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Focal reducer for SCT

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No need to apologise Olly... thats just the one I thought I had seen before...

Do you often have probs with CN ?


No, not really but I find it too big! I'm a country bumpkin and so I don't often go to CN. The net can be iffy here.


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  • 12 years later...
On 20/12/2010 at 15:36, BlueAstra said:

My 'working distance' survey says:

Meade 4000 x0.63, 45mm

Celestron x0.63, 105mm (x0.7 @ 50mm, x0.5 @ 225mm)

OpticStar x0.63, 105mm (same as Celestron?)

Antares x0.63, ?mm

TelVue TRF2008 x0.8, 56 +/- 4mm

This may be of interest:

Weasner's Telescope Accessory Reviews

I'm having this issue at the moment with not actually knowing the BF of the antares FR 😕 I've been using 105mm but rotational star trails at the edges suggesting its too far away. I've managed to get it down to 87mm with the e tra buts I have have but not tested yet

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10 minutes ago, labtech1122 said:

not actually knowing the BF of the antares FR 😕

It is not difficult to measure the focal length of the reducer and telecsope directly. On moving mirror focusers the telescope fl changes too**.  Then calculate* the back focus that matches FR=0.7. Whether that is actually the ideal is another matter !

*you can look up the formulae in any respectable source. ** so the calculation may be iterative.

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On 20/12/2010 at 14:08, starscy said:

Could anyone suggest a focal reducer for an 8" f/10 Schmidt-Cassegrain Telescope? I read that some focal reducers have issues focusing with 2" star diagonals.

With a moving mirror focuser you will have lots of back focus, so it will reach focus - but the prime focal length also gets longer as you do so and this is not optimal. As you are thinking visual (star diagonal) you might get vignetting (due to the baffle tube inside) if trying to reach very wide views. However it could be cheaper than buying widefield eyepieces for very occasional use. The Antares amongst others is affordable, look for used? worth a try.

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