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Author Topic: RGB/NB Relative Exposure Times  (Read 718 times)

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Offline BlueAstra

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RGB/NB Relative Exposure Times
« on: 09:07:33, 03 February, 2017 »
My SXV-H16 CCD camera has a quantum efficiency of 0.53, 0.306, 0.26 at the narrowband wavelengths of O3 (500nm), Ha (656nm), S2 (673nm). I have been taking equal length exposures in O3, Ha, S2, but should I weight the exposure time to take account of the relative spectral efficiencies, eg O3 = 1, Ha = 1.73, S2 = 2.04 ? I guess the characteristics of NB sources complicates this question, but the query equally applies to RGB sources. Should I take into account the spectral response of the CCD when shooting RGB and balance the exposure times so there is a flat response to each colour? Is there a way of compensating for the different efficiencies in PS or Pixinsight if you have taken equal time exposures ?

Offline chris.bailey

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Re: RGB/NB Relative Exposure Times
« Reply #1 on: 11:10:23, 03 February, 2017 »
An interesting question!

The answer is probably yes but.

Most narrowband targets are strongest in Ha and weaker in the other two. There are therefore fewer photons to convert to electrons in say OIII compared to Ha in the first place so QE is to a large part irrelevant compared to the abundance of photon arrival. Most narrowband images are rarely sky limited so if anything you need to go longer with the channels where the photon arrival rate is lowest to have a chance of shot noise breaking away from the read noise never mind overwhelming it. There is also the issue of different filter peak transmission to take into account. For Baader NB filters, there are graphs around that show the SII peak transmission is over 90% but the OIII is a few percent below that.

With RGB there is perhaps more of an argument. I have tried it in the past to try and help colour calibration in post but found, if anything, it was worse. Maybe in a truly dark site it may work but with modest light pollution it becomes a bit of a nightmare.

I personally like PixInsights Colour Calibration routines i.e. neutralise the background (neutral grey) and then adjust the strength of each channel to get true white. Its much easier and more effective than trying to do it at capture time.

I work on the premise of as long as you dare, as many as you can approach.

Chris
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