actually looks green, that's already the problem.
For the photometry: negative values should work just as well as positive values, if not, that would be a bug in the Excel sheet. I really would not care about this magnitude constant in IRIS, leave it at 0.0 .
Maybe you can post the instrument magnitudes for (say) eps, eta and zeta Aur that you get, so I can later compare them with the ones I got at home.
>Where could I be wrong ??
It's color correction that seems to be wrong: Your numbers indicate you used a TC of about -0.2, which seems to be wrong. IIRC I got something like 0.12 or 0.13. Note the sign for the color correction is wrong I guess, so when using 0.12 as TC (==> correcting in the other direction) you should be about fine, so I suggest you check the color correction part of your test.
But I suspect your manual procedure is also not entirely correct: If I understood correctly, you are setting the IRIS magnitude constant so that eta Aur comes out with instrument mag = cat mag = 3.172 .
This is wrong IMHO, because it neglects that you'd also have to do a color correction for eta Aur! The instrument mag for eta Aur should be such that it will be cat_mag + TC * (B-V) _eta.
Or am I missing something?
One other minor thing that comes to mind is aperture size: When using cfa-to-rgb conversion, the green image will end up having the same size as the original exposure, and from what I remember the defocused star images will not fit well into the inner aperture circle with default settings, you have to ~double it IIRC.
CS
Heinz
Hello Heinz!
Well this post should go to three threads (this, simplified reduction, iris tutorial), but i think there is no need to post it so many times...
" >By performing the whole procedure at the tutorial I end up with an image in green.
>> jJust to make sure: what you should get by following the tutorial is a B&W image representing the green channel (named "final-g" in the tutorial). If you get a picture that actually looks green, that's already the problem. "
Ok, I may confused you by writting this.What I meant was that I end up with an image containing only the green channel, not that it is green (sorry about that!).
My final image is 16bits, B&W , coming from the RGB separation. I did one more time the image processing and I find these values
rho -11,517
lam -11,993
mu -11,889
eta -13,577
zeta -12,851
eps -12,980
So I went on to put these values to the various spreadsheets. On your version I end up with the value (guess what?) 3.70 !!! hooray! [ m = 0.6 , b = -16,73, without using zeta] But in the last version (Reduction Beginner) I can calculate the same m , b but i do not get a result because of some errors. I don't put values for all the stars which seems to be a problem how to transfer the values of m and b for the final calculation of epsilon aurigae (take a look in this!).
So from what I can say up to now... it works!!! (I will look it again in case something is wrong...).
Note: I tried also the final output of processing to split it as split_CFA r g g b (we dont really care about r,b pixels) so I end up again with a "green" image (i mean green channel only). I made aperture photometry also there and i end up with something different ( a negative m) which is not quite right! So I thnik that your proposal to use RGB split works better for sure.
Best wishes,
Grigoris
There is a small change to the spreadsheet that I just uploaded that should fix the no-result result. Basically we switched from using linest() to slope() and intercept() to get the necessary information. Could you download the new beginner sheet and retry?
Brian
Brian,
Where is the new spreadsheet posted?
Tom
It's a replacement for the file in the beginner tutorial (you might have to clear your cache to get the new copy):
http://www.citizensky.org/content/calibration-beginner
Brian
Hi Brian,
I think this needs some additional tweaking.
Even tho slope() and intersect() will no longer produce errors, it is kind of error prone that you have to explicitly delete cells in E16-E21 if you don't have values for the corresponding Instrument magnitude cell.
It would be better if the cell clears itself, see my "demo version" in the thread where the xls was initially uploaded.
Oh, and eta and zeta Aur would be nice :-)
CS
Heinz-Bernd
It seems like I'm making more posts with "whoops" in them than before... I should have blanked-out the data before posting them to the CS website. I'll have to fix that tonight.
Brian
Excellent! I'll move the IRIS tutorial over today/this evening (scheduling around the IRTF observing time and a nap of course).
Ah, I knew I would forget something! I would have thought this would have been encoded inside the file... Tom, do you recall the date?
Hi Brian,
In case anybody is interested, here is the complete info on the sample images:
Camera: Canon 20D mounted on a photo tripod (no clock drive).
Lens: 70-300mm zoom lens set at 70mm and placed slightly out of focus by manually moving focus adjustment against the infinity stop.
Exposure: 5 sec, F/4.0, ISO 800
Date/Time: 26 Jan 2010 / 0115 UTC
Tom
I'll add that content into the readme file... once I can recall how to remove/add files from a zip file via. command line on a remote server!