A travel setup, using camera lens

[themos]

Hello, 

I thought I'd share my travel setup as it could be useful to others. 

1. The objective is provided by the USSR-made "Photosniper" TAIR-3-PhS 300/4.5 lens. Focusing is by an extending front element, can be manually stopped down.
2. The lens's mount (42x1 Male thread) is converted to a Male T-thread
3. Female T-thread to 1.25-inch eyepiece adaptor (similar)
4. TeleVue 2.5x Barlow lens
5. Celestron erect image diagonal, 1.25-inch
6. Celestron Plossl 32mm eyepiece

You may have some of these bits already! I originally used a 12mm Nagler eyepiece instead of 4+5+6. I called that combination the "Apollo-Soyuz", because of TeleVue's origins in the Apollo program (Al Nagler provided optics for a training simulator). It provided a monocular of 25x67 spec with an 82-degree AFOV. The drawbacks was an inverted image (bad for wildlife watching) and a straight-through light path (bad for viewing sky objects high up). This design gives an erect 23x67 magnification with a 52-degree AFOV. The Barlow was necessary to give enough back focus for the 90-degree erecting element. The lens has a standard tripod thread, 1/4-20 UNC or 3/8-16 UNC. Length: 15 inches(assembled), 10 inches (lens). All it needs is a nice geared head to carry it on a tripod for astronomical observation, or a monopod for wildlife.

Thanks for reading!

[themos]

The above numbers for magnification were derived from the theoretical formula. In reality, I measured an exit pupil of 1.7mm which indicates a magnification of about 40x. I think that the extra light path distance between the Barlow and the eyepiece is throwing the theoretical formula out of whack.

[happy-kat]

That must be quite small overall and weight light about 1.5 kilo?

What is it like on objects with high contrast?

[themos]

It's about 1.8 kilos, all assembled. The Jupiter sattelites were nicely visible through some haze last night. A bit of chromatic aberration can be controlled with stopping down on bright targets.

[happy-kat]

Neat way of utilising an old prime lens. Good find the m42 to t thread then to 1.25 eyepiece adaptor.

[happy-kat]

Did you do some testing first to see what sort of distance was needed to reach focus with an eyepeice?

[themos]

Hello, yes. I started before I had any adaptors by just hand-holding eyepieces up to the rear of the lens. The M42 lens mount provides infinity focus at 45.46 mm from the flange. Once you get an image in the eyepiece you can sort of tell if there will be enough room for an adaptor to hold it in place.

I am still puzzled about the excessive magnification. Does anyone know how it can be calculated?

[Stu]

Hello, yes. I started before I had any adaptors by just hand-holding eyepieces up to the rear of the lens. The M42 lens mount provides infinity focus at 45.46 mm from the flange. Once you get an image in the eyepiece you can sort of tell if there will be enough room for an adaptor to hold it in place.

I am still puzzled about the excessive magnification. Does anyone know how it can be calculated?

I'm not sure of the calculation, but the extra mag will definitely be because you have the barlow before the prism, not after it. I assume it does focus if it is afterwards?

Nice little setup anyway, very well thought out and put together

[themos]

I haven't tried putting the barlow next to the eyepiece yet. I will be surprised if it works because the rays are bound to be divergent at that point.

[Astrobits]

I did something similar back in the 1980's. I used my Tamron 200-500 zoom lens. It was interesting to watch the nebulae "appear" as I zoomed from 200 to 500. As the f/no remained constant through the zoom range ( I think it was f/6.9 )so the actual aperture in use increased hence the increased light grasp.

Nigel

[themos]

I think the way to calculate compound lens quantities is the Matrix method in Paraxial Optics, for example, here

[themos]

Finally found a good resource that explains how the increased magnification comes about and shows the calculations.

The distance, d, between primary lens (of focal length F) and Barlow lens (of focal length f) for magnification M is d = F - f*(1-(1/M))