Thoughts on a camera lens as guide scope

[DarkKnight]

I'm about to have a crack at guiding and have been doing my homework on guide scopes. Initially I'll be imaging with a DSLR and my current lens.

Being an OAP and not flash with cash, the thought crossed my mind that I may be able to use one of several Canon FD lens that I have, including a 70-210mm f4.5 or a 200mm f4 as a guide scope. I'm assuming the optics would be on a par with most guide 'scopes.

I'd be looking at pairing it with a basic guide camera, something like an ASI or QHY.

Anyone done this exercise and could give me some pointers on the adapters etc needed, and importantly, if it's feasible.

Cheers

Kev

[bojan]

FD 300mm F4 is excellent lens for AP (with external aperture to step it down to F5 ~ f5.6

[Merlin66]

Kev,
I've been using an Ol' Zuiko 200mm f4 lens as an electronic finder on my C11 for many years.
I've coupled the lens with my ATiK16ic (because it was available) - worked very well....
Recently needed the ATiK16 for spectroscopy imaging so replaced it with the DMK51 I had......again the results were very good.
I made up a 1.25" adaptor using the body of a lens extension tube and a machined aluminium holder, epoxied in place.
(This is illustrated in Astronomical Spectroscopy for Amateurs, Fig.12.13, p 154.)
I think if you search eBay you'll find some lens to T thread adaptors, which, with the addition of spacers could do the job.
Re suitable cameras...I think the ASI120 FOV is pretty small, I'd be looking for a larger chip - maybe the ASSI174 or similar.
Finding mounting rings or a good method of rigidly aligning the electronic finder is another interesting challenge!!
Just my 2c

[DarkKnight]

Thanks bojan.

I have the excellent Nikon 300mm f4 + 1.4 T/C for imaging.

@ Ken.... Thanks for the input and in particular to hear what I had in mind is feasible. I had to do a refresher on the designations for small sensor sizes, so thanks also for that.

I have an unused lens mount ring that I can get drilled and threaded to fit three alignment screws as per your illustrations.

This is what I love about forums, getting the right advice to turn ideas into realities.

Cheers

Kev

[DarkKnight]

Just been looking at the specs on the ASI174MM and ASI174MC and can't see any difference although I'm sure there are some as the MM is mono and the MC is colour. :shrug:

[JA]

Hi Kev,

Using the 300mm f4 would be good idea from the mounting perspective as it has a good mounting collar, but do consider that depending on the size of the sensor in the autoguiding camera, there will be a field of view crop compared with the 300mm at Full Frame and the apparent focal length of the lens will be much longer than the 300mm FF Focal length by a multiplier equal to (FF Diagonal [~43mm]/Autoguider Camera Diagonal [??mm]). You could butcher a rear lens cap to try out the camera on the lens, bearing in mind the CCD image plane needs to be about 46mm from the lens mount flange for focus.

Best
JA

[DarkKnight]

@ Ken

I checked the specs on the ATiK16ic (from Craig Stark's review) and he states that it has a Sony CCD sensor measuring 4.9 x 3.6mm which if I read it correctly is pretty much the same size as the 1/3" CMOS sensor, 4.8 x 3.6mm, used in the ASI120.

Ken, in my ignorance I'm probably missing something obvious here, maybe CCD vs CMOS or something, but I'm a bit confused and freely admit that it doesn't take much to confuse me in this area.

@ JA

Thanks for your input. Something else to consider.

PS: I should probably add that I'm intending to use my HEQ5 Pro with a Losmandy Side by Side plate, camera and imaging lens mounted on one side and guiding lens and camera on the other side.

[Merlin66]

Kev,
Sorry mate, I was actually using the ATiK 316icS which has 6.49 x 4.83 chip. The DMK51 is 7.0 x 5.3 mm
My suggestion was only based on getting a maximum FOV from the electronic finder....
You can use CCDCalc to verify the probable outcomes...
http://www.newastro.com/book_new/camera_app.html

[DarkKnight]

Thanks for clarifying Ken.

I've just had another look on ZWO's website and they have the ASI178MC ( https://astronomy-imaging-camera.com/products/usb-3-0/asi178mc-color/ ) with the latest Sony sensor sized 7.4 x 5mm.

And it's $200 cheaper than the ASI174. :)

[DarkKnight]

Thanks for the replies.

I put it in the 'too hard basket' and bought a WO 50mm 'scope and a ZWO ASI120MM-S