Which CCD Camera Lens to use?

I've aquired a monochrome CCD camera (Jai CV-M4+CL) & a frame grabber card.
I thought it might compliment the ETX125 telescope somewhat, at least so as to familiarize myself with what's involved in Astrophotography,

I wonder if you guys cold help me out & give me some heads up as to which CCD Lens in conjunction with the telescope.

As the camera is 1.4Mega Pixel, do I need a high resolution lens?
What's the rule of thumb here, What are the considerartions?

Any help you guys can provide me o this would be very much appeciated.
Cheer & thanks.

Hi crouges

It all depends on what you want to do with your camera + additional lens and your ETX telescope. I was in a similar situation recently and I did a little bit of research into the equipment aspects and what could be achieved using a sensitive monochrome video CCD camera in combination with available lens designs and with a telescope.

I decided to purchase two Watec 902DM2S monochrome ½” CCD video cameras which have high S/N ratios and very low lux ratings of 0.0006 which was to be used for automated meteor patrol in conjunction with specialized software.
In order to get the best out of the camera (with extended IR capability) I found the ideal lens to use is a fast f - ratio aspherical type with auto iris capability. These lenses can enhance your camera low light performance by 50%, I therefore purchased two “Computar” Aspherical lenses of 2.6mm F1.0 (144 x 97 degree sky coverage) and a 12mm F0.8 (39 x 24 degree sky coverage), Quality is excellent and they are reasonably priced. You can record directly from the camera onto the PC (with an appropriate frame grabber, I use a K-World USB Frame Grabber) in AVI format real time viewing of the night sky. You can take; for example, 10 second video clips or longer and stack all the frames using RegiStax software to bring out fainter objects into a bitmap / Jpeg image etc.

In summary for your lens question…
1. Use fast f - ratio lens of aspherical type, you must match the lens to the CCD, for example is your CCD chip 1/3”, ½” or 2/3”.
2. You must also find out from the camera specification if there is auto iris control and if so is it DC or Video enabled (if may have an auto detect circuit for use with either one) or you may decide not to use this feature at all.
3. You must also find out from the camera specification if the camera accepts C-mount or CS-mount lens, I think there are spacers available for switching between the two.

If you want to use your telescope with your new camera for planetary and deep field work you will need a T-adapter to match your camera. For example I removed the visual back of my Celestron telescope and fitted a focal reducer with a T-adapter for use with a CS-mount camera (which the Watec 902DM2S is), Celestron provided all the items I needed in order to do this and I am sure Meade can provide the same service.
If you are using your new camera in conjunction with your telescope I would recommend the following piece of software www.coaa.co.uk/astrovideo.htm which has many useful features and is user friendly for video astronomy, best of all, enjoy your new toy.

Some of the other guys and gals here may be able to provide additional information for you as I am only starting out in this area myself.

Clear skies
Mike

Mike,

Thanks alot for taking the time & your advice, I most certinly need it & appreciate it.

I wonder if I could ask you to tell me a little more of, what is termed 'fast f - ratio'. Also rather than getting a 2.6 mm etc, is there any advantage is a varifocal lens>? Also do you have a site than I can review these lens?

In the case of getting magnification, do the same rules apply, 'focal lenght of the telescope / focal lenght of the eyepiece ? I'm wondering as the ETX has a focal lenght of 1900mm & if I use a 2.4mm, I get a magnification of
X 700 or more, which may be too much for my little scope.
Am I on the right track,

