Up Perescope

[mswhin63 (Malcolm)]

Wondering is it possible to join 2 diagonals via and extension tube to produce a periscope eyepice or will this cause problem with eyepiece focal length problems or is there a periscope type eyepiece readily available.

[scopemankit (Chris)]

The focus would occur soon after the fist diagonal and then the focal cone would become divergent. To correct this. a relay lens or two may have to be used to recapture the focus.

[bojan]

"Periscope" type eyepiece is what is used on binos, only Porro prisms (http://en.wikipedia.org/wiki/Porro_prism) are used instead of mirrors (to erect the image).
Why would you want to do this? As Chris already mentioned, the optical path is rather long in assemblies like this.

[mswhin63 (Malcolm)]

Not sure of the term, never thought to type "prism" in the search.

I have a back problem that makes it difficult to bend over effectively (getting a lot of Physio atm). I basically have to do the splits when viewing low (reminds me of my martial arts days).

I was just looking at a way of viewing with bending my too much. I know I expect to recover, but not sure when as I have neglected my pain problems for many years until now.

I remember seeing a device in the movie "Knowning" but wasn't sure whether it was just a prop.

"edit" - I was hoping to use off the shelf products to make it. I am not worried about image inversion.

[mercedes_sl1970]

Hello Malcolm

Was just thinking about your issue of difficulties with bending over. I'm sure the issue of height change in alt/az is a problem for quite a few people. Two options (apart from getting some other type of scope) come to mind - an adjustable chair or a raise-able platform/mount. The fomer would be simpler and the latter would mean more of a commitment in terms of space, etc and any arrangment for a dob would need to be of considerable size.

My thinking for this relates to my own situation - I have MS and for the past 2-3 years have been using a 100mm 45 deg set of binoculars in alt/az. The binos plus mont come to around 20kg. The constant shifting around, heavy weight, changing position, etc ended up being quite tiring - mainly in terms of my legs. I did make a cats perch chair which worked well, but found it a tad irritating shifting it up and down (that's just me). As an alternative, I considered a large manfrotto tripod but was appalled at the price.

I then rigged up a wind up pier out of an old hills hoist clothesline - basically cut down the upper/lower halves, plus the internal spindle mechanism. It required a fair bit of trial and error but, imagine a shortened clothesline base on an EQ6 portable pier (all home-made) with large castors. It could then be wheeled around to appropriate spots on our deck. It wasn't elegant, but... did the job. A coat of white paint did give it a vaguely respectable appearance. (Note this has since been partly replaced with a secondhand Samson quick-set mount - easier for taking in the car!).

The clothesline mechanism I picked up for about $10 at a local recycling place - nobody seems to want them these days. And, they are exceptionally strong and well built. The lateral movement in the top section was overcome by drilling/tapping a hole and using a hand tightened bolt to lock the shaft (if that makes sense).

Anyway, it may not be suitable or feasible, but possibly some sort of adjustable platform could overcome the height changes inherent in a Dob's alt/az configuration? It would no doubt need to have a fairly wide base to avoid toppling. There was another post recently by Houghy (?) about building an electrically raise-able pier which looked very interesting.

Andrew

[mswhin63 (Malcolm)]

I will consider all the options, I may grab some cheap diag just for the hell of it, I expect there may be some light reduction associated with diags but if the concepts work (ie: no image spoils from the long tube) then a good quality diags for reduced light loss. I have thought about inserting a long erecting or barlow lense instead of a tube to maintain good image projection. I usually like to experiement anyway, part of my nature.

Still the chair sounds like a good idea although the car is looking a bit full with a transport mount for the 12" coll DOB. I may get the of-the-shelf chair any way.

[Blue Skies (Jacquie)]

Originally Posted by mswhin63

I remember seeing a device in the movie "Knowning" but wasn't sure whether it was just a prop.

Ah! YES that was JUST A PROP!! There is no way you can physically extend the light path of a newtonian. It is like a piece of string - bend it anywhere you like but you cant extend or shorten it.

Best idea/s if bending over is a problem is a chair and the small table base to raise it as well, just an extra foot in height is a big difference and sitting to observe is quite relaxing, plus you get to see so much more because you're not concentrating on standing up and can put all your concentration into the eyepiece. The stellarchairs are just great, worth the extra freight to get them to WA.

[scopemankit (Chris)]

Malcolm,

The type of telescope you need is a Mersenne. It gives out parallel light which can then be viewed, via a diagonal, with a small (but good) refractor. Clyde Bone published the manufacture of a massive one in Sky & Telescope, Sept 1999.

Find that copy and look at the theory and see if it would suit you. Clyde built the scope and said he would be able to use it from his wheelchair in his old age!

I can't find my copy, so you will have to find that issue yourself.

Good luck!

[mswhin63 (Malcolm)]

I knew that the light would need to be straightened before or after a diagonal. A diagonal has only a short distance to cover after a standard focuser so the path could be short enough to use a diagonal on a Newtonian while afterwards other barlow lenses could be used to provide an extended point. (albeit small)

The lense used Mersenne to straighten the path was he kind of thing I was looking for, but still haven't been able to locate anything as yet.

Thanks for the information, am still interested in this method of observing as it intreged me when watching the movie. I myself prefer to stand when observing, although strange I have multiple injuries that dont make it easy for me to stand. Maybe it is what I am accustomed to over the years standing up while electronic servicing.

Even if it become to difficult or near impossible then I had fun trying.

[mswhin63 (Malcolm)]

It has been an interesting learning curve on optics for me, it seems the key to Mersenne telescope is aspherical lenses , not practicle for most people and typical not to design for the lower market.

Interesting though.

[Wavytone]

The Mersenne is just a pair of confocal parabolic mirrors - concave primary, convex secondary - producing an afocal beam coming through the primary, followed by a smaller telescope some distance behind the primary mrror - a fast lens with an aperture large enough to collect the beam. If the Mersenne primary was say 30 cm aperture I'd expect a 60-80 mm aperture will be adequate for the refractor.

The Mersenne is aplanatic, anastigmatic and achromatic, it contributes magnification without producing an image. In a practical implementation the abberations from the small telescope at the end are what will be evident in the final image.

If the refractor was f/5 and the Mersenne magnification x3, the result is f/15.

You can definitely use two or more folding flats (or star diagonals) to bend this beam where you want it.

The only downside is the number of surfaces involved and the losses resulting, but it could be viable...

As for making the pair of mirrors, it is no harder than making a classical cassegrain provided the optician has a way of star testing the complete system on an optical bench.