Focal Ratio and performance?

[Capt-Crapulence]

is it true that the lower the focal ratio, the 'faster' the telescope is, and therefore the better it is for Deep Space Objects?

I'm looking to buy my first telescope. I've been looking at the Skymax 127 Mak-Cas, which has a speed or focal ratio of f/11.81 ... ? is that the correct terminoligy for what i'm on about? lol correct me if i've already cocked up...

To the point: I would like to be able to get as crisp views as i can (on a beginner's budget) of our solar system, but would of course also like to be able to get the best views i can of DSOs. The Skymax 127, from my research so far, seemed to be a great telescope with pretty good specs and some nice gizmos, particularly for a beginner, and on my budget.

I assumed that I would quite comfortably and simply be able to change filters and eyepieces, in order to manipulate the field of vow and magnification, when wanting to change viewing between planets and DSOs.

However, after reading that lower focal ratios are better for DSOs, I'm wondering if i'll be shooting myself in the foot by going for the Skymax 127, or a telescope with a high focal ratio, if i'm interested in good views of DSOs? or am I right in thinking that a simple change of eyepiece can make the difference in planetary and DSO viewing?

any advice/hints/thoughts/comments/help would be muchly appreciated.

Thanks.

[rowan46]

yes and no. If you are imaging a fast scope is best but if you are using it visually apparture is what you need a 127 mak under a dark sky will give ok views of quite a lot but 5" under urban skies is quite limited. The high focal ratio gives a narrow field of view which can be mitigated with focal reducers and wider eyepieces. But If you really want better views of deep space objects you need a bigger scope. this one doesn't have the electric gizmo's but it's miles better optically

http://www.firstligh...-dobsonian.html (other vendors are available)

I should qualify that as the 127mak has very nice optics. By miles better optically I mean for dso's

[Capt-Crapulence]

ahhhh ok thanks! I suppose if I wanted to do any imaging I'd need an Equitorial mount anyway?

There is a fair bit of Light Pollution where I am, but I was hoping to use the scope for general garden viewing, but also to hop in a car and take it to darker skies.

One of the reviews of that Dob say it's actually quite easily transportable, so it certainly seems like it ticks the boxes! thanks for bringing it to my attention.

have you used it personally, or have a telescope on a that sort of mount? Is it easily leveled on uneven ground, or do you need some kind of adaptor(s) for the base if you were to put it on grass for example?

[AstroJOE]

Haven't used the Don but certainly agree with a lot of what has already been said. I have a 127 mak and live in a semi rural location. It's fantastic for the moon and planets and some of the brrighter DSO's like the Orion Neb but it struggles with the fainter ones. I managed to get reasonable look at Andromeda from a dark sky site but from my garden the view was pretty non existent. For visual astronomy apeture rules which is why I ended up getting an 11inch SCT. Depending on your own preference and budget the Dob might not be a bad call.

[ollypenrice]

For visual use the difference between a long FL scope with a long FL eyepiece and a short FL scope with a short FL eyepiece giving the same magnification is really not something to get excited about.

But the long FL will have a smaller, even much smaller, field of view. Now beware focal reducers. It is by no means certain that a focal reducer in a Cassegrain of any kind will really give a wider FOV because the baffle tube in the scope imposes the final limit in many (Most? All?) cases. I don't know how this plays out for all the scopes out there but I suspect the problem is widespread. A 2 inch long FL EP often reaches the field limit of a Cassegrain.

For deep sky imaging a fast F ratio is utterly essential. If I were very rich I would use nothing slower than F5. F7 is already starting to get painful on some targets.

For planetary imaging a long focal length is essential and the effective focal ratio (extended by Barlows) can be incredibly slow because your targets are so bright. Note that on the planets a cheapo webcam will greatly out perform a DSLR. Strange but true.

[ronin]

Fast and Slow are photographic terms, it relates to the intensity of the prime focus image formed on the sensor.

If you had 2 scopes f/5 and f/10 aimed at the same thing, Orion Nebula, and had eyepieces in each such that the magnification was 100x then the image you see would be the same brightness.

So to a fair extent if you are visual then ignore fast/slow.

The "problem" wih DSO's is that in general they are DIM, very DIM.

