5" GoTo.. or 6" EQ mount?

[themos]

I got mine at U.K. Astronomy Buy & Sell - it came at £330, I think, and it was only a few months old.

[pvaz]

If you are still unsure about having no computer help, you can buy an Orion xt6i (6") or xt8i (8").

They are "push-to" dobsodian reflectors. No motor to keep track but they tell you where to point.

I don't know the price in pounds. In euros (in Portuguese sites) the xt6i is around 500€ and the xt8i, 650€. That may be a bit above what you have planned, but maybe someone here can point you to a cheap UK Orion seller.

There's also a promotion for a Xt10i here: xt10i

It's only +30€ then the xt8i I just ordered. I only found out bout it the day after.

[pvaz]

Erm... I just checked the rates, the Brit. Pound really went down against the euro recently. I thought 1 pound was like 1.35€, seams its only 1.09€ now.

[Macman]

Thanks again. I have checked out and see there is a good deal on a (discontinued) Celestron Advanced C8-N - but comes with only a 20mm Plossl eyepiece. Can someone please advise what the 20mm eye-piece can magnify at and what other eye-piece I need? I aim to use the scope for a planetary, DSO and nebulae looking?

Also, as it's discontinued, should I be concerned by this?

[ollypenrice]

Hi,

To calculate the maginification of a given eyepiece in a given scope you just divide the focal length of the scope by the focal length of the EP.

While it would be nice to observe the planets at over 300x it is rarely possible, or at least worhthwhile, because the 'seeing' (level of atmospheric turbulence) does not allow a good image to form.

I would suggest about 200 x as a decent planetary/lunar compromise if you only havelimited eypieces. However, for the deep sky you would want the wider field of view afforded by a magnification of 30x or even less.

In an ideal setup you'd have the EP options to take you from about 15x up to 300 in incremental steps. One dodge is to have a Barlow lens which doubles the focal length of any EP/scope combination. (I say doubles because most people go for a 2x Barlow but others do exist.)

A propos the Dobsonian mount it has rightly been said that they are very simple. However, they are - or can be - by far the BEST alt-az mounts as well. Often made of wood, which has little tendency to resonate, they are very 'dead' when bumped and yet very easy indeed to move. (We have a big one, twenty inch, yet it moves at the touch of a finger. If you bang your head on it it's your head that resonates!!)

In terms of getting the most out of your evening GoTo mounts can be equatorial or Alt-Az. The equatorial ones do need to be roughly polar aligned, which takes a bit of time and might cost you a few minutes at the EP. Alt-az ones just need to be level.

Olly

[acey]

would the 5" scope be able to pick out individual stars in the Hercules globular cluster M13?

Just to add a couple of points to all the excellent advice that has been given.

You want to see stars in M13. I've done this with an 8-inch dob but you won't do it with a 5". Anyway it sounds like you've moved onto the idea of 8-inch.

I wouldn't recommend getting a dob of larger aperture. Plenty of people go for 10 or 12 with their first scope and are perfectly happy but I went for 8 and didn't regret that. Easy to carry the whole lot in one piece (handy when it happens to be in the wrong part of your garden), rarely needs collimating (don't even think about it for the first year or two), and it'll show you hundreds of DSOs with an interesting level of detail (once you learn how to tease it out through averted vision etc).

But if you really want to see DSOs then the single most important tihng is the quality of your sky. Ideally you want to be able to see the Milky Way with the naked eye. Then with practice an 8-inch will show you spiral structure in M51. But if you're in a light-polluted place where you struggle to see all the main stars of Ursa Minor, you'll struggle to find M51 with the scope, and when you get there, there won't be much to see.

Planets are fine from a light-polluted site - with my 8-inch I resolved the disc of Uranus from my back garden. But to see stars in M13 or arms in M51 I had to drive to the darkest place I could find.

When I first got the 8-inch I thought it was huge. After a year or two it felt just right, and after about 7 years it felt too small. So then I got a 12 inch, which currently feels huge. But it still fits easily in my car.