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Help with tutorial

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ptsouth97's picture
ptsouth97
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Joined: 07/17/2011
Posts: 6

Hello,

I finished the IRIS beginner tutorial, but my results seem to be off.  I feel pretty confident about steps 1-5, but I may have made a mistake in step 6, photometry.  In Step 6 a, the tutorial mentions configuring the magnitude constant, but I do not see where this is explained.  As a result, I left the magnitude constant at 0.0.  Maybe this is throwing off my results?

Here's what I got for Instrumental Magnitudes for the Comparison Stars:
Iam Aur -10.747
rho Aur -10.339
mu Aur -10.671
ome Aur -10.616
sig Aur -10.364
58 Per -11.190

I only took these values from one of the final separated green channel files.  Is that right?

m = 0.111
b= -15.555

Average Check V Mag = 4.743, which is more than +/-0.01 from the 4.705 V Cat

Finally, my average V Mag for the target star was 3.741

These values were a result of the average from all 3 of the separated green channels.  Again, is that correct?

Another thought I had was that I may have made a mistake when getting the instrumental magnitudes because I had difficulty centering the crosshair right in the middle of the stars.  How sensitive is that part?

Thanks for your help,
Blake

Roger Pieri's picture
Roger Pieri
User offline. Last seen 35 weeks 3 days ago. Offline
Joined: 08/29/2009
Posts: 136

Hi Blake,

This version of the tutorial has been somewhat improved but not at 100%. A new version should be on line soon. There is a remaining confusion at level of RGB separation.
In fact there are two options:

 - stacking the images from the series (an addition of all), resulting in ONE IMAGE, separate R,G and B from that image, make the photmetry on that SINGLE resulting G image. Send the results to the speadsheet.

Command: RGB Separation...  

Or,

 - not register, nor stack at all, but directly go to RGB separation of the sequence, you get R,G and B images FOR EACH OF THE IMAGES OF THE SERIES. Next you make the photometry of EACH G IMAGE, all useful stars. Then you should calculate the Average and the Standard Deviation (SD) of the results for each star. Next go to the spreadsheet using that averaged results.

Command: Sequence RGB Separation...

The endresult is the same for both methods. The first needs less work if you have many images, the second enables to calculate the SD that's very useful for assessing the process and improve it, you can also locate images showing issues and eliminate them.

One good way is to mix both: processing (stack) images by group (let say 5 groups of 5 images of 10 seconds each)

The Magnitude Constant is only an option that enables you to see an "approxinate" result before having applied the speadsheet calculation. The XLS works equaly well with or without it.

Magnitude Constant = Catalog Magnitude - (signed) Instrumental Magnitude

Photometry Apertures: The new version of the tutorial develops more that point. The "inner circle" (where the photo-electron count from the star is made) should be choosen large enough to get all the signal from the star. You should adjust the "Threshold" level and the Black Level (upper and lower sliders of the Threshold box) upto the point you well see the background and perceive the noise texture. Then clipping the star image strongly enables you to see the lowest "foot" of the star signal, its "footprint". The inner circle shall be large enough to include all that signal. If you do so the centering of the circle on the star will be much less sensitive and you will be sure to measure all the flux coming from the star. You should determine this on the star having the largest "footprint" of the image and then keep it for all stars of the same image.

You will probablly find other theory about that but they can't apply to DSLR lens that have PSF very variable accross the image and shape (PSF) far to fit a Gaussian...

The external annulus (where the background is measured) should be large to average the noise on number of pixels, this up to the point a bright star would not be intercepted (faint star are well eliminated by the "median" option, but don't use it if there is no risk from faint stars)

Accuracy: +/- 0.01 mag is a good accuarcy, the best catalog we have (Tycho 2) is in no case better than 0.009 !!!  and sometime much worst...  I do not remember the case but your 3.741 seems ok to me at that epoque of the eclipse.

Clear Skies,

Roger

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zsperko
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Hello,

I'm newby on this forum. I read the "DSLR photometry tutorials".
In the Iris-Beginner section, at the 3.1.1 Creating a Formal Bias
I founded this: To generate a Flat master frame go to Digital Photo Tab and select "Make an Offset..." (Bias frame). 

To generate Flat master? Is it sure?  It is the right "To generate a Bias master...", isn't it?

Zsolt

Roger Pieri's picture
Roger Pieri
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Joined: 08/29/2009
Posts: 136

Sorry Zsolt, this is a typo, you are right. It should read "To generate a Bias Master" !

Clear Skies !

Roger

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zsperko
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Hello,

In the "DSLR photometry tutorials", read the following

Step 5 - Selecting green channel:

....

From the Digital Photo menu, choose Sequence RGB Separation

and the picture is

but in the Iris 5.59, if  I choose this menu item, than I see this

What should I write in Generic name cell ?

Zsolt

Roger Pieri's picture
Roger Pieri
User offline. Last seen 35 weeks 3 days ago. Offline
Joined: 08/29/2009
Posts: 136

Hi,

As I said hereabove this is a mistake of the tutorial, the command shall be "RGB separation" and NOT the sequence one. You are just having stacked the images. Then the result is a single image.  By the way "sequence" shall not be used. The picture is ok, it's well the one for "RGB separation" (all IRIS versions)

Clear skies !

Roger

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zsperko
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Thank you!
and I'm sorry, it was careless of me.

Zsolt

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