The Iceman’s Luna CCD Image

[Dennis G]

The numerous replies received to my submission, Colour compared to Mono

DMK CCD imager has shown me that I’m not alone requiring similar information and. with this in mind I’d like to lean on you Iceman Mike with your expertise and ability to explain in detail how you set up the telescope, recorded the crater Clavius and processed it, and in doing so cover the questions below. Something along the lines of your planetary article, keeping in mind the elderly slow learners.

Replying to one of my questions you wrote, a Astronomik (?) red filter was used to capture this magnificent picture. Why a red filter and details of the filter, such as name and where it can be purchased. For those who cannot afford a full set of filters and adapter, can the red filter screw to the 1.25 inch adaptor that comes with the DMK mono imager and be used as a single filter and recorded as AVI file.

For those who gave their time to reply to my questions, I finally decided on mono and as I’ve never recorded in B&W, I tried it with the Toucam. In my opinion Greyscale looks too deadpan blue and I gave the attachment a slight colouring in Photoshop. If the DMK mono is better than this first attempt, I’ll be happy with B&W. Thanks all

[sheeny (Al)]

G'Day Dennis,

I'll take a guess here... my guess is Mike used a red filter for two reasons:

1. to reduce the bandwidth of light reaching the camera sensor in order to improve the sharpness of focus with a Powermate (lens) in the optical train; and
2. red light is scattered less than blue light through the atmosphere (this why the sky is blue, and we can see green laser beams but not red ones) so the effect of bad seeing is less... ... maybe?

It's a bit of trade off at times with filters. Sometimes a red filter may increase the exposure time unacceptably, so in those cases a better option might be a UV/IR filter that lets the entire visible spectrum through but controls the UV and IR which will be more out of focus (when a lens is in the train).

My rationale on filters is:
If the seeing is really good, a red filter will give a sharper image with a slightly longer exposure.
If the red filter is not sharp, it may be the seeing is changing during the exposure, so more light is required to shorten the exposure... so go to a UV/IR filter, and maybe a faster frame rate to catch the few sharp frames you can.
If it's still not sharp reduce focal length or give it away for the evening.
I'm sure there are more expert people on the forum than me when it comes to filters and planetary imaging, so I expect them to straighten out any misinformation I've created.

Not that I have an Astronomik red filter, but I believe they are available in 1.25" and would screw into the DMK nosepiece if you don't have a filter wheel.

Al.

[Dennis G]

Thanks Al, for your reply. A question: is the UV/IR a single filter or two?
I have a 50 mm UV from my Video days Some time ago I purchased a horrible green moon filter and discarded it. Using the holder I cut down and ground a light reduction filter I had to fit. I could do likewise with the UV if it is used as a single filter.

[iceman (Mike)]

Hi Dennis

Al answered perfectly the question of why I use a red filter when capturing the moon.

A UV/IR filter is a single filter. If you get an astronomiks set of filters (LRGB type II, around $350), they block UV/IR as well. They only allow the red wavelengths and block everything else.

If you're getting a monochrome DMK, then if you ever want to capture colour images you'll need a set of RGB filters. The astronomiks are among the best but they're not cheap. You won't need any other filters on top of that. They are 1.25" and can screw in independently to the nosepiece, or using a manual or automatic filter wheel, then you store them in the filter wheel and change filters when the need arises.

The astronomik's come with the "L" or "clear" filter, which blocks UV/IR only and allows all other wavelengths through. This is effectively your UV/IR filter. It will allow more light through and allow you to use a shorter/faster shutter speed when capturing the moon.

But as Al points out, in good seeing the red filter can help capture a sharper image by blocking other wavelengths as it's less scattered by the atmosphere.

The clavius image I captured is probably my best lunar image to-date, so while it's good to strive for that, it's certainly not something I can expect, or you can expect, to capture each time out at the telescope. The seeing was very good that night, and I used a 4-panel mosaic with my 12" telescope and a 5x powermate, so the focal length was over 10.5m for each panel.

With your 10" lightbridge, you won't get near that focal length so it will be a less-magnified image but that can also help you to get a sharper image when the seeing isn't so good. At longer focal lengths, you really need the seeing to be very good to capture sharp images.

Good luck with the DMK and keep asking questions.

[Dennis G]

Thanks Al and Iceman for your replies. Much appreciated!