So a "fast" scope will have a longer focal length and this makes higher magnifications easier. it can make them compulsary, but high magnifications mean a dimmer image of what is already a dim object.

I bet half the reason for the 127 Mak was the fact that it could give high magnification. The catch is that you probably cannot get 30x out of it which would give a brighter image but a smaller one. This small bright set up is what you need for DSO's at least initially as you have to find the damn things. After that magnification may help or hinder, you have to find out.

For DSO's you need a few long eyepieces with the 127: 25mm 30mm 32mm, I have a 40mm for my Mak and someone in the US makes a 53mm plossl that I have considered.

[Dunkster]

Size matters when it comes to DSOs... the big SCTs and Maks with their high/slow focal ratios are just as capable as similar aperture scopes of other designs, and even a C11 can take in about 1 degree FOV ... which is most things in the sky, excepting those that are probably better viewed through binoculars or a small refractor anyway

One of the differences will be the focal lengths of EPs you buy to accompany it

[Moonshane]

focal length effectively determines magnification at the same aperture and magnification is one of the factors of field of view. scopes of the same aperture will broadly give the same image at the same magnification (but using a different eyepiece of course).

many people worry about focal length and having a huge field of view but the fact it that with very few exceptions, all DSOs will fit into a field of 1 degree.

my biggest scope has a focal length of 1840mm and with a telrad and right angle finder finding most visible DSOs is quite straight forward and all but maybe 5 fit in the field of 1.1 degrees.

[rowan46]

ahhhh ok thanks! I suppose if I wanted to do any imaging I'd need an Equitorial mount anyway?

There is a fair bit of Light Pollution where I am, but I was hoping to use the scope for general garden viewing, but also to hop in a car and take it to darker skies.

One of the reviews of that Dob say it's actually quite easily transportable, so it certainly seems like it ticks the boxes! thanks for bringing it to my attention.

have you used it personally, or have a telescope on a that sort of mount? Is it easily leveled on uneven ground, or do you need some kind of adaptor(s) for the base if you were to put it on grass for example?

You just need a piece of ground flat enough to put the base on and it's footprint is not even as large as the 127 mak with its legs extended. I don't own one but have used many at starparties. I have a c5 which is similar to the 127 mak and an 8" dob blows it away for views.

[Capt-Crapulence]

thanks for all the feed back guys, I really appreciate it!

Am I getting over-obsessive in trying to find the 'perfect' beginner telescope for me? .... probably.

Should I just be diving in and enjoying the pros of whatever Telescope I get, instead of getting hung-up on the cons? considering i'm not going get an 'epic' telescope on my current budget .... probably.

I really like the idea of the Skyliner 200p Dob. It's a bit of a BBW of beginner telescopes ... perhaps lower F/ and less stock Magnification, with more apperture, is more beneficial to me as a beginner?

For the difference in price between the Skyliner 200p Dob and the Skymax 127 Mak, I could probably get an Eyepiece or two for the Dob?

I know a few basics for Eyepices, but I know not what brands to trust or price to pay. advice here would also be greatly appreciated!

Cheers,

Pete

[swamp thing]

Hi Pete

Personally I wouldn't worry about extra eyepieces for now, just get yourself up n running with the scope.

I believe the skylines come with two eyepieces to get you started anyway.

[zedzed]

If you go with the 200 Dob then I've heard that it's worth upgrading the finder to be the right angled one - otherwise it's tricky to look up the finder scope attached to a big tube. It's £65 for that, but you could sell the finder it comes with for £30 and get some of the cash back.

[Capt-Crapulence]

cheers guys, yeah I think i'm'a probably crack on and get stuck into the fun bit

half the fun is learning as you go i guess!

Good shout on the view finder Zed! I can see that being a pain without the right angle

yo Zed, what's the viewing like where ur from? I heard you can some times see the northern lights from parts of Scotland?!

[Dunkster]

There's always upgrades "necessary" for those of us that obsess about our quality time... "Maybe it would have been better if..."

A 200p should last you a long time, unless you get the fever not that the 127 isn't a good scope for what it is, but it can't compete against an 8" scope for DSOs.

