RGB, LRGB newbie - where to start....

[JohnH]

Hello,

I am taking the plunge into using a mono ccd camera having used a DSLR and OSC ccd previously.

I have a manual 1.25" filter wheel and I have RGB, UV/IR cut and neb filters. They are standard quality filters though as I want to start out without to much expense.

So I am looking for the idiots guide here. I think it best to start with a few mono (L) shots to get familiar with the kit.

Then I shoot RGB at 30s or so of a G2V star to get my filter weightings.

I guess I need will flats for each colour as well as L?

I will need darks to match the L and RGB exposures...or do I keep exposures the same and simply stack more for L?

Then I have the basics for first run...what advice tips do you have for a newbie? I will be using AstroArt4 for capture/processing...

Big learning curve for me...

Would also like to try Ha - is there a recommended beginners Ha filter - the scope is the 127 ED and the camera has a Sony CCD: ICX285AL chip which has a QE of some 55% at the Ha wavelength.

[jase (Jason)]

Refer to this link for a "how-to" approach to G2V calibration - http://www.iceinspace.com.au/forum/s...75&postcount=8

Ideally, you should shoot flats for each individual filter as dust formation between the filters can be different. Some take a different approach, by "attempting" to keep their filters clean and simply use a unfiltered or clear flat the represents all other filters. If the FITS header does not contain a filter name, then it is usually applied across different filtered light frames. This can work considering the overall optical train configuration is identical regardless of filter i.e dust on optics or vignetting, but I'd question the accuracy. Personally, I take flats per filter. This risk of scratching filters keeping them clean does appeal to me.

Use the same darks across all filters. So in other words use similar exposure times and build a library. My dark library consists of 300s, 600s, 900s and 1200s masters (sigma reject combine of a minimum of 12 subs). Take them during the day if you can set your cooling point low enough. Don't waste precious night time hours taking darks. Refresh the library often as pixel states change. IMO a library that is a month old is needs to be refreshed. Some take darks on the night of imaging, this gives the best accuracy. Depends of far you want to take it and if you can perceive the benefits. For photometry and astrometry, sure, darks every night you take a light, but for pretty pictures. hmm not much gain IMO. Of course this assuming the camera is temp. regulated. Perhaps start with 600s luminance and 300s RGB if the camera is ABG. Much shorter if NABG

Play around with lum shots to get a feel for the processing. Or take some RGB only frames to see how your colour balance is working out. Straight RGB is good for bright targets, but for the dim, you'll want to introduce luminance data. There are good resources on the web for LRGB techniques.

Above all, have fun. Mono imaging with filters takes patience, but yields a superior result esp. when using Ha or specialised NB filters.

[JohnH]

Thanks Jase...that is what I wanted....the camera is actively cooled but not temp controlled so I think I will do darks at the end of each session - it is not too much of a chore as I will be asleep!

ABG, NABG were new terms for me (learning)...the chip is the Sony ICX285AL EXview HAD CCD these are ABG designs I beleive although the camera maker does not make a specific claim about the cameras capabilities in that respect. I guess I will have to try it and see how I go...

Any advice re a suitable "budget narrowband filter setup" or is that an oxymoron?

[jase (Jason)]

The Sony ICX285AL is an ABG chip, no need to worry about blooms. If I'm not mistaken, its a 1.4mp array with a pixel size of 6.4micron. These are small pixels so you may want to validate you arcsec/pixel sampling. The chip can be binned so you can get the optimal sampling you require. Typically, many take luminance data as 1x1 binning (in other words, no binning) and the RGB as 2x2. The luminance contains the resolution. The RGB is simply there to colour the luminance as such the 2x2 bin doesn't compromise the resolution of the 1x1 bin luminance when layered. A 2x2 bin will boost sensitivity.

Narrowband filters aren't cheap. The smaller the window bandpass, the more expensive the filter is due to the tighter manufacturing tolerances. The Ha filter is a good item to have in an imaging arsenal, especially for those full moon nights. It makes imaging productive...collect Ha data when the moon is around, then complete the task new moon with LRGB data. You can also collect R filtered data providing the moon isn't too bright or in close proximity to the area being imaged.

[JohnH]

Jase thanks for all the help. I got the camera under the stars last night an (because of the nearly full moon) went for a luminance fram of M8 (yada yada). I used my DGM Optics NBP filter as a poor man's Ha well it passes Ha and OIII as well. I am pleased with the detail showing for 8*240s shots (flats but no darks were used).

Next to add some colour....