Low profile focuser decisions

[jambouk]

I've just got an old 254mm blue Skywatcher newtonian up and running which I bought many months ago and only have now got around to using.

I want to do some DSLR imaging with it, but need a low profile focusser (and I also want a dual speed).

There are three options available to me:

£125 http://www.firstlightoptics.com/skywatcher-focusers/skywatcher-dual-speed-low-profile-1252-inch-crayford-focuser.html but I'm not sure if it will be chunky enough to hold the DSLR nicely.

£200 http://www.harrisontelescopes.co.uk/acatalog/Baader_Steeltrack_Newtonian_Crayford_Focuser.html but not sure if it will fit straight onto my scope without the need for additional holes / or plates to make it fit, and whether it will be too heavy and sag.

£200 http://www.firstlightoptics.com/moonlite-focusers/moonlite-cr-dual-rate-tri-knob-crayford-focuser-for-newtonians.html second hand version of this with a base plate which will make it fit my OTA without the need for extra holes.

Any comments?

Thanks

James

[TwoPi]

Can't comment on the first two James, but I have the MoonLite CR2 and it's a quality bit of kit. As you say, the Newtonian installation baseplate ensures you don't need any extra holes.

I'm visual-only so never tried attaching my DSLR to it, but it is rated for 2.7kg of load so can't imagine you'll have problems from that perspective.

Good luck with whatever you go with.

[jambouk]

Thanks. I'm drawn towards the Moonlite.

With regards draw-tubes, does the standard 50mm one even stick into the OTA? I wish there were pictures with measurements of how much draw tube was inside the OTA when fully retracted, and how much was out when at maximum stretch for each of the draw tubes lengths.

Decisions, decisions.

James

[Moonshane]

although I am not an imager if you are imaging, I'd personally suggest the Baader rather than the Moonlite. The Baader has steel runners which cannot mark like the drawtube of the Moonlite and I heard good feedback from various sources that the Baader is less likely to suffer from slipped when under load (i.e. when gear is hanging off it rather than sitting on it). I suspect a lot of Moonlite tubes which have the 'standard marks' described in for sale ads have been overtightened to the point where bearings fail or at best scrape the anodising off the drawtube.

even though I am visual only, I'd probably buy a Baader again rather than a Moonlite when it comes to replacing my other dual speed focusers (Revelation).

[TwoPi]

Fair point Shane makes, I've not experienced any slipping with mine but then again never had more than about 1kg of gear sitting in it.

To answer your question James re the 50mm draw tube, no it doesn't stick into the OTA even when fully retracted. I believe however that the 38mm version is usually recommended for imaging?

[jambouk]

Thanks.

That is one thing I don't understand.

Say the focuser needs to pulled out to 20mm to achieve imaging focus, then surely both the 38mm and the 50mm draw tubes can achieve this, and neither project into the light path inside the OTA; so why is the 38mm draw tube better for imaging? the 50mm one would also allow visual or afocal imaging with an EP and a camera on top of that which my Baader EPs allow).

James

[Moonshane]

I think it's likely to be connected to flexure. the longer the drawtube the more pressure is potentially on the whole system in terms of staying true? might be complete rubbish but it seems logical. personally, I believe that almost every aspect of astronomy is about compromise to achieve what you want. the perfect scope for imaging is not likely to be the perfect one for visual so you either compromise on one in between or on spend and buy both.

[TwoPi]

Can't help you there I'm afraid James as I've never investigated imaging.

At a guess I'd say it was because for visual work you want the focal point of the scope to be 'inside' the drawtube so that when the eyepiece sits inside the focuser it is a small distance from the focal point to bring the image into focus, whereas with prime focus imaging you actually have to get the sensor to the focal point of the scope and so the focal point needs to be slightly beyond the drawtube. I could be talking complete rubbish here though as I've never looked at this properly - sorry.

I'm sure someone helpful & more knowledgeable will be along soon though!

[jambouk]

Thanks again both.

Whatever the logistics, i think i need the 38mm one for dslr imaging. I'm going to get an old barlow and remove its optics and convert it into a 2" extension tube for visual stuff.

Thanks again.

James

[dweller25]

Have you seen this - it's a rack and pinion .......

http://www.telescopehouse.com/acatalog/Revelation-Superfocus-2-inch-Newtonian-Rack-n-Pinion-Focuser.html

[jambouk]

No i hadn't seen that. I'll have a better read, hough it doesn't seem to come with any bits to mount it.

James

[laser_jock99]

Have you seen this - it's a rack and pinion .......

http://www.telescopehouse.com/acatalog/Revelation-Superfocus-2-inch-Newtonian-Rack-n-Pinion-Focuser.html

Beat me to it!

[jambouk]

Cheers.

I'm going to go down the crayford route as it's one i've got some existing experience with, with my maksutov. But thanks again for showing me wider options.

James