Focal reducer?

[animaal]

I've just replaced the visual back on my 8" SCT with a Baader 2" Clicklock adapter. That makes it nice and easy to swap between a 2" diagonal, and my DSLR to which I've attached a t-ring and 2" nose piece (attached at prime focus).

Given that my mount is alt-az, and I haven't yet got a wedge, I'll be keeping the exposure duration to a mimimum. I gather that a focal reducer might help with better results in the same exposure time. What kind of reducer would work best for my setup? most of the examples I see are threaded for an SCT. I've seen a 0.5x Antares, and similar from GSO, that have 2" threading. Are they my best option? I imagine I can screw one into the camera's 2" nosepiece. I'd rather not be removing and reattaching the visual back on a regular basis.
 

Thanks.

[Leveye]

Don't get the Antares, Didn't work very well on my 8" SCT. It might also be made by GSO so that cuts that one out as well. Personally you might want to get on Astromart and find a used Celestron reducer thats made specifically for the C8. Unless it's a Meade and then i really can't say what's the best for that scope. I like Taiwanese Synta quality better than Chinese GSO.

[animaal]

Thanks Leveye, that's interesting. The Celestron reducers I've seen thread onto the SCT. So I'd need to add/remove the visual back when I want to do some imaging. I was hoping to avoid that. But if the best soutions require it, I'll have to reconsider.

Thanks!

[DoctorD]

Hi 

If you want to continue to use your DSLR then a dedicated SCT reducer is probably the best option - the Meade or Celestron F6.3 should be fine - steer clear of the F3.3 as it is only suitable for smaller CCD chips and will produce severe vignetting with a DSLR or larger CCD.

With AZ only you are going to be limited to 30s or so exposure depending upon where you are pointed in the sky - however my experience is with much lower resolution cameras (720x576) so I'm not sure what I'm pact using a DSLT will have.

HTH

Paul 

[Guest]

Hi,

You do need to go for the dedicated SCT reducers, mainly because they have an extra field flattening lens in them also, which you need on an SCT when using a focal reducer, the others you mentioned do not have this and are just reducers.

Also with either the meade, Celestron or antares SCT reducer, the distance from camera sensor to reducer lens is critical, it need to be 110 mm, bear in mind that on a canon DSLR the sensor is 45mm inside the camera, so from the front edge of the camera to the centre of the reducer lens assembly it need to be 65mm for the proper 6.3 reduction, all the SCT reducers are f6.3 except for the 3.3 mentioned earlier, that is just for CCD imaging.

Also,the reducer should be fitted as close to the back of the scope as possible, preferably straight onto the back, then an extension tube with the correct spacing.

These reducers can be used at shorter spacing so but the focal reduction will be less and the image may suffer a little, but do not use it a a longer distance, although you will get more reduction, the image will be severely vignetted and degraded.

Hope that helps and makes some sense.

MM

[Tinker1947]

If you use the celestron 6.3f reducer, there is a Moonlite extension that has a SCT thread this screws into the FR the other end goes in the baader Visual back, the DSLR end of the FR will take another Baader Click Lock this then goes onto the extension attached to the DSLR the spacing for the FR is 105mm from the lens (DSLR end) to the Camera sensor......

http://www.firstlightoptics.com/adaptors/moonlite-smooth-bore-2-inch-to-sct-adaptor.html

Attaches to FR DSLR side

http://www.firstlightoptics.com/adaptors/baader-click-lock-2-for-celestron-meade-sct.html

[Guest]

That looks rock solid, but the reducer should really be right up against the back of the scope, or the f ratio will be off

MM

[Tinker1947]

That looks rock solid, but the reducer should really be right up against the back of the scope, or the f ratio will be off

MM

Its a Non Edge Scope, but if it still needs to be right on the back of the scope it would need a visual back with a SCT fit, the one supplied has a 1.25" thread, as its set up the distance between the rear of the FR and the DSLR Sensor is 105mm thats the important part although the distance can go down to 85mm but not sure what changes in the FR then,,,,,,thinking about it not being attached to the Visual back it just means its slightly under the normal FL as the Mirror would need to be around 25mm closer to the Secondary...???

[Guest]

Yes that is correct, it is just a case of working out correct focal ratios and not assuming it will be 6.3, the reducer will work fairly well from around f5 to f7.5 I believe, again distances will determine this reductions, but to get very close to 6.3 the reducer should be directly on the back of the scope with 110mm distance from sensor to the centre of the reducer lens assembly, so 105mm ish to the back of the reduspcer glass.

The reason it is measured to the centre of the lens assembly is that the rearmost glass nearest the camera is the field Flattener lens, so you need to measure to the back of the reducer lens and not the Flattener.

This is the way I understand it

MM

[animaal]

Thanks, that all makes sense. I think I'll leave it for the moment though, maybe get better without the focal reducer first. No point adding the complexity untl I need to...

[Guest]

Yes it is a good idea not to complicate things, just get out there and enjoy the hobby, we all get wrapped up,in the science and forget to enjoy sometimes.

MM

[laser_jock99]

. I like Taiwanese Synta quality better than Chinese GSO.

.....err GSO are Taiwanese http://www.gs-telescope.com/

and Sytna are from China http://synta.en.gongchang.com/contact