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Making the most out of SC6agt setup

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I hace a SC6agt scope with celestrons reducer (6.3). I have a T mount and a eye piece projector.

I am using a cannon 450d (unmodified). Mainly I have been using the T mount focusing the telescope as a lens for planets and deepsky.

By using the T mount and star diagonal I can change to a larger field of view which is nice for deep sky. compared without the star diagonal. How will this effect the focal length and ultimatly the exposure time?

would i be better using evepiece projection? (how does this effect the exposure time?)

Thanks Paul

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and to aswer my own question......

Adding a star diagonal to a Cassegrain type telescope with moving primary or moving secondary focusser, can indeed effectively increase the focal length of the telescope.

Any increase in distance between the eyepiece and a secondary which has power, changes F.

The principle is the same as moving an eyepiece towards or away from a Barlow lens, the effective F of the telescope changes, hence magnification changes.

The word 'effective' I use in these cases, symbolising a change in F without the corresponding change in the linear length of the converging beam. In real terms the change in F is true.

Types of telescopes where this is true -

All SCTs (with moving primary focussing) e.g. Celestrons and Meades.

All Maksutov Cassegrains (with moving primary focussing) e.g. Synta Maks (Skywatcher, Orion). Intes-Micro Mak Cass, Mirage Mak Cass, Meade ETX's and LX 7" Mak. Celestron 4" Mak. Questars (I think, although not sure about the precise focussing method, it is many years since I took one apart).

Cassegrains with moving primary or secondary focussing e.g. Tak Mewlons.

Of course this means that all of you out there with SCTs have telescopes with no one focal length. The possibilities are endless.

This discussion was brought up briefly on another thread 'Mak replacement for Newt' in this Equipment Discussions category.

Some numbers are mentioned in this discussion.

Every Cassegrain type telescope with movng primary or secondary, has a premium position behind the primary where an eyepiece should be placed. This position roughly corresponds to the correct position for an eyepiece, focussed at infinity, where Lower order Spherical Aberration is at its least. This is also what some manufacturers of these type of instruments regard as the 'back focus position'.

E.g. With Celestron, the C8, 9.25 and 11 (not sure about the 5 or 14), have the correct eyepiece position some 90mm to 100mm behind the rear cell. At this position an eyepiece focussed at infinity is working (roughly) at f/10.

An eyepiece placed directly into the visual back means the f/ is less than f/10, F is reduced and magnification is reduced. The opposite is true if the eyepiece is pulled further away from the rear cell.

I have worked out (by simple true field measurements within the apparent field of an eyepiece) that placing an eyepiece 120mm further back from the normal back focus position, on a 5" and 8" mass-produced SCT, decreases the true field by about 20%. This means that F has increased by 1/0.8, which is 25%.

So, e.g. in the case of a C8, a 2" screw-fit to push-fit adaptor, attached to the visual back. then a 2" push-fit Star Diagonal added, then the 1.25" adaptor in the diagonal pulled out by about 1", and a 1.25" eyepiece inserted and focussed at infinity, F is no longer 2032mm but has increased to approx. 2540mm. f/10 becomes f/12.5. A 10mm eyepiece changes from giving 203X to 254X.

You can take it too far and introduce mechanical vignetting, by pulling the eyepiece too far back, so using the system as a weak Barlow has its limits.

However, you can genuinely claim that your telescope has no focal length, or that it has many focal lengths.

So, the diagonal itself does not have a focal length, as it does not have power, it is plano. But, it does contribute to a change in F and f/ in mirror focussing Cats or Cassegrains.

Best regards


whoever chis is

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