RGB Exposure Time Ratios

[BlueAstra]

My original equal time exposure ratio for RGB on a target gave a PS histogram peak of 13:9:8 on a target. This is an SXVR-H16 with Baader RGB filters. This implied I should use a 1:1.46:1.65 ratio for RGB exposures. I looked at the spectral response of the CCD camera and it's approximately RGB = 30%, 45%,48%. This implies a 1:0.67:0.63. However, when I look what other people are doing, Peter Shah uses 1:0.7:0.87, Dietmar Hager 1:0.77:0.92. I believe the correct way is to image a G2V star, but I'm not sure how the ratios used in practice (around 1:0.7:0.9) tie up with the CCD response and my results, although I guess my measured results depend on the colour content of the target. Can anyone illuminate me further?

[AndyUK]

Earlier this year I tried an experiment using a white light full spectrum Gerd Neumann light panel so see if I could cheat on G2V... In short, the ratios it came up with were bizarre and the resultant image a total mess. I then tried a white card on a flat dull day, but I haven't used the ratios I obtained as I haven't tried taking an RGB image since!

From what I've read, the ratio's are dependent upon the camera, the filters, the OTA optics and of course the object and it's altitude in the night sky

Although I'm lazy (hence the light panel / white card experiments), I've now come to realise that there really isn't a shortcut in astronomy / imaging... . I have this link ear-marked for G2V stars - If you're using EQMOD / CdC it might be a little easier to locate them (so that you know you're on the right star), but not having tried this out myself yet, I have a feeling that I'm probably never going to QUITE know if the star I'm calibrating on truly is the right one... It's a pity that one of the named stars isn't a G2V otherwise it would be easy!

[RikM]

I used a white card and got reasonable ratios with a daytime white balance. Then did a real G2V and got slightly different, but close ratios. I have a LP filter permanently installed on my coma corrector, so the ratios are a little weird but they work for me. I use 1.32:1.3:1.0. (My flat times also match these ratios).

I took 6x 1min images through each filter of a G2V star near the zenith. Bias subtracted and stacked them, then using the info tool in Maxim with a radius that matched the star, I compared the intensity and calculated the ratios. I picked 1min (and checked it on the fly) to ensure the star wasn't saturated. When shooting an image, I take equal length subs for each colour and colour combine in Maxim with the G2V ratios used as the colour weightings for each channel.

If I didn't use the weighted colour combine, I would shoot subs of different lengths for each colour to match the G2V ratios e.g. 400s for red, 390s for green and 300s for blue and then colour combine at 1:1:1. The reason I don't is that it means I can use just one master dark rather than three as I am habitually lazy.

[BlueAstra]

Earlier this year I tried an experiment using a white light full spectrum Gerd Neumann light panel so see if I could cheat on G2V... In short, the ratios it came up with were bizarre and the resultant image a total mess.

I tried the same thing previously and also had a bad result. I used the flat panel to find equal intensity times for R:G:B and got 1:1.07:0.53. This led to R,G being similar, but B was very low on my test exposure (PS histogram peak R:G:B = 26:29:17). I looked at the spectral intensity of the panel and found out it emits a lot of B, so its not got a flat spectral output suitable for exposure time calibration.

Next step - how do you easily find G2V stars of reasonable brightness?

[themos]

http://web.njit.edu/~gary/322/G2V_stars.html

[themos]

You can save that to a text file and use CdC to create a customised catalog.

[themos]

Download these two files and put them in folder "g2vhip" under "cat" folder of CdC

https://docs.google.com/open?id=0Bws-dmFqKZThdzU3UVBwWVJ0Uzg

https://docs.google.com/open?id=0Bws-dmFqKZThcU44RmdqUWxlZ00

Then go to CdC, Setup -> Catalog -> tab Catalog , click Add and point to the .hdr file

[BlueAstra]

Thanks for the catalogs. I'll have to look into using CdC. I currently use MaximDL, but it looks fairly straightforward to use the CdC catalogs.

[AndyUK]

And thanks from me too for the catalogs . There appears to be a nice mag 5.08 G2V star between Leo Minor and Ursa Major that looks as though it should be fairly obvious - All I need now is a clear sky... when I'm not working away from home