6" Newt is equal to a 10" Newt on planets, Alt/Az .. ?

[Joseki]

I've been going over some basic numbers and I came up with something I was surprised at. More experienced people might say "well that's obvious" of course, or "that's not what I've found".  The reason I've been looking at this is that I'm planning to buy a Skywatcher 150P and I wanted to figure out how the views might compare to a 250mm Newt f6.4 I have.  - I'm ignoring coma and the additional cost of eyepieces (and I'm ignoring maks and refractors).

Ok, my premise:

Mounted with an Alt/Az, no tracking.

Just visual observing, no astrophotography.

Optimum magnification for planets (on an Alt/Az, no tracking) = about x200.

Just in terms of viewing Planets and our moon.

Using the "basic telescope math" in TLO;

The maximum useful magnification M_max = 2.5 * A

or rather for x200 you only need 80mm or to be safe, x300 = 120 mm.

If you assume a 30% central obstruction then the surface area of light collected is equivalent to a circle with diameter  squareroot(0.7) times the total diameter, i.e. 150mm * 0.84 = 125 mm, i.e. the 6" Newt collects the same amount of light as the 5" refractor (in a basic geometry sense).  I don't know how that relates to the maximum useful magnification but if x300 requires 120mm (as above) it seems wise to assume you need 150mm (sorry for swapping units around).

So in theory, the 6" Newt is fine for all the magnification I want (i.e. without tracking or an equatorial platform).  I'm looking at the 150P as I want to use it on a AZ4 mount.  I can accept the coma and requirement for expensive EPs that will probably go with using this scope for planetary or at least just ignore the differences in focal length.

The other thing is, would a larger mirror give me more detail?  - the equations suggest not.

TLO suggest resolving power is = 120/A

i.e. for a 150mm scope the resolving power is 120/150 = 0.8 arc seconds (or should that be 120/125 = 0.96?).  For a 250mm mirror its 0.48 (or 0.57).  So the 250mm resolves more detail? - well actually TLO state even on the best of nights the atmosphere limits resolution to around 1.0 - so according to that both the 150 and 250 are atmosphere limited (on average?).

....

Anyway that's the rather simple maths I went though but does it match up to experience?  I'm going to end up with them both side by side for comparison in a while (since the 150 will be useful anyway), though any differences seen could be due to a large number of factors I guess.  I've read of people resolving detail below the theoretical resolution limit and if you observe long enough you might catch patches were atmosphere limited seeing is well below the 1.0 value (I'm guessing) - so perhaps the bigger mirror allows for occasional glimpses of wonderful detail? - that blows away the 150?

Also, I'm curious as to whether anyone's upgraded from 150mm to 250mm and remember their impression of viewing planets with the new scope. - that's basically the same question though.

ok - I've re-read that post so I hope there's no stupid mistakes since I can't edit it after I hit post (and I really need to get some sleep).

[Joseki]

btw I was thinking that a Skymax 127 would make a good planetary grab and go but I'd like the option to have a bit more aperture to take to a dark site and I like the mechanical simplicity of the Newtonian telescope.

[John]

I moved from 10" F/4.8 to 12" F/5.3 newtonians and noticed a pretty consistent improvement on planetary and lunar resolution. I don't know how that fits into your calculations though ?

A 10" F/6.4 newt would only need a 20% central obstruction.

Prior to the 12" F/5.3 I also had a number of 150mm F/8 refractors (no central obstruction at all) and a 152mm F. 5.9 maksutov-newtonian (19% CO) and the both the 10" F/4.8 and the 12" F/5.3 could show more planetary detail.

