Decided on a first scope -- any comments?

[smolloy]

Hi all,

I have decided on the Skywatcher Skyliner 250PX for my first scope, and would really appreciate any comments on this decision.

I have been spending quite a bit of time outdoors with my 10x50 binos learning the night sky, and am starting to get a good idea for things that interest me. I like challenging myself to find particular planets, and to see if I can observe their moons (in the case of Jupiter).

I also like hunting down different Messier objects (although I have never made a record of what I have seen) -- tonnes in Auriga, Orion, and the Andromeda galaxy. The one next to Triangulum has evaded me thus far.....

This scope has a 250 mm aperture, and I believe this would allow me to push to 500x magnification with appropriate eye-pieces. I see that the eye-pieces that come with this would get me 120x and 48x, but that can be fixed easily enough with the purchase of a Barlow and another eye-piece.

One concern I have is that the f-ratio (f/5) is pretty fast, and that I might get better contrast, etc., with a slower scope. On the other hand, there's no way I could afford a 250 mm scope with a slower f-ratio, since that would require catadioptrics, etc. Right?

I don't plan on any imaging with this (except maybe for a bit of webcamming when I get confident), and will just be slewing around the sky by hand looking for stuff. Will that be easy enough with this?

I take it that a tube this size will be pretty difficult to move around? (That's not necessarily a bad thing, but I'd like to know what I'm buying.)

I'd appreciate any comments or corrections you have.

Thanks,

Steve

[Stephen]

It's a good choice

500x is fantasy land though, 250x on the best nights (which will be few and far between I'm afraid!)

[John]

It's a great scope but 500x will be far too much. The reality is that 240x is about the limit on a good night but it's the seeing conditions that limit that, rather than the scope.

The 250PX is actually an F/4.7 which is a fast scope. Orion Optics will make a slower newtonian but it would cost around twice as much as the Skywatcher. You should still get decent contrast and resolution as long as the scope is kept in good collimation.

I could just about lift my 250PX in one piece but lifting the tube and mount separately was much easier. It's very quick to put back together.

[NickK]

You'll find your eyepiece collection growing

Seriously, the after market EPs will make this shine.

[kniclander]

it's an awesome scope but dont get carried away with the high mags. you probably won't often want to push it above about 200 especially with a dob. don't see why you'd get better contrast with a slower scope but as you say, at that aperture, anything other than a dob is in a different ballpark pricewise.

[Ags]

The 250 is a powerful scope, but I doubt you would use 500x on many nights. Manually tracking at such high mags is difficult, and the air is rarely stable enough to sustain an image at such magnification. 150 to 250 magnification is more commonly used in most scopes, but only for a few targets, mainly the planets and the moon, as well as double stars and some tight clusters.

A big dob such as you are contemplating is more often used at lower powers to observe deep sky objects. The low magnification and high light gathering means that the sky background (aka light pollution) is very bright in the eyepiece. So dobs are often taken to dark sites where LP is less of an issue.

The speed of the scope does not directly affect contrast. The contrast is the balance between aperture (light gathering) and magnification (stretching and dimming of the image). A large aperture however struggles to reach sufficient magnification to darken the sky background: with my mak 100x gives a black sky, but the dob would need 250x to show the same dark sky. Stars would be 6 times brighter in your scope, so you will see much more even though the background might be a little gray sometimes.

The last time I looked into this, the 250px tube weighs 15kgs, or a big bag of dog food in my money.

[smolloy]

Thanks for the really speedy replies!

I read somewhere that you could count on 2x magnification for every millimetre of aperture, which is where I got my figure of 500x (that, and the manufacturer is claiming that as well), but I will bow to your wisdom and lower my expectations to 240x (twice that of the included eye-piece).

I live in a relatively badly light-polluted part of southern Sweden, and the comments that Dobs are normally transported to dark-spots for viewing has concerned me a little. I understand that the big aperture and low-mag will mean a brighter sky-background for that scope, but since the stars (or planets or DSOs) I'm viewing are also brighter, don't these effects cancel out?

Perhaps I should also be lowering my aperture-ambitions to allow me to afford something with higher magnification and/or a mount that is easier to control?

Steve

[nightfisher]

maybe consider a 200 dob that can be transported to a darker sky, however the 250 will travel okay in a car, in 2 parts, just decide if you feel up to the job

[Ags]

I don't mean to put you off! The 250 would be a great scope - capable of high mags, high resolution and strong light gathering. Many people do use dobs in suburban settings, and they are more capable even under strong LP than a smaller scope of any design.

Light pollution is a problem not so much for stars but for galaxies and nebulae: these remain dim objects regardless of scope size (the surface brightness of extended objects is limited by the structure of the human eye, not the size of the telescope). So, to view the dim fuzzies, you want to get far away from LP if possible.

[bish]

Great scope. I wouldn't worry too much about eyepieces to start. If you are used to 10x50 there will be a huge amount more to see - and if you take it to a dark site a whole lot more. When you've used it for a bit you will have a better idea of what eps you want.

[John]

The magnification limit we are talking about is due to the limits imposed by typical seeing conditions, atmospheric disturbance, that sort of thing. It applies to larger scopes than the 250PX as well. Smaller aperture scopes will have a correspondingly lower practical max magnification. Very rarely the conditions allow more than this but only very rarely - a couple of times per year perhaps ?.

[ollypenrice]

Forget 500x as the others have said. I never go beyond 200 in our 20 inch Dob because the tracking becomes too much of a hassle and you end up seeing less.

The reason you get slightly more contrast in a slower scope is that the secondary mirror is smaller but the effect is marginal and often exaggerated. It is easier to figure slower optics though, so at a fixed price slightly slower might be slightly better. Once again, this is really only talk! Enjoying the scope is main thing.

Olly

[angusb1]

For what you want to do I think you have chosen a very suitable scope. You will find it is capable of showing you both faint DSOs and detail on planets whilst being portable by one person in a normal size car if you want to take it to a dark site. A planetary newtonian would be better for contrast with its small secondary mirror but would need either to be very long and unwieldy if it was 10" aperture, or be have less aperture if it was the same length as the 250px, and would not be as good on faint DSOs. You won't find it difficult to track objects and with the practice you already have from using your binoculars you won't struggle to find objects either. I would consider a red dot finder or telrad if you can afford one and definately buy a cheshire so you can collimate it. A laser collimator is handy but you need to collimate it as well and if I was only going to use one collimation device at that price it would be a cheshire.

[smolloy]

Thanks again to everyone.

I'm not really looking for a very transportable scope since I really doubt I will have that many opportunities to get out of the LP soup here (busy with work, family, etc.).

I see from the online manual for this scope that it should be possible to collimate it without buying extra kit, but experience will soon show how true that is, so thanks for the tips angusb1.

I've put an order in for this scope, and will be getting the bill today or tomorrow, so any last minute advice (before I pass the point of no return by paying the bill!) would be gratefully received!

Cheers,

Steve

[NickK]

A Baader Neodymium filter will go some way to help cut out light pollution.

[dweller25]

Good choice Steve.

Plan on buying some very good quality wide angle eyepieces and you will have a superb setup.