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[novalab]

Couple of months ago I was doing some Exoplanet photometry in the vicinity of M13. When I was done, I thought to take few shots of M13, with a little twist. I imaged it in LRGB as well as with photometric (BVR) filters. The below pic is a regular looking M13 color G-star calibrated LRGB image:



To highlight the blue stragglers in the cluster, it is possible to display the same image but highlighting the temperature information. This can be done by shooting thru BVR photometric filters instead, and displaying it as LRGB. The “L” channel is a simple sum of the BVR filters, creating the LBVR image below. The BVR images were G-star corrected.



As you can see, the image looks very different, and the young blue stars are quite easy to see when presented in this way. The only processing done was the DPP, in order to dim the central GC glare.

The scope was a CF C11 on CGE-PRO, working at F7.4 thru a Starizona SCT Corrector. Focuser is a 3” Motorized Clement, with temperature tracking. Filters used were Astrodon Photometric and Astronomik LRGB. The seeing was decent.

And, yes, the date is correct: the new year’s eve, but I also have stuff fron xmas

I hope you find this interesting and useful.

[Paulie pchris00]

Hi Nova, and welcome to the forum.

I find this quite interesting, but forgive me, I'm not an astrophotographer. I'm assuming the BVR is blue, violet, and red filters? Perhaps you could elaborate for those of us not so AP literate? Thank you, and I appreciate the post.

[novalab]

Hi Paulie,

Thanks for welcoming me to the forum.

The BVR are actually photometric filters (Blue, Visual and Red). They appear deep blue, light green and very deep red. There are actually 5, including the UV and IR filters, to form a famous UBVRI sequence. Theses standardized filters are used to exactly measure object magnitudes, so that two observers using vastly different equipment will always arrive at the same number. You are probably familiar with the “B-V” index, to describe the color of the star. This is nothing but a blue mag – visual mag, as measured thru the B and V filters. Thus, higher this number, bluer the star, and thus hotter. You can actually calculate the star surface temperature from this value. Since the radiation profile of any object has a peak at some wavelength, and this peak moves towards the red with the temperature dropping, for some very cool stars like brown dwarfs, we will find it useful to measure the color using V-R or even I-R indexes. Some very hot energetic events are measured via U-B indexes.

Measuring the star photometric magnitude is more complicated than just using the right filter, as all CCD cameras, and the in-between optics have vastly different spectral response. Thus the system first need to be calibrated by observing many stars of known UBVRI magnitudes. Further, as the star travels lower towards the horizon, it is passing thru more air. This dims it, and it has to be accounted for. a secondary effect is that the dimming is more pronounced in the blue, and less in the red, and the least in the IR. This is called atmospheric extinction. So this is actually a pretty complex measurement..

The reference to G-star has to do with the absolute color. The G stars (like our Sun) are assumed to be white. The Sun only looks yellow due to our atmosphere, but if you were traveling in space it would look white to you. It is possible to calibrate any system, to show such stars as white. Thus the colors are normalized, and appear as they would if you were in space. This is a pretty standard thing in AP. Here I carried it a step further, by doing something similar with the L-BVR images. So, any white stars you see are like our Sun, and the overall golden appearance of M13 suggests that is composed of very old stars… that give birth to young ones (blue-white).

Pretty cool what you can do with a bit of colored glass

Chris

[Paulie pchris00]

Thank you for the explanation. I find it very cool that you can do science measurements, and seem to enjoy it.

[patrickm]

Wow.

Wow.