Under poor viewing conditions a 6" scope with a small / no CO can get close or even equal a 10" newtonian on the planets but under normal or good conditions the aperture shows more. Those periods of seeing what the aperture can do are really worth having in my opinion 

But thats just my experience - others may vary

The mak-cassegrain 127mm's are nice scopes and very portable. A 200mm F/6 dobsonian would make a nice partner scope for one of those   

[Joseki]

Ok, thanks - I think perhaps that shows you have to take those equations with a pinch of salt.  I'm glad to hear it's worth keeping a bigger scope around.  I'm not very keen on trying to sell off anything (at a loss).  You could be right, a 127 Mak and 200 dob might be a good combination for my budget and habits - it's confusing stuff.  I think the 150P would probably be okay for me too, especially since I'd only have to buy that one scope with my current setup.

[Stu]

Resolution does increase with aperture. The size of the airy disk produce reduces as the aperture increases which results in greater ability to resolve doubles and finer planetary detail.

There is some good explanation here:

http://www.rocketmime.com/astronomy/Telescope/ResolvingPower.html

This assumes good seeing conditions. In poor seeing, I have seen small scopes give much better images than larger ones as they are looking through a narrower column of air.

Just to directly answer your question about detail in different scope sizes....I've recently upgraded to a 12" scope and can see a definite increase in planetary detail over an 8".

To prove the point about the impact of seeing conditions on different sized scopes, I have viewed Jupiter through a 16" dob, and then with it masked down to 170mm ie clear aperture, no obstruction. The views through both were fabulous, ultimately the 16" gives more detail, but if the seeing is poor, the image stopped down is more reliably stable and clear.

Stu

[Mr Spock]

Many years ago I had a Helios 6". Comparing that to the 10" Newt I used to have and the C925 I have now is no comparison at all - they both show vastly more detail than the 6".

[Naemeth]

It's not all about magnification - as others' have said it's about resolution. Mind you, there are some nights' where the seeing is so poor that my Vixen 80M (80mm aperture) gives views equivalent to my 10" dob (250mm aperture).

[swamp thing]

I have viewed Jupiter through a 16" dob, and then with it masked down to 170mm ie clear aperture, no obstruction. The views through both were fabulous, ultimately the 16" gives more detail, but if the seeing is poor, the image stopped down is more reliably stable and clear.Stu

^^ This

[AndyWB]

I went from a 5" to a 10". Under good conditions the extra resolution of the 10" knocks spots off the 5" looking at Jupiter. Under bad conditions, well, there's very little in it. With planetary you really don't need more light, it's the resolution.

Viva La Resolution!

[ronin]

TLO may say 2.5*A but it is usually stated as 2*A (to do with the exit pupil and the eye, not the scope) and I find that 1.5*A is more realistic.

So that would throw everything out.

Concerning light grasp you have not taken the reflectivity into account. Ignoring what a manufacturer says the best you are likely to get is around 90% and you have 2 mirrors so 2x 90% losses and I have seen it said that 70% to 75% is likely to be more realistic for reflectivity.

If you want to view planets through a 150P reflector get the PL variant.

If the idea is will a Skywatcher 150P rivel a 250mm Newt f6.4, the answer is no, the 250 will basically walk all over it. You are comparing a 150 newtonian to a 250 newtonian.

[Joseki]

Thanks for the advice everyone.  It's reassuring.  I guess I'm going a bit crazy at the moment with the weather and the fact that the 10" is currently in pieces (polygon wooden OTA is completed now though, just got to install thermocouple probes, side fan then the optics and rebuild the base) - the other small scopes I have just aren't up to planetary views (compared to how I remember they looked through the 10").  I guess it's worth the effort to complete it.  I'm starting to think 1 big scope and 2 small ones (one for high mag, one for widefield) might be best (plus a really big ultraportable one day when I save up and have time to go out more) - so may come down on the side of the Mak 127 as John suggested.

[RikM]

I have both a 150P and 250PX and there is no contest; the 250 easily gives the best views. I actually find that only 1x per mm aperture seems to be the limit for most observing where I live except for the moon and doubles.

[AndyWB]

 I actually find that only 1x per mm aperture seems to be the limit for most observing where I live except for the moon and doubles.

Funny, that's just what I was thinking the other night. I figured that it was 'cos the seeing is rarely that good, but I don't often use the 130mm at more than x130 either.