Red Green Blue filters

[Jammy]

I'm looking at getting a mono guiding camera, but would also like to use it for planetary imaging.

I have a Celestron eyepiece set that comes with a set of filters.  Are the red, green and blue filters in the set the same ones that you use for imaging, or do I need to buy additional ones?

They are numbered - red No. 25, green No. 58, and blue No. 80A.

Does anybody know what the numbers refer to?  Is it Celestron's catalogue, or is there a filter numbering system?

Regards

[Oddsocks]

Hi Jammy,

The filter numbers are a throwback to the earliest days of film, they are called Wratten numbers, here is a link to wikipedia that describes the history and what each number means.

http://en.wikipedia.org/wiki/Wratten_number

Wratten filters are absorption filters, they absorb the wavelengths of light that you don't want and pass on the ones you do, they will be fine to begin with, they are not very efficient and you don't get all of the target wavelength through the filter, modern Dichroic (interference) filters are much more efficient (and much more expensive).

http://en.wikipedia.org/wiki/Dichroic_filter

The filters you have were in standard use ten years ago and will work fairly well for bright targets like the planets, you will need to think about a filter wheel though, it is very difficult to centralise a planet on the sensor and keep it there, once you start having to remove the camera to change filters so that you can construct RGB colour images then the whole thing becomes much more difficult, so use the filters you have but look to buying a new or used filter wheel to make it easy to swap between them.

William.

[Jammy]

Thanks William. All becomes clear. I'll give these a try and see how I manage before getting any new ones. The filter wheel is a good idea.

Do you know if these filters have IR/UV block for imaging planets?

I do have a separate IR/UV filter if I still need to use it.

[Oddsocks]

Having looked at the Celestron web site this evening the filters supplied with the eyepiece kit seem to be for planetary visual enhancement so they are probably not IR blocked, Wratten 25 is definitely not IR blocked.

You would need to keep your IR blocking filter in the camera nosepiece at all times or you will struggle to get a balance between the three RGB filter exposure times.

Also I think the blue 80A may not be dark enough, normally the blue filter as part of an RGB set would be Wratten No. 47.

But have a go, there is very little blue colour in the easiest planets to image and you can always adjust the colour balance in post processing.

William.

[Jammy]

Thanks William.  I didn't think they'd have IR block.  Now to try them and see what the results are like.