A few basic questions...

[Neil M]

Hi,

I'm considering buying a telescope in the next month or so, budget is around £150. My initial interest would be observing the moon and planets. I've been drawn towards the Skywatcher range, and having browsed this forum (very informative) for a good few hours, I thought I'd list some very basic questions with the hope of being guided in the right direction! Help on any of the below queries would be most appreciated.

1. How useful is a motorised telescope? Particularly with reference to the Skyhawk Explorer 130 and 130PM. What's confused me somewhat is the difference in focal lengths between the two - the 130 being 900mm and the 130PM being 650mm - I don't really fully understand the difference. There is also the Skyhawk 1145PM, how large is the difference between a 114mm and a 130mm reflector (don't say 16mm!). Only having used binoculars before I've no perception of how fast objects move through the field of view.

2. I'm only 8 miles NE of the centre of Birmingham and so light pollution is an issue. I'm also bang opposite a sodium street light (I presume it's sodium, it's a nice healthy orange). I'm sure I saw somewhere that you can obtain filters which help cancel out the effect of sodium lighting - how effective is this? How much effect can light pollution have on viewing the planets? If I'm going to go ahead with the telescope it needs to be from my back garden, i.e. convenient!

3. As I'll be observing from the back garden, the field of view is restricted with respect to objects near the horizon. I guess this is the same for most people and so is not a big issue? Presumably objects near the horizon are distorted more by the earth's atmosphere anyhow?

4. I've mentioned the Skywatcher range above - I'm completely open to suggestions however, within budget!

Many thanks,

Neil.

[barkis]

Hello Neil.

With your budget of £150 I would suggest you consider buying from the Astro For Sale forums. SGL has one, and some really good bargains crop up from time to time.

I would concentrate more on the size of the objective, ie, the mirror, or Lens, depending on the type of telescope..

Mirror scopes of course, are the reflecting type, and lens scopes are refractors. Reflectors are normally cheaper to buy, when you compare size for size.

The most popular buy for a first time buyer, is more often than not, the 6" f5 reflector. The f number, is simply the scopes focal ratio, that is, the mirror or lens diameter, divided into the focal length.

So, if the scope was a 6" f5, then the focal length would be 30 inches.So, dividing the focal length by the mirror diameter you get f5.

The mount for the scope, would usually be a Dobsonian, which is a simple frame in which the scope tube sits, and allows free movement in both altitude and azimuth. That means, up and down.

Object tracking is manual, ie pushing the instrument by hand to keep track of what you are observing.

Buying a telescope with tracking motors, and on an equatorial mount, will be at a cost of smaller optics as a rule.

That is why for a first scope, I recommend either a 6" f5, or an 8" f5 Dobsonian. You could get one for the budget you have, and maybe even have some change. Remember. Good second hand instruments do come up, so keep an eye open on the for sale boards. They may be used, but invariably, they have been well cared for.

Ron.

[crovax]

I am quite new to scopes so I can shed some light on "motorized scopes" problem only. I have reflector sky-watcher 130mm and initially I was not going to make it motorized but now that I've faced this problem...

So for example when you're looking on Saturn at 180x, you can't go in the house to tell somebody "hey, check out what I've found" because it will totally run away while you're in house. Also when changing EPs I've found out that you need to do that fast. So when I centered it in 10mm eyepiece I had to quickly change to 10mm+barlow. Again, may be I am facing those problems because I am newbie but on Tuesday I am going to take my motor from the local store. I guess all the problems are not worth 40usd that motor costs.

[Doc]

I am quite new to scopes so I can shed some light on "motorized scopes" problem only. I have reflector sky-watcher 130mm and initially I was not going to make it motorized but now that I've faced this problem...

So for example when you're looking on Saturn at 180x, you can't go in the house to tell somebody "hey, check out what I've found" because it will totally run away while you're in house. Also when changing EPs I've found out that you need to do that fast. So when I centered it in 10mm eyepiece I had to quickly change to 10mm+barlow. Again, may be I am facing those problems because I am newbie but on Tuesday I am going to take my motor from the local store. I guess all the problems are not worth 40usd that motor costs.

This is where widefield eyepieces come in handy as the image takes longer to drift accross your fov.

Scopes with go-to or motors have most of their design expenditure tied up in the technology side of it. This leaves very little for the mirror and mount. I personally would rather have a larger aperture and better mirror then any electrics.

At the end of the day it's the mirror that allows you to view deeper into space not how the scope drives.

[crovax]

Goto is no fun. I was talking only about a small 40usd EQ2 motor that moves your RA. All the rest I prefer doing manually instead of lettign goto steal all the fun. I mean learning constellations, stars and finding the way there is much more interesting than pushing buttons.

