Reflector Optics

[Lost In Space]

Simple question. I want the simplicity, cost effectiveness and power of a newtonian scope, without the awkwardly placed eyepiece. So, I was wondering if any of these designs may work to bring the eyepiece to a more comfortable viewing position?

[DavidU]

Blake, the problem with both designs is that the light cone with a F7-F8 requires a 1st mirror over 50% of the diameter of the primary.
Also you have drawn the light cone going to a focal point then going back out.
The lower drawing is very vaguely like a Shmidt Newtonian with out the corrector.

[DavidU]

like this

[Lost In Space]

Oh ok :P

So what would you say the best type of telescope for a first time builder is?
I know newtonians are simple to buy the parts and construct, but in the future when I want to build a large scope for visual and astrophotography, I want one with a nasmyth focus, so I figure I might as well try to build a small version now.

I was thinking cassegrain or gregorian with nasmyth focus, but I can't seem to find anywhere that sells cassegrain mirror sets, or gregorian. :shrug:

[torana68]

So what would you say the best type of telescope for a first time builder is?
a small Newtonian

I know newtonians are simple to buy the parts and construct, but in the future when I want to build a large scope for visual and astrophotography,

... you'll want a bigger Newtonian then (edit what do you want to get photos of ? fuzzies or real things like planets :) )

I want one with a nasmyth focus,

I like this one as well, purely for giggles as an extra mirror means less for your eye, the extra cost would buy me a bigger newtonian mirror.:shrug:

I was thinking cassegrain or gregorian with nasmyth focus,
google is your friend , if you have uS$2k Royce will sell you a good set, anything odd will cost you. You might be better off with a ..cough... refractor :sadeyes: ....cough...

[Rod]

Hi Blake,

Two designs you could research are:

low rider dobsonian like one of these:

http://www.biophysik.uni-freiburg.de/Reiner/ATM/lowrider/lowrider_e.html

or a chiefspiegler (not to be confused with a schiefspiegler) recently described in Sky and Telescope. A chiefpeigler is an off axis scope that uses two corrector lenses to address the aberrations caused by tilting the main mirror. A flat secondary focuses the image part way down the tube. I know a 20 inch F8 version has recently been built and the observer does not need a ladder. There is a yahoo group called spiderless devoted to the design. Ed Jones who designed it has a site here:

http://home.isoc.net/~ejones/ (http://home.isoc.net/%7Eejones/)

Regards,

Rod.

[Ian Robinson]

For a big newtonian , or dob , if you want to keep your feet on the ground when looking through it , or keep your feet close to the ground , ladders , soft ground and dark conditions are an accident waiting to happen IMO , folded newtonian system is a good idea.

Big primary, flat secondary (about 1/3 size of primary) and regular sized for the primary and f number tertiary diagonal.

[Lost In Space]

Sounds good to me! :P

I agree with your safety concerns. As Theodore Roosevelt said, "Keep your eyes on the stars and your feet on the ground".

By folded Newtonian, do you mean a 3 mirror Newtonian or a 2 mirror Newtonian?

[nightstalker]

I enjoyed having a chat to this scopes owner /maker last but didn't chance upon a look through it.

wonder if hes going to bring the bigger version this year.

[skies2clear]

You didn't say roughly what size (aperture and f ratio) you wanted.
If it isn't too big, say up to 10" or 12", you don't have to have your feet off the ground. Or even larger apertures if the f ratios are fast (f5 or less).

The Newtonian is so popular because of some of the reasons you stated, ie. simple, cost effective, performance, etc.

As soon as you consider Cassegrains, schiefs.. and other designs, you get more and more complexity and cost. As soon as you fold the optical path more, you compromise performance in several ways (more reflections = less light; larger central obstruction (not the schiefs) = less contrast,,,).

In any case, if you wanted cassegrain optics, there is at least one place in Oz that can do it (Deep Sky Optics).

[Ian Robinson]

3 mirror (the secondary is a flat that's aligned normal to the optical axis) and then a tertiary that's a regular sized diagonal between the primary and the secondary.
That's what I'll build one day after I've made my jump from 10" --> 14". The next step will be 22" and that will be a folded f3.5 if I ever get there.
This what I calculate for my future beastscope.
Summary
Primary Mirror Diameter (D1)550mm
Focal Ratio 3.5
Mirror Focal Length 1925mm
Telescope Tube Length 1118mm
Eyepiece Height 661mm
Optical Flat Diameter (D2) 256mm
Optical Flat Position (L) 1118mm
Elliptical Flat Minor Axis (D3) 136mm
Elliptical Flat Position (To Optical Flat) 457mm
Obstruction Ratio 46.6%
Angular field 1.31 Deg
Designed for 35 mm imaging and a focus point 350mm from c/l.

Each of these 10" --> 14" --> 22" folded give an increase in 100% of light gathering (based on area of unobstructed primary mirror).

[Lost In Space]

That's going to be one awesome scope Ian! :P

Just on the note of central obstruction, if say, you were to have an 18" folded Newtonian with a central obstruction of 33% (5.94"), that would leave an unobstructed area with a diameter of 12.06". Does that mean, that I could go with a 15" unfolded Newtonian with a central obstruction of 17.33% (2.6"), leaving an unobstructed area equal with a diameter 12.4", and get the same results? :question:

[Ian Robinson]

Yep , it'll be a beast , and I'll have to save my pennies to do it. I'd be buying the optics and critical tube hardware and will need a much more capable equatorial to mount it on. Talking lots of dollars.

Light gathering area probably.
Wrt contrast ?? I'll leave that to more knowledgeable folks to comment on.
Wrt how well the system will illuminate a sensor ? play with http://www.catseyecollimation.com/designie5.html and you'll get illumination vs field diameter.

Download the XLS program here http://www.oldham-optical.co.uk/index.htm and play with it and it'll give you an idea what will be practical and will work. I'm in no way an optical expert, but the guy who wrote the program is.

Obviously if I was only interested in looking through the beast I'd need a smaller secondary (and less obstruction) but I'll want to do mostly imaging.
A higher f no will also reduce the secondary size.
I considered a classic newtonian but I don't want that long a OTA, and I really don't want to deal with the cost of going for fancier arrangements.



Will make do with my 10" newt and Atlux for now. Almost ready to upgrade my OTA to a DIYS carbon fibre tube (have the cloth, just need to make my mandrel) as soon as it warms up a little more),ill make my OTA considerably more manageable and lighter, then I'll get cracking on the roll off roof shed.
The current 10" and Atlux are real chore to lug out of the garage and set up for any imaging and I'd rather avoid doing all that heavy lifting at the start and end of sessions.
Been getting more use out of the old CG5 (D/A version), my old telescope (the guidescope) and my 135mm f2 and 300mm f2.8 lenses for imaging. Haven't had a chance yet to try out the autoguider, but now I've modded the G5 handpaddle to interface with the autoguider.