Clear + RGB filter usage

[rogerg (Roger)]

I finally got around to buying a Astronomik clear MG filter for my CFW-8 filter wheel. This means I now have par-focal clear/un-filtered capability, which is great. I've found even the IR filter of the Astronomik standard set for CCD imaging take a huge punch out of the sensitivity of my ST7 to the point that I'd be happy to sacrifice the colour for the sensitivity.

I expect to largely use the clear on it's own - not combine it with RGB data. The clear suits my purposes of faint galaxy cluster hunting.

But if I were to use it with the IR cut RGB filters, does anyone have any advice? I expect I'm going to get data in the clear image that won't exist in the RGB, because the RGB's are IR cut. I suspect this might lead to the extra data again being lost because there's no RGB to represent it. I expect that I would use the clear as the luminance channel. I'll be interested to see what this means for the resulting image.

Anyway, I'm really happy to have the high sensitivity back ... enjoyed cruising around a few galaxy clusters and individual galaxies last night, taking 2-7 shots of each (5min exposures) just to see what's there, getting lot of faint stuff.




Roger.

[Terry B]

Using a clear filter with the IR component in the image as a luminescence image makes it very hard to balance the colours. It isn't to be recomended.

[jase (Jason)]

The Astronomik MC clear filter will not have any impact on RGB images and will still colour balance fine in an LRGB composition. The only time where you'll have issues colour balancing is when the RGB set are not UV/IR blocking. The luminance does not matter. You'll certainly get benefits image with a luminance filter on galaxies and widefield work. Its hard to beat a narrowband Ha filter for nebulae type targets. Chris Schur presented an interesting slide deck at AIC2007 regarding Ha imaging of galaxies - pretty cool.

I find it interesting that you can make a sensitivity comparison between the MC and UV/IR blocking filter. How have you determined this? Can you share unprocess fits files between the two filters so we can measure star intensity values?

Seriously, the difference between the MC and UV/IR filter from a sensitivity perspective would be minimal to none. We are still talking about visual wavelengths here. Sure the characteristics of the ST7 chip will allow you to go near infra-red, but you really need to use NIR filters to get any benefit. In fact, if you're using an optical design that uses lenses, you'll struggle to get clear NIR wavelengths due to the optical coatings. This is the reason why most (if not all) research is done with only mirror based instruments such as RC's as they transmit all wavelengths equally.

Personally, I'd also question the benefits of the Astronomik MC filter in comparison to unfiltered. Unfiltered would probably be a better option. There is also no reason filters need to be parfocal either. Sure it makes imaging a little easier, but if you're using FocusMax its easy to determine filter offsets for fast focusing.

[rogerg (Roger)]

Interesting comments jase, I can understand your points.

It definitely feels like I get less image data when using the Astronomik Luminance filter than without, but I have to admit I haven't done any scientific testing of it - I will do that and see what the results are, I'll be most interested.

Focussing is an issue for me, it's very rare that I can perform an auto-focus on the FOV that I'm imaging. There's never anything bright enough (small FOV, faint galaxies, purposefully removing bright stars that might bloom, avoiding it trying to autofocus on a galaxy, etc). So refocusing for each filter isn't practical.

You mention determining an offset for focusing - not sure how I would integrate that with my imaging runs in CCDSoft or CCDCommander.

Considering those focusing issues the "sure it makes imaging a little easier" means it's more like "it makes imaging a lot easier" for me

Roger.

[Terry B]

Sorry for the incorrect info. Yes it is the colour channels that need IR blocking.
As for the IR component and the sensitivity of the ST7. I have a KAF0401E chip in my CCD camera that is similar to the ST7 (mine is non antiblooming).
I have tried imaging through a near infrared photographic filter on my VC200l. The scope has correctors in the draw tube so is not exclusively a reflector. The result was lots of signal in the near IR region with similar sensitivity to using a green filter when imaging a star field. I certainly have found a significant reduction in sensitivity when I image through a IR blocking filter but it does simplify focusing as the clear filter makes the colour filters par focal.

[jase (Jason)]

I'd be interested to see the results. The UV/IR Astronomik filter is reported as having a 97% transmission rating. I would imagine the MC would have similar values.

You're using focusmax right? Use the acquire star function. It will plate solve to find a suitable star, slew and center it, then begin to auto focus routine. Once focuser is in the critical focus zone, the telescope is then slewed back to the target. No brainer. I've used it on narrow FOV's before on my previous scope - C11 (~2800mm). Works fine.

Not sure about CCDSoft and CCDCommander for filter offsets. I'm using ACP which supports it. I know MaximDL also supports it, but you need to configure it manually. In ACP you simply perform and inital focus run through all filters (even if parfocal), ACP then calculates the difference based on the clear filter. Thus, you only focus once on the clear filter, then all other filters will reach focus, even if not parfocal.