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One of the problems CCD camera users face is actually aligning the wanted object with the relatively small CCD chip itself. If astro CCD cameras could be made with a 35mm single lens reflex camera body this would be fairly easy. Until that day we have to add an external flip mirror unit to achieve a similar 'view finder' result. There are few flip mirror units on the market, some such as the Murnaghan Instruments and Taurus units appear to be rather expensive. The UK manufactured mirror unit by True Technology Ltd is quite robust, compact and relatively cheap at approx 139 pounds sterling (delivered) plus some 30 pounds extra for the SCT to Starlight Express CCD camera adaptors. The main body of the flip mirror is threaded in the usual 'T2' threads that normal photographic accessories use. One of the limitations of flip mirrors is the extra light path 'lost' due to adding some 3 or 4 inches of pipe between the CCD camera and the telescope. Newtonian telescopes may not have sufficient focus travel to use a flip mirror. There is plenty of focus adjustment with most SCT's, certainly my Meade LX6/2120 can focus easily with a flip mirror added. Given all the advantages of a flip mirror, are there any disadvantages? Well yes there are, it needs to be very accurately machined if perfect alignment is to be achieved and secondly, a single lens reflex camera uses a ground glass screen to focus the image; a flip mirror finder simply has a normal eyepiece. This means that you cannot guarantee when the eyepiece is in focus the CCD camera will be. These two 'disadvantages' aren't quite as bad as it might look. Firstly using a CCD camera with a chip around a quarter of an inch square requires very high alignment accuracy - you aren't going to get that using parts from three different makers. Getting at least part of the wanted object on the CCD is sufficient to adjust the telescope on the next exposure. The nearly par focal eyepiece and CCD camera can be used to roughly focus the scope. Find a suitable dim star and focus the CCD, then leave the focus knob alone for the rest of the session. I've painted a couple of white dots on the focus knob to (hopefully) assist in getting the CCD into focus the first time. Obviously there are bound to be changes in focus with temperature variations etc. but it's better than not knowing where to start. The unit arrived within a couple of days of sending a faxed order. The instructions are more than adequate. The only disappointment is my favourite 20mm Erfle eyepiece wouldn't focus and fit into the tube! As a temporary measure I've been trying an old 40mm Kelner which is sitting well out of the tube (as shown above). Note, a standard Meade 4000 series, 25 mm eyepiece worked perfectly. Another minor niggle is the chosen object doesn't quite sit in the middle of the field of view of the eyepiece; this is explained in the instructions and is a function of errors in the manufacture. In practice you soon get used to knowing where to place the object for it to hit the CCD. Used in conjunction with a set of 4000 step digital setting circles, I can centre any deep sky object on the CCD 'every time'. Using a CCD camera without a flip mirror would be just too difficult to contemplate. Update Since writing the main part of this article, I've bought an infra red blocking filter that screws into the TTL flip mirror assembly. The cost is around 35 pounds sterling (available from True Technology). In theory the filter should give sharper star images when using a telescope that incorporates a lens (i.r. focuses at a different point to visible light, when using a lens). I've also fitted an eyepiece projection barrel in front of the flip mirror assembly. Results from the latter look encouraging, I'll let you know in due course what effect the i.r. blocking filter has (little effect, only serves to reduce the already poor sensitivity).
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