Cheers & thanks again,

Hi Crouges,
First of, I am not an expert on optics or CCDs, I can only offer what I have learned by myself, there is much more experienced people here on this forum that may correct what I am about to write and pass on their knowledge, I think some people here on the forum own ETXs presently and are doing excellent work with them.
Can you be more specific in what astronomical work you would like to undertake, this is an important point which you must be clear on, I / we may be able to give answers which are more pertinent?
The relatively small aperture (125mm in your case) of the ETX and its long focal length (f15, that’s ~1900mm) mean that ETX range is not a prime choice for deep-sky visual / astrophotography work, especially on faint galaxies. I would also have concerns with tracking and unguided exposures on such objects. All is not lost however, where it does come into its own and excels is planetary work, the Moon, brighter star clusters and double stars (camera requires shorter exposure times on these objects). Here are some pointers to consider for astrophotography work with your current setup.
The telescope’s focal length and pixel size calculation for your ETX125 in combination with your camera (Jai CV-M4+CL) Works out at the following regarding the number of arc seconds covered by each pixel and the result is 0.70 arcsec per pixel and your field of view is approximately 12 x 16 arcmin based on the following information…
Telescope: 125mm F15 (focal length rounded to 1900mm)
Camera: 6.45um x 6.45um pixels with an array of active pixels 1360(H) X 1024(V) ~1.4mp
Typical nights seeing conditions are between 2.5 and 5 arcsecs (from where I live), on occasions of good seeing it can go below 2.5 arcsecs some nights, so your telescope camera combination is well below this, you should really aim to have optimal resolution, which should be just under the seeing conditions, a good figure would be to aim for 2 arcsec per pixel, a good quality focal reducer fitted to the ETX should help in this regard, for example if you fit a f6.3 focal reducer this will bring your resolution to a more useful figure closer to 2 arcsecs per pixel and increase your FOV, if your camera has binning capabilities (in effect clustering existing pixels into effectively a larger pixel) this can help also.
Because of the camera and telescope combination you currently have, it follows that each individual pixel is seeing too little sky, therefore you have to increase the exposure time which would cause me concern regarding the ETXs tracking capabilities for unguided exposures as I stated earlier. Although there maybe software to help with tracking by feeding off the live image from the main CCD and updating the AutoStar tracking system in real time. This can all get complex when considering FWHM, Nyquist theorem etc.
A good website to visit to get the most from your ETX is www.weasner.com/etx/menu.html , lots of really good information in there.

If you want to use this camera with matching lens for stand alone sky patrol, for example, meteor work, you will need the correct type of lens to suit your 2/3” CCD camera, because your CCD is 2/3” (sorry, I thought it was ½” initially) I would recommend a fast (about f1.4) Computar Megapixel lens (fast lens basically cuts down on exposure time), I believe there is a f1.4, 12 – 36mm MegaPixel varifocal C-mount lens available from Computar. Hope this helps!
Clear skies
Mike

thanks again in taking the time to help me out. I can't say I understood all of it, but look forward to the learning curve ahead.

I finally bought a lens from Computar, 12.5mm-75mm, F1.2.
I'll have to wait a few days to get it & I'm eagar to get started.

To kick things off, I thought a some planetary would be pretty interesting.
I'm aware of the limitations of the etx, but hopefully it will lead to bigger
& better scopes in the future.

Good advise all round, appreciate it, & clear skies to you.

Hi crouges
Glad I could be of some help. That’s a good quality lens you have purchased. As a suggestion for getting you started in imaging the night sky by video means (initially starting off without a telescope) and practicing with software you will be using for capture and processing, I would offer the following general ideas….


1. Attach your new lens to your camera (set lens to 12mm setting which will give you a wider field of view and set the iris to maximum opening, i.e., f1.2, this will give you an image area of approximately 47x30 degrees of the night sky, as a comparison, the moon has an angular diameter of 0.5 degrees)
2. Place assembled lens + camera on a sturdy tripod and point at a rich star field, include a number of bright stars if possible (a dark site is always preferably)
3. Connect necessary power and signal cables to camera
4. Plug signal cable from camera into PC (Frame Grabber Unit)
5. Initialize whatever software you are using for video capture
6. hopefully you should have a live image on the screen (you may need to adjust focus at this stage, there should be an adjustment ring on the lens (near / far)
7. Using your video capture software, start recording, try a 10 ~ 20 second video clip saved in AVI format (note that a 20 second clip may take up a few hundred meg of memory on your PC, especially if you set it around 720x576 higher resolution).
8. You now have an AVI file which contains many video frames, this is where the processing comes in to integrate those hundreds of frames and bring out finer detail such as fainter stars. You can use a free program called “Registax” in order to help you with this aspect.


You could also try to use the lens set at full 75mm which will give you 8x5 degrees sky coverage, try the same procedure again (try slightly longer exposures) to see what happens, its all about experimentation to see what does and doesn’t work. Once you are familiar with your new camera and gained experience with your video capturing and processing techniques you should find the transition of imaging through the telescope that little bit easier (hopefully).

Recommended software and helpful advice for budding Astro-video astronomers..
Registax: www.astronomie.be/registax/html/download_v4.html (Free)
AstroVideo: www.coaa.co.uk/astrovideo.htm (free trial period applies)
K3CCD video capture: www.pk3.org/Astro/ (free trial period applies)
Helpful advice / ideas: www.qcuiag.co.uk/

Clear skies
Mike