Reflector Specs.

[Colin_Fraser]

Hello all,
I have been doing more research for my first scope.
I have just been informed it may very well be Christmas before the Skywatcher 14" comes to Australia. So I am now looking to get something smaller.
These are the specs between the 12" and 14"

Focal length 1500, 1700
F/Ratio 5, 4.7
Secondary Mirror both 70mm
Highest Practical Power both 610x
Resolving Power both 0.38
Faintest Stellar Magnitude 14.9 and 21 (whopping 6.1 increase)

I think there may be a mistake with the claim of 21 for the 14" because there is only a difference of 1.3 between a 6" dob and a 12" dob.
Seems like the specs are so close it would hardly make a difference to a beginner like me.
Anyone have any thoughts on this? Is the difference really that critical?

Thanks in advance.

[ausastronomer (John Bambury)]

Hi Colin and welcome to IIS,

Those specs you have researched from highest practical power down are not accurate. This is a fairly common thing.

Highest practical power on 98% of nights is "seeing" limited and not limited by the aperture of the telescope. "seeing" in astronomy terms is the steadiness of the air, which usually is the limiting factor in the quality of the views. Generically, people refer to a scopes maximum magnification as being 50X per inch of aperture. ie. 600X for a 12" and 700X for a 14". This can also vary depending on the quality of the optics. An exceptional small aperture scope will do a lot better than this and a poor large aperture scope will not get close to it.

Resolving power (the ability to detect faint detail), is by the nature of the physics always better as the aperture of the telescope increases. However, the seeing conditions will rarely if ever allow either scope to resolve to the limit of their resolving power.

The faintest limiting magnitude is again dependent on many factors in addition to the aperture of the telescope. However, a big scope will always go deeper than a small scope.

In summary a 14" telescope will outperform a 12" telescope in every respect on most nights. It will go to slightly higher powers, it will resolve slightly smaller details and it will show slightly dimmer targets. Targets will also appear slightly brighter in the larger scope. However, the differences between the two scopes are not large and in some cases fairly minimal. They arec ertainly detectable to an experienced observer. They are a lot less detectable to an inexperienced one.

Of critical importance is the fact the 14" scope is a lot more difficult to store and transport than the 12".

If I could only have one telescope, I would be happy with a 12"/F5 as a great "all rounder"

The added bonus is it costs a good bit less.

Cheers,
John B

[Colin_Fraser]

John,
Thanks for the in-depth reply. You certainly backed up everything I have read. I have only looked through an 8" dob and was blown away by it, so would be crazy not to be completely happy with a 12" dob.

[astro744]

The maximum power based on 0.5mm exit pupil is 710x for the 14" and 610x for the 12".

The theoretical maximum resolving power of a 14" must be better than a 12". Dawes limit = 4.56/D in inches. eg. 4.56/14=0.32, 4.56/12=0.38.

Rayleigh criterion is the other indicator of resolving power and leads to a slightly higher result but nonetheless cannot be equal for both 12" and 14".

Faintest visual stellar magnitude of 21 for the 14" is grossly in error. The value should be more like 15.3, slightly more than the 12". Note magnitude is a log scale and a small rise does in fact lead to many more fainter stars. Note too a better indicator is light gathering capacity which can be compared by the squares of the primary mirror areas. The 14" has 1.36x the light gathering capacity of the 12"

Note the f4.7 of the 14" would be far more demanding of your eyepieces unless they were highly corrected and in any case will show more coma (a function of the primary mirror). A Paracorr will correct for coma but at 1.15x to the focal length/ratio so f4.7 becomes f5.4, f.l.= 1700x1.15=1955mm. You may find that coma is not objectionable on an f5 system provided you still use highly corrected eyepieces corrected for eyepiece astigmatism.

[andrew2008]

I just got the 12" skywatcher dob and couldn't be happier. I did consider the 14" but cost and size put me off. Grab the 12" and with the leftover cash add an Argo Navis DTC and you'll have many, many years of observing to keep you busy.

[Colin_Fraser]

Thanks for the reply astro744. I got the specs from Skywatcher and they did look a bit inflated. Thanks for the explanation.
Looks like I'll get the 12" dob as well Andrew. The tracking dob is my best choice now. I'll just do a bit of research on best type of filters and eye pieces. Getting close to purchase time