In fact I haven't tried out the motor yet (I'll do that next weekend) and if it sucks, then the only fix for the problem is widefield EPs you mention. However I believe they are a bit more expensive and widefield EP collection will take much more than 40usd (I mean price difference vs non-widefield).

[SteveP]

Neil

Welcome to SGL

Buying your first scope can be a little unnerving at he start but you've done the right thing by followingthe threads on sites like this to get a better understanding of what to do.

I'll try to answer your specific questions

How useful is a motorised telescope?

Very useful. I wouldn't be without one although there are advocates of non tracking mounts - known as Dobs. Unfortunately with your budget, you will be limited and at this stage my advice would be to forego tracking and use the cash on the optics

the difference in focal lengths between the two - the 130 being 900mm and the 130PM being 650mm

I wasn't aware that the 130 had such a long FL. It's FL which determines how much the image can potentially be magnified. Thus all other things equal, a 900mm FL will magnify over 40% more than the 650mm

how large is the difference between a 114mm and a 130mm reflector

I won't say 16mm! The aperture determines the light gathering ability of the scope. The bigger the better as a general rule. A 130mm gathers about 30% more light than a 114mm - it's a function of the surface area of the aperture - radius squared x pi

I'm sure I saw somewhere that you can obtain filters which help cancel out the effect of sodium lighting - how effective is this?

They can be ... but at this stage I would concentrate on learning about how to use the scope and then consider whether a filter would be necessary

Presumably objects near the horizon are distorted more by the earth's atmosphere anyhow?

Yes they are distorted. The higher in the sky, the better the viewing usually. You wouldn't be alone in having restricted viewing ..especially in the urban areas

The 130PM scope is avery good starter scope and a number of people here have started with that model. But a word of warning. If you decide that astronomy is for you, you will outgrow it and you'll be wanting something bigger. That's astronomical life

One other bit of advice. Why not give the guys at FLO a ring. You'll get good and impartial advice from my experience.

Good luck

Steve

[crovax]

All this FL talks reminded me about one misterious question I don't know answer on yet.

I own this scope (130mm, 900m)

SKY-WATCHER TELESCOPE

I payed for it 300 USD.

Also there was this scope in the shop (130mm, 650mm)

SKY-WATCHER TELESCOPE

and it goes for 315 USD.

I am really missing something becuase from what I know the second scope is not better. Its only difference is shorter FL, right? Then why does it cost 15USD more than a better scope?

[SteveP]

Crovax

I'm guessing a bit here but the slightly more expensive one is 'faster', ie it is F/5 vs your f/7 ish. The mirrors on faster scopes tend to require a little more machining to allow the light to be reflected more acutely. I note the F/5 version has its mirror described as parabolic. Maybe that's the reason. Aletrnatively, it's just what SW think the market will bear!

[kniclander]

I think the 6 inch dobs will usually be F8 not F5 - at F5 the tube would only be about 30 inches long so you'd have to sit on the flor to use it. I think the 8 inch dobs are usually F6.

The skywatcher 6inch (F8) dob is an ideal starter scope

[crovax]

Yeah, really, now I see. Well, may be that mirror type explains this whole thing however I don't have idea what it affects. Moreover until now I thought that primary mirror in reflectors have the same shape, you know the one designed by Newton in shape of C letter.

[insomnia]

I have the skywatcher 130pm with 650 focal length. The main difference is that the 650 has a superior parabolic mirror, hence more expensive.

[kniclander]

Yeah, really, now I see. Well, may be that mirror type explains this whole thing however I don't have idea what it affects. Moreover until now I thought that primary mirror in reflectors have the same shape, you know the one designed by Newton in shape of C letter.

I am no expert but logically a long FL mirror and short FL mirror can't have quite the same shape as they have to focus light at different points, ie one nearer and one further away. I think it's harder to make a decent short FL mirror because it has to be more curved. no?

[The Warthog]

I added motors to my EQ3-2 after about a year. I had been using a GEM for 6 years or so with manual controls, but now I wouldn't be without the motors. They allow me to watch a target for a long time with my hands in my pockets, or drawing, or to go inside for a cuppa and come back with the target still in the scope.

One important difference between a 114mm scope and a 127 or higher is that most 114mm and smaller scopes have spherical mirrors, while most larger scopes have parabolic mirrors. This makes a great difference to your ability to get the most out of your scope.

[Neil M]

Thanks for all the advice - very much appreciated. Should additional funds become available (which would mean caving in to my wife's constant nagging for an iphone!) would the Skywatcher 6" Dobsonian, Skyliner 150P, be a better bet than the Explorer 130PM?

What are the differences in practical terms? Those dobs look like quite hefty beasts.

Thanks again, Neil.