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SCT Reducer / Moonlite Focusser and DSLR

Rossco72

By Rossco72
in Discussions - Cameras

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Rossco72

Rossco72

Posted
Posted

Hi all,

I'm lookiing for some help from all of you knowledgeable folks out there. I have been connecting my Canon 450D to my C11 using a Moonlite CS Focuser with the f6.3 focal reducer threaded to the draw tube. I simply cannot get the image to achieve focus as I run out of adjustment on both the primary mirror and the Moonlite.

The distance on the Moonlite from the reducer to the end of the drawtube is a fixed 71mm. I believe my DSLR is 55mm from the front of the T-Ring to the CMOS giving a grand total of 126mm between the reducer and the CMOS, is this too much? I seem to recall the ideal being listed as arounf 105-110mm!

Without the reducer the camera will come to focus ok when simply attached to the SCT star diagonal (with the Moonlite and reducer assembly replaced by my visual WO star diagonal)

Am I missing something or is this all down to the offset between the Reducer and the CMOS? Is the Moonlite only suitable for CCD cameras where the sensor is nearer the front?

Cheers in advance for your thoughts

Stuart

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Merlin66

Merlin66

Posted
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Stuart,

certainly sounds like it.

The 110mm is a "recommended" spacing for "optimum" performance.

What about mounting the reducer in a 2" > SCT nosepiece and putting it into the focuser?

You'll need some SCT>2" and 2" to T thread adaptors to build up the spacing...(or use the 2" to EOS adaptor to minimise vignetting)

When you use a filter wheel etc etc this get even more untidy.

Ken

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Rossco72

Rossco72

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One of the reasons for going for the Moonlite Moonlite Focusers - MoonLite CS Dual Rate SCT Crayford Focuser for large 3.25 inch thread

which wasn't cheap was the built in use of the reducer, I am now wondering if this was purely for CCD imaging and will never work for my DSLR's?

I haven't had a chance yet to try it out with my SAC10 but I guess that's the next step when I get another chance.

Cheers

Stuart

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Merlin66

Merlin66

Posted
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Stuart,

Looking at the Ad etc, it certainly reads like the feature of screwing the reducer into the front of the focuser is a "feature".

If the "fixed" distance is 71mm as you say, it doesn't leave much backfocus for filter wheel, flip mirrors etc etc (never mind a DSLR)...

Hmmmm......................

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Psychobilly

Psychobilly

Posted
Posted

If the "focusser" has used up 71mm already and the DSLR t-mount to sensor distance is 55mm then your outside the 110mm required for the f6.3 or am I missing something? (apart from my marbles which I already know about)

Normally the f6.3 reducer is used with a 55mm SCT-T adaptor fitted

Billy...

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Psychobilly

Psychobilly

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So afer you have fitted the reducer to the front end ... you for a reducer to drop the thread down agian?

I would be inclined to try the reducer on the back end of the focuser fitted but as already mentioned this will need a a 2" to SCT adaptor... and the standard 55mm SCT -T adaptor between the reducer and the camera...

It just depends if the mirror has enough travel then though - I would have hoped it would have...

I would be inclined to set the moonlite so its racked nearly all the way in with a few mm of travel left ... then try and focus using the main mirror focus knob... Pity the CPC doesnt have a mirror lock :)

Billy...

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Rossco72

Rossco72

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Well I have already fitted a Feathertouch micro focuser to replace the standard C11 mirror adjustment knob. Mirror shift isn't too much of a problem.

There is no means to connect the reducer to the back of the Moonlite as it is just a normal 2" compression ring at the rear (without of course using a 2" to sct adapter, and even then, it would totally defeat the purpose of the Moonlite having all of this hanging off the back, it would be more secure attached to the SCT surely?)

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SteveP

SteveP

Posted
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Rossco

For the reducer to work optimally, the focal plane of the chip needs to be approx 110mm from the rear of the reducer - usually called Back Focus. In your case with a DSLR you will need an adapter or two to fill the gap betwen the reducer and the chip. The Moonlite will then fit on the scope side of the reducer. To use it between reducer and chip is to use it at best as a very expensive variable adapter and at worst not be able to reach focus as you've found

Steve

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SteveP

SteveP

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Merlin

Good question! To be cynical, I'd say to sell more focusers!

When Meade/Celestron designed these reducers they were designed to fit on the visual back of the OTA ie downstream from the internal SCT focuser and the imaging plane was optimised at 110mm, ie providing ample backfocus to allow for filter wheels, CCD's and AO units.

When i had an SCT, I used an Optec focuser and Optec reducer which were specifically designed to be used on an SCT. Optec went to great lengths (and supplied very precise adapters for the CCD) to stress to their customers that the focuser attached to the visual back and the reducer was positioned downstream of the focuser. This ensures that the reducer/chip can be set at the correct distance and they functions as a single fixed unit irrespective of the focusing position

Does that make sense?

Steve

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SteveP

SteveP

Posted
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I was a little hard on Moonlite earlier with my facetious comment! I think the real reason is to minimise the strain on the focuser's drawtube and possibly to avoid having to machine it to incorporate a way of attaching the reducer. Unfortunately they have introduced other problems and cut across the original design of the reducer

Steve

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Psychobilly

Psychobilly

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Wouldn't coarse focus normally be achieved by moving and then ideally locking the mirror and the rear cell focusser then only needs minimal travel to attain and maintain fine focus?

It looks like you possibly have the wrong Moonlight to use with the SCT ... at least if you want to fit the reducer into the front end of the drawtube :)

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MartinB

MartinB

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I got a moonlight SCT focuser with the same flange which houses the reducer. The scope wouldn't come to focus when using my active optics unit. In the end I changed to a lower profile flange and satisfied myself with mounting the reducer in a more conventional way. The reducer comes with a push fit adaptor. The replacement flange cost £25.

BTW the figures given for this reducer are a nonsense. I use mine at about 120mm and plate solving shows this to give an F ratio of 7. My scope is a 10" LX200ACF which gives an excellent flat field. Theoretically the reducer is also a flattener which in theory means it should badly distort the field of my scope. It doesn't! It produces minimal curvature. In otherwords this reducer isn't spacing critical, the flattening is negligible. You would need a BIG spacing to get it down to F6.3.

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  • 3 years later...
Corpze

Corpze

Posted
Posted

Any news on this topic? I am thinking of getting the tall flange for the fr and 0,95 travel drawtube with my nikon Dslr... But i have second thoughts now....

Baader steeltrack might be better?

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