Is a 5" mak enough for the skies of ireland?

im a beginner in the field, looking for my first scope . ive been using a cheap reflector for a while but i only had it on loan. Im thinking a 5" reflector would be good, ( starmax have a good one) but is it enough for the skies of ireland? people say that the skies of ireland will allow a max magnification of 200-250 , and a 5" primary, with a focal length of 1500mm and a 6mm eye piece would allow this ( at the scopes upper limit). should this be enough or should i save for a while longer and try for a 6"? my main plan is to view planets, but occasionally to give deep space objects a look also

A mak is a good choice for planetary viewing, but do keep in mind that the seeing should not really limit the size of the scope you use generally. Remember that the bigger the scope the brighter the object will be through the scope, this is especially inportant when looking at clusters and nebulae, also clusters are resolved with a 12" scope are are not with a 6" scope at any magnification.

If you have a big scope and feel its been effected by bad seeing, you can always stop it down to 8 or 4 inches.

so there are many deep sky objects i wont see with a 5". im sure theres still plenty i can see with a 5" though? i dont want to limit my self just to planets.

Even in Ireland, the useful magnification still depends on aperture on many nights. While there are certainly some nights that won't support even 200x in any scope, there are also a few nights that will support everything you have.

Over the last year or so, my records indicate I've observed planets at the following mags:

around 180x: 1 night
around 250x: 6 nights
around 400x: 12 nights
around 500x: 2 nights
around 650x: 1 night

This data is slightly biased by the fact that I'm mostly a DSO observer. If the seeing is bad, I often don't even bother with the planets. But you can see that even in a single year there are opportunities for more magnification. (Keep in mind that these observations were taken on a 16" SCT, which could support even more in steadier skies.)

Aside from magnification, there's also resolution. The resolution of any scope is relative to its aperture. (The edges of the aperture cause diffraction which smears detail below a certain size. This size is directly relative to the aperture causing the diffraction.) Note that atmospheric seeing also smears small details so a scope capable of resolving 0.5 arc-second detail won't do much good when the seeing's 2 arc-seconds.

To put things in perspective, the nights I used 200x were probably around 2 arc-second seeing, while the nights I used 500x or 650x were around 1 arc-second seeing. (I saw a lot of 0.5 arc-second nights in California, but I've never seen one in Ireland.) You need to get up into the 8" to 9" range to take advantage of sub-arc-second seeing, but a 5" has a resolution in the range of 1 arc-second, so it's not a bad size for Ireland.

OK, just when you thought we were getting somewhere, I'm going to throw another wrinkle in. Seeing on any night is variable. I spent 2 hours one night chasing Red Spot Jr. on Jupiter. It was a mostly 1 arc-second night, but there were many moments of better, and exactly two moments, each probably 5 - 10 seconds long, where the seeing approached 0.5 arc-seconds and I was able to see the little spot. So if you like to really study planets, then you could make use of more aperture than 5", but only for brief moments (granted, though, these moments are the ones planetary observers chase, and to some of use are well worth the wait).

Wrapping up, a Mak is an excellent choice for planetary. But if you could swing a 6" or 7", it'd be well worth it.

-- Jeff.

so there are many deep sky objects i wont see with a 5". im sure theres still plenty i can see with a 5" though? i dont want to limit my self just to planets.

Yeah, if you're also interested in DSOs then I'd go with a more general-purpose instrument (say a 6" or 8" SCT).

-- Jeff.

im a beginner in the field, looking for my first scope . ive been using a cheap reflector for a while but i only had it on loan. Im thinking a 5" reflector would be good, ( starmax have a good one) but is it enough for the skies of ireland? people say that the skies of ireland will allow a max magnification of 200-250 , and a 5" primary, with a focal length of 1500mm and a 6mm eye piece would allow this ( at the scopes upper limit). should this be enough or should i save for a while longer and try for a 6"? my main plan is to view planets, but occasionally to give deep space objects a look also

The Orion Starmax has a focal ratio of f/12.1 so while fine for planets is not ideal for deep sky. Beginners are far better off starting with a simple 6 to 8-inch Dobsonian, preferably the latter. These usually have a focal ratio of f/5 to f/8 providing a wider field of view for deep sky viewing yet still giving superb planetary views. You can buy a 6" Skywatcher from Greenwitch for £179 or an 8" for £279 including two Modified Achromat eyepieces, which are not of great quality but will get you going. Add a good high power Plossl eyepiece, Sky Atlas 2000 and a red torch for night vision and you've got a nice beginner setup with which you will see loads. Dobs are really simple to use and setup. Replace the MA eyepieces when funds allow.

Don't get hung up on magnification. Aperture and quality optics are more important. Most deep sky observing is done at relatively low powers but no matter what magnification you're using, it's best done with the largest aperture you can both handle/setup comfortably and afford. A big 'un will always outperform a little 'un! Even on good nights, planetary views with the 127mm Mak at 250x will look poor compared to an 8-inch Dob at the same magnification as it has two and a half times the light-gathering power and far superior resolution.