Lens as scope

[daroma (David)]

What sort of results /problems are involved using a (top quality) telephoto lens as a visual scope?

FL is shorter than regular scopes so magnification will be limited. We're talking 300 or 400mm FL and aperture f2.8 - f4.

Would the visual quality be similar to that of a quality refractor of similar diameter?

Any comments/advice welcome!

David K
melbourne.au

[Dennis]

Hi Dave

I suspect usability will be the main issue. That is, most telescopes come with a focuser, a mount for adding finder scope (a small, wide field, low power scope to locate objects in before centering in the main scope), means for attaching to a mount, etc.

I have photographically tested a Canon 400mm F5.6 wide open and this leads me to believe that optically, it would make a fine visual instrument.

However, I would need to purchase an eyepiece adapter so I could insert eyepieces to use it as a ‘scope. I’m not too sure if a simple adapter will do; that is, would the eyepiece need to be very close to the lens in order to achieve focus. Would the eyepiece reach focus? Most astronomers have several eyepieces and you would need to check that each eyepiece came to focus on the lens.

Mounting rings for telescopes come in standard sizes and you may require some modification to mount a tele lens?

Also, would you want a nice piece of L glass to be dripping in dew, or outside in –ve temperatures? Notwithstanding their tough design and build quality.

I have focused the 400mm F5.6 using Remote Live View on a 40D and it worked extremely well, although it would have been nice to have a motorised focuser which can be fitted on most telescopes to assist in focusing. The 400mm F5.6 won’t accept an off the shelf motorised focuser, although Bert (Avondonk) has designed a very nice belt drive system for focusing his lenses via a motor drive.

Cheers

Dennis

[Ian Robinson]

I have a suspiction PRO-OPTICS (USA) and EDMUND (USA) ??? sell a kit that do this for most any telephoto.

Can't speak for the image quality - once you put an eyepiece rather than a film or chip in the system - as never tried it.

Check your old S+T mags from about the last 1990s , you'll find it advertised there.

[AlexN]

I have looked through my 300 F/2.8 with an eyepiece via an adapter bought from moggadapters.com.au

The view was rather good, extremely wide at 300mm, focusing done via the lens focusing ring.. (which was VERY fiddly)

I have also organised a M42 T thread to EF lens adapter, which you could then adapt an SCT type diagonal + ep.. which would also work...

As Dennis mentioned, the idea of L glass dripping in dew the way my telescopes sometimes do scares me a little... that being said, a dew heater on my 300mm's front element and on the rear was enough to keep the whole body of the lens warm enough to resist dew...

Bert's belt drive setup would be easy enough to duplicate, even with something as basic as the skywatcher electric focuser. take off the clamp that would normally go onto the focus knob shaft, put on a small pulley, run a rubber 'o' ring from the pulley to the focus ring of the lens.. tap a button to move focus in and out.... Futhermore you could then adapt it to run of a FCUSB from shoestring for computer control...

For imaging I think DSLR lenses are the goods with their very fast apertures, however for visual, it would be cheaper to buy an APO scope of the same size.
300mm F/2.8L 117mm front element = >$5000
400mm F/4L - ~129mm front element = >$5600
400mm F/2.8L "" >$8900
WO FLT132 ~$5000


EDIT : !!2000th Post!!

[Wavytone]

I have a 500mm f/8 mirror lens and 135mm f/2.8 telephoto left over from my (long gone) SLR, the 135mm works OK as a 9X finderscope and the 500mm is great for eyepiece projection of the sun through a 20mm Kellner.

You can use camera lenses at low power for wide angle views but they're a bit of a waste:

(a) they are not designed to be diffraction-limited, so don't expect good images at high power.

(b) in focal lengths up to 200mm they are designed to provide very fast focal ratios - but faster than f/4.5 is a waste with most eyepieces.

[bojan]

I have MTO-100A , Rubinar F10/1000 and Macro Rubinar F8/500 (all catadioptric maksutov design).
They are excellent visual instruments. photographically a bit unsharp.. but only at pixel level.
Portability is excellent (very light and short for focal distance).
Of course, I removed focus stops, so they are no limited now for film (sensor) focal plane only, I can focus them with almost any eyepiece.