Mirrors, faster, slower, f, etc.

[tims]

Not sure if this is the same publication as described below but go to:
http://www.mso.anu.edu.au/cas/documents/NEWTNOTE.DOC
for a copy of "Newtonian Notes" by Peter Francis.

Tim

Originally Posted by ngcles

Hi Afro & All,

..........

I’m not sure where you would get “Newtonian Notes” nowadays. The last time I saw it was at BINTEL. Others may still carry it. It used to be only about $20-odd, about 150 A4 pages, cheap type-face, hand-drawn illustrations but had a wealth of good, practical advice and wisdom on how to design a Newtonian. The other book mentioned on building dobsonians is also mandatory reading but tends to concentrate more on building the thing and engineering it.


Best,

Les D

[Wavytone]

If you are trying to decide on f/ratio, bear in mind there are two limits:

- the lowest useful magnification (and longest useful eyepiece focal length), which is set by the focal ratio of the scope, when expressed in terms of eyepiece focal length this rule equates to 6mm x the focal ratio ; and

- the highest useful magnification, which is set by its aperture (x1 to x1.5 per mm of aperture).

Now look at eyepieces available on the market... Lots to choose from in the range 7 - 30mm, a few above 30mm to to 55mm in 2" barrels and a few below down to 2.5mm.

If you choose say an f/4, an f/7 and say an f/12 scope for comparison, then work out the eyepiece ranges, you will find that an f/7 scope allows a 10:1 range between highest (4mm eyepiece) to lowest magnification (42mm eyepiece) with readily available eyepieces. With f/4, it's fine for low powers but forget high power. With f/12, high power but forget really low powers.

So... questions:

1. Why do people opt for Newtonians faster than f/7 ? Size, and the difficulty of making a stable mount for it. It's a lot easier if the scope is f/4 to f/5.

2. Why do people buy SCT's (f/10) from Meade/Celestron ? Because they are very compact, and at f/10 give good views of the planets.

3. Why doesn't anyone build an f/7 Newtonian ?
Well... I don't know. 30 years ago they did, and they work spectacularly well from personal experience, BUT they are long.

Ultimately the choice comes down to what you want to observe - if its planets - go for f/10 to f/15.
If its widefield views of the Milky Way, f/4.
If you want an all-rounder that can do both well, choose f/7. For this reason a lot of the f/7 refractors are probably Nirvana, from the perspective of a visual observer.

Note also you should also choose eyepieces to match the scope.

[norm]

Great thread Af.

Even greater discussions/knowledge sharing between Mental and Les

I enjoy keeping my feet on the ground when observing. I'm 6' using an f4.7 14.5" truss dob. My feets firmly on the ground and on the odd occasion at zenith I may need to go on tippy toes....my point here is think about the portability/convenience aspect here. Ask if you want to be lugging another item out during observing time, whether it ends up being just a stool, step ladder or ladder.

I don't use a parracor (to reduce/remove coma aka seagulls), but having plonked one in, I do see the subtle differences of stars on the edge being pinpoint. To that end, one is on the way. I prefer using premium eyepieces when viewing as well.

At F5 I think you can most definitely get away without one.

Cheers,
Norm

[Geoff45 (Geoff)]

Originally Posted by Bassnut

Beware charitans that use "fast" f6 or less scopes, you will lose your self in boring done-that widefield rubbish that pollutes our LCDs and dillutes the pure pursuit that is true "deep sky" astrophotography

C'mon Fred, you can't just quote focal ratio. An f5 scope and camera with 6 micron pixels will give exactly the same speed, image scale and resolution per pixel and the same difficulty of guiding as an f10 scope of the same aperture with a 12 micron pixel camera. Also, remember the "myth of focal ratio" and also here
Geoff

[Bassnut (Fred)]

Who has 12 micron pixels?. Id had a few when I said that , I was kidding (a bit). The trend is towards faster scopes (wider FOV) and bigger cameras (even wider) which has more to do with beautiful hi-res photography, which I like and appreciate, rather than "exploring the deep sky".

By definition, the wider the FOV the more generic images must be, other than in technical quality, which is now where effort seems to be directed.

[Geoff45 (Geoff)]

Originally Posted by Bassnut

Who has 12 micron pixels?.

See here . Finger Lakes ProLine 9000 with 51.9mm diagonal and 12 micron pixels. Yours for only $US10495.

[torana68 (Roger)]

I am wondering, I understand the issues with fast mirrors, but what about the other end? Ive read of f10 and more but is there a point where "something" starts to suffer (probably image quality if im on the right track) with mirrors slower than F10? at what point ?
ta
Roger

[Bassnut (Fred)]

Originally Posted by ghsmith45

See here . Finger Lakes ProLine 9000 with 51.9mm diagonal and 12 micron pixels. Yours for only $US10495.

Yes, I know, sigh, now show me how many 1000s of IISers that have one he?. Im talking about main stream amature imaging, not obscure specialist gear.

[wasyoungonce (Brendan)]

Well I have to say that using a fellow IIS'ers 8", F4 scope for astrophotography has ...opened my eyes!

The image scale is smaller but the sub exposure time is..."most excellent" compared to my C8.

But the considerations are:

Will my G11 be able to handle a 8" or 10" short FL scope given the longer tubes;
These short FL scopes are cheap!
The short FL scope really do need a Baader MPCC or TV Paracorr.
Much shorter subs on a short FL scope...and better pixel scale especially wrt using a DSLR.

This really has me pondering. Should I change direction & maybe get an F4/F5
newt?

Anyway I don't expect answers...just musing and reading the fine input buy others!

[Bassnut (Fred)]

Well , an F4/5 newt is an easy way to get awesome pics, cheap, shorter exposures, less demanding guiding (FL wise anyway) and an 8" should be fine on the G11. I had a 12" f10 SCT on it and although marginal I would think, despite the long tube, an 8" newt would be viable. The MPCC aint expensive. You may end up over sampling with a DSLR, but thats a minor problem, better than undersampling

There are many technical "advantages" in short FL imaging, especialy mount wise, its just less demanding all round, just be ready for same as pics, not that thats important in the scheme of things I guess, depends on what you want, horses for courses.

[wasyoungonce (Brendan)]

Thanks your you input Fred.

Problem I see at the moment is I'm already badly oversampling with my 450D.

C8 @ F10 is around .54 arc secons per pixel. Well my mount is currently around 5-6 arc seconds guiding accuracy according to PHD & PECPrep. Obviously I'm over sampling badly but so far the stars are reasonably round but oversampled.

With a .63FL reducer I'd get .85 arc seconds per pixel, better but not great.

With a F4, 80" I'd be at 1.34 arc seconds per pixel...much better but still outside my mounts ability..yet probably acceptable.

I guess in the future I'd get a shmick camera but to be realistic I'd probably stay with the DSLR for the next 2 years & eeek the best I can from it.

Which makes an F4/F5 Newt look real nice for little outlay! Including an MPCC.

The last thing is the learning curve. I suspect I'd be better suited to a shorter FL scope ...kind training wheels..until I earn my stripes.

But all that said...I still love my C8 and probably need to explore it's potentials before I make drastic decisions.

edit:

I'm really want to concentrate on DSOs, mainly Galaxy & nebula...which cover about all of them.