How to use Meade f/3.3 Focal Reducer with SPC900NC & Celestron C6-SGT

Posted by XTSee on 1st August , 2011

I have a Philips SPC900NC PC webcam that I use for astrophotography with my Celestron C6-SGT XLT SCT Goto telescope. I have modified it with the Steve Chambers LX (Long Exposure) mod and wanted to try out a fairly good easy starting target to experiment on its capabilities.

Pleiades seemed to be a good choice, but after having a go, I discovered that the field of view of the webcam was far too small to capture the whole of the Pleiades. So after reading up on the capabilities of focal reducers to;

a) give a wider field of view, and
b) to give a brighter faster image, i.e. shorter exposure times,

…I decided to get a Meade f/3.3 Focal Reducer.

The Meade focal reducer will screw directly to the backplane of the C6 SCT tube (any Celestron SCT), by unscrewing the visual back (the holder the star diagonal slots into) from the backplane, screwing the focal reducer onto the backplane, and then the visual back onto the focal reducer.

The SPC900NC webcam (see pictures) has an AC414n 1.25″ nosepiece adapter nozzle fitted for easy use in lens holders, into which I also have an infra-red filter screwed. The nozzle/webcam alone is then inserted directly into the visual back, i.e. without using the star-diagonal, nor any barlow or other extension tube.

One of the difficulties of setting up the webcam is focusing the scope! Of course the laptop and software can be set to preview the image coming from the webcam, but when learning it is all too easy to miss the focal point with dark or dim stars.

When I first got the focal reducer I wasted a couple of evenings by not understanding whereabouts the Meade f/3.3 focal reducer would focus, or how much of a difference it would make to the field of view. This was made more difficult by not knowing the region where focus would occur, and so I was continually focusing so fast (by hand) through the actual focus area, that I began thinking the Meade FR would not work with my scope+webcam, or that I required some kind of extension tube.

Therefore from experience I recommend it is best to do a practice run in daytime, by lining up the scope on some distant landmark, then working out how many turns of the focuser is required at least so you know a ball park figure for when working in the dark.

  1. Unscrew visual back from backplane of the C6-SGT
  2. Attach FR direct to backplane
  3. Screw visual back to FR
  4. Insert webcam fully into visual back (with IR filter screwed in)
  5. Unscrew C6 SCT tube focuser knob (anti-clockwise) fully until back-stop
  6. Setup/start webcam viewing software (I use K3CCDTools with WcCTRL utility)
  7. Screw focuser clockwise until focus achieved, counting the number of turns (note I chose clockwise as this is the lesser number of turns required from the back-stop, than anti-clockwise would be from the front-stop).

The focuser on the C6-SGT requires approx 75 finger half-turns to go fully from front-stop to back-stop.

Focus occurs roughly between 25 and 35 clockwise finger turns (half-turns of the knob), i.e. sharp focus approx 28 to 32 half turns. This may vary depending on how far away your daytime landmark is, and whether you are using a different Celestron telescope. Achieving final sharp focus requires VERY fine focusing. Pros will no doubt recommend a motorised focuser (more expense). Bear in mind also that the focal reducer captures more light, so makes for a much brighter image, and you will need to reduce the gain on the webcam as otherwise the image may be overexposed preventing you from seeing your distant landmark.

On a large object (e.g. pylon in distance across other side of valley) this method is easy, but with stars for a proper observing session it can be quite tricky, so locate a good bright star and centre as accurately as possible with an eyepiece/reticule and the scopes EQ tracking on, before replacing the eyepiece with the Webcam/FR combo!! Note: Webcam+FR gives normal image (right-way up).

By doing this I now know that the Meade f/3.3 works fine with my setup, and that the field of view is approx. 3 times wider than the SPC900NC chip can obtain by itself.

Now I can have a proper go at Pleiades.

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One Response to “How to use Meade f/3.3 Focal Reducer with SPC900NC & Celestron C6-SGT”

  1. Morris Adams (1 comments)

    That is amazing. Normally I would buy a telescope of higher focal ratio in order to obtain a smaller more close up field view of stars and nebula instead of working with “fast” focal ratio. Thanks for sharing this post.

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