The main thing is to get out there enjoying the sky... when the weather lets us

[Capt-Crapulence]

yeah I think you're right there! Will have to just get it and enjoy it... when i get it ... will be a month or so till I can actually afford it lol

Yeah! sheeessshh! bought some cheap binoculars to get me started a couple of months ago - they succeeded in inducing a bit of astro-fever - but I've only been able to use them a few times thanks to the clouds! .... 2 months of clouds? that's excessive even for England!

[Capt-Crapulence]

I meant to ask!

Does anyone know if the Skyliner 200p Dob has some kind of slow controls on the mount?

[ollypenrice]

I don't really agree with an earlier post saying that a degree is wide enough for most things. I don't find that to be the case at all. There are many splendid clusters or starfields which cry out for a wider view, along with one galaxy and a fair few nebulae. I suspect that people discount them not because they're not there but because they won't fit in their scopes, which is a circular argument... I do have a 10 inch SCT here and often feel 'boxed in' by it. Another good point made above is that a wide FOV makes finding things easier.

I'd be sure to have a look at any scope you're considering because they might not be what you want to handle and move around, or they might. In the Christmas Astronomy Now my personal suggestion for a beginner scope under £500 was the 10 inch SW Dob, with the caveat that nobody should buy one without looking at one first! Once you become an astronomy obsessive a 10 inch Dob can come to seem like a pair of opera glasses but that isn't how they strike normal people!

In the end, though, all the scopes discussed here will bring enjoyment.

Olly

PS, No slow motion controls on a Dob but the movement is very sweet apart from close to the zenith, an area known as 'the Dobsonian hole.'

[Capt-Crapulence]

mmmm the price jump from 8" to 10" is fairly substantial ..... although ofc, why have an 8 when u can have a 10 right?

I have been looking at some scopes more in the region of £400 anyway ... i guessed I'd have to think long and hard about how much I want to spend!

that said, the 50% higher price does equate to 50% more light, and thus perhaps makes it a more long-term keepable telescope ...

is a Dob no good for photography, due to the type of mount movement? has anyone used a Dob with a camera?

[Capt-Crapulence]

oh yeah, what's the 'Dobsonian hole' ?

[Moonshane]

because the dob is an alt-az mount (left right/up down only) and the 'sky moves' (it is obviously the Earth that moves) through a curve (right ascension), at the zenith (i.e. right overhead at any point in the night) you find it hard to move the scope when pointed straight up. in reality it's not that hard you just need to turn the tube to move left or right, rather than turn left right - it's hard to describe really but if you were to try and use a dob at the zenith you'd see what I mean - it feels weird.

[Capt-Crapulence]

ahhh right ok, I think I can imagine what you're describing.

so I guess now I need to decide between 8" or an extra £150 for a 10" ....

There's a couple of 'tester' / 'sample' or 'come along and use a telescope' nights being run by a local astronomy shop next week, so I'll have to go down there and get a feel for some different types of telescope ..... won't be great viewing conditions, but hopefully there'll be a Dob I can get a feel for!

[Moonshane]

definitely worth it. the advice of users and experienced ones at that is always better than any sales talk. hope you enjoy yourself, and don't foret to check out dobson's hole when you do. you'll understand then - it's not a big issue though. just wait a few minutes and the object is not in the hole any more.

[Capt-Crapulence]

Thanks Moon-Meister, will look forward to a bit of hands-on with some Telescopes.

Do you personally think the extra 2" on that Dob is worth the price difference?

[Moonshane]

my advice would be dependent upon whether or not you envisage this as your 'last ever scope' - is there such a thing??!!

if yes then I'd say that 10" is a great aperture for longer term use and is in a very useable and movable package. so get the 10"

if no and you possibly plan to get a larger scope in the future then the 8" would be a good first step to 'whet your appetite'. the move from 8-12" for example is more beneficial than the move from 10-12". in this case the 8" would be recommended.

either scope will be great.

[Capt-Crapulence]

cheers m8, yeah I hadn't really decided just how long-term it will be, over than not having no plans to get other telescopes any time soon.

Wanted a nice telescope for as long as I've had a memory lol, so maybe I should splash out and enjoy making up for lost time?

I think i'd like my second telescope to be something expensive that can give me nothing less than 'fantastic' views of the planets.

in which case, a Dob purchased now will probably see good use for a long long time ....

Will chew it over, but I am quite tempted to try and get the money together for the 10" ...