Jump to content

First scope - skywatcher 200p or 250px


Recommended Posts

In general, that's not true (it is theoretically possible, but it rarely ever happens). What happens is that the larger scope has a less aesthetically pleasing --less stable-- image, but you usually still see more details using the larger scope, and vastly more in the brief moments of better seeing. For the smaller scope to really see more, you almost have to hand-craft the convective cells in the atmosphere for it (and yield a speckle pattern that slowly shifts on the small scope but is blurry on the large scope; if you actually use small scopes and star test them at high powers, though, you rarely see such a small but shifting speckle pattern).

Of course, some people prefer seeing an aesthetically pleasing image to seeing a lot of detail but in a distractingly unstable image. It is, of course, their prerogative.

One aspect of "seeing" that really does cause more problems on large scopes is the seeing in the tube, and the couple of millimetres right above the mirror in particular. That's because large scopes usually have a mirror at the bottom that's thick and has trouble cooling. Improper thermal management is the bane of large scopes (and that's the reason I never pick a large scope when I only have ten minutes to look at a planet).

But too many people don't try to determine if the seeing's really bad or if it's jus their scope, despite the fact it's a lot easier to do something about cooling the mirror than changing the seeing.

What smaller scopes are good for is things like splitting bright doubles at low magnification (on large scopes, the larger exit pupil means more aberrations in the eye to make stars appear uglier, but of course increasing magnification solves thant handsomely and makes the split easier) and firs tand foremost observing things at low magnification (which are too large to fit in the field of view on the large scope with a magnification that makes the sky background pleasingly dark.)

I have encountered situations in which an off-axis mask showed more detail than the entire aperture, but these are extremely rare; so rare that I discarded the mask. Now the only thing I use is a tuned apodising mask for my 400mm Dob.

As for splitting double stars, that's usually only valid at equal magnifications, and the human eye's aberrations is the cause (it's generally unproductive to use exit pupils above 2mm, i.e. magnifications not at least half the millimetres of aperture).

But there's no reason to stick to the low magnification on the larger scope, so that's saying that the larger scope will be beaten by the smaller one if it has to run with its shoelaces tied.

Minor nit: North is not always at the bottom (if you observe above the North celestial pole but with the scope pointing North, North is "down" naked eye but close to "up" in the scope.

Even "up" is not always at the bottom, as you don't always have the same orientation with respect to the eyepiece and as not all focusers are exactly horizontal. But yes, the view is mostly rotated close to 180°.

Thanks sixela - don't know where I'd be without you :)

Link to post
Share on other sites
  • Replies 81
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Posted Images

I have read something about this. It is some thing to do with the smaller aperture cutting through the particles in the air better than a larger aperture can. The wider the aperture the more of the dust and c**p the light has to pass through.

Well, each part of the wavefront passes through different "c**p", and all parts on average pass through just as much, so I don't think that argument is correct. What can happen in theory is that disturbances in the wavefront are more correlated in the small scope than the large, which can move the image rather than blur it, but I have often watched for the effects and it simply doesn't happen that much (at least not in a way that makes the smaller scope better).

See

Autocostruttori: How different apertures behave under the same seeing

Autocostruttori: The effect of seeing on different apertures

Autocostruttori: Aperture masks with seeing

Autocostruttori: Realistic simulation of atmospheric seeing

Autocostruttori: A close double in bad seeing

Autocostruttori: Simulation of bad seeing

for some interesting discussions.

I will try to find the article and post the details.

There are actually a number of articles that come to wrong conclusions, because they claim seeing amount to focus position shifts in parts of the wavefront and so small f/ratio scopes are more affected; in case you coma across them, read

Telescope Optics Topics

Edited by sixela
Link to post
Share on other sites

Thank you Sixela, i'm sure those links will come in handy.

i think you just melted my mind though lol. I still thinking about going for the 250px.

One last question though, i found a 10inch dob by orion, same spec apart from it being an f4.5 where the skywatcher is a f.5

Much difference?

Also i've looked into the cooling solutions for the dobs. the fan mods seem quite straight forward.

Link to post
Share on other sites
One last question though, i found a 10inch dob by orion, same spec apart from it being an f4.5 where the skywatcher is a f.5

Much difference?

Is that Orion as in Orion Optics, the UK manufacturer ?.

Link to post
Share on other sites

Hi

Just thought would put my 5p worth in having owned a 200p then upgraded to 250pds.

200p gives excellent views, very mangagable from size perspective and easy to get out and put away.

250pds is quite a lot bigger, heavier and slightly more challenging to handle. Visually you get more to view from bigger apeture but not excessively more.

One point you have to remember is the aperture vs magnitude relationship starts to get non-linear greater than after ~8", hence doubling apeture doesnt double light grasp.

Overall to summarise would suggest going for 200p as first scope then consider upgrading after if your keen. If you enjoy it then certainly upgrade, but personally would have been a less enjoyable experience for newbie going straight to 250pds. Only been into this for about 10 months so stand to be corrected, above is just my opinion. thanks J

Link to post
Share on other sites

I have a 150p and an old but large eq mount, it still shakes in the wind! Gona build myself a dob mount when the garage warms up simply because I can make it much stiffer. Tickled pink with the 150, limits are viewing conditions. Got it from FLO, changed my mind for a 200 and rang to change, the guy tried but already dispatched, arived the next day. Very friendly, very helpful and prices normally same or less than ebay. Can't recommend enough.

Link to post
Share on other sites
One last question though, i found a 10inch dob by orion, same spec apart from it being an f4.5 where the skywatcher is a f.5

Much difference?.

Slightly more expensive

Smaller footprint

Underframe is all metal instead of wood/plastic

Similar optics.

Focuser feels better on orion

If you can wait till Feb you could get a good deal at the astrofest

Link to post
Share on other sites

Aout the optics quality:

OO does have the occasional lemon in the lower bin (especially if it's got a couple of narrow zones partly missed by the interferometer's coarse sampling and if there's a huge defect in the middle that is not completely covered by the test because of the setup, but you need to be fairly unlucky), but they'll usually stand by their mirrors quite differently from Shuzou Synta if you get the mirror independently tested and it proves to be so-so. But so do Shuzou Synta and GSO (the two main mass market mirror makers), and their lemons can have quite ugly zones too.

But if you get a "1/6th wave" Pro grade mirror, even the lemons are usually (at least in my experience, and including other mirrors I read about) as good as the good Shuzou Syntas (I saw one German complain that a zone on the mirror meant the Strehl ratio was "only" 0.93 instead of the claimed 0.98 --the OO interferometry report's protocol underestimated its effect--, but it completely missed the point that it's a good mirror regardless). And my experience plus that of several German and Dutch ATM Dob builders is that those mirrors (some Pro grade, some "regular" grade) are definitely better on average than GSO and Synta mirrors, especially in the larger sizes. The secondaries are also a tad better.

I don't like the Orion Optics Crayford focuser that much, though (but then it's certainly not worse than Synta's cheapest Crayford focuser).

Edited by sixela
Link to post
Share on other sites

Thank you all.

250px for me i think. the orions are closer to the mead prices and i just cannot justify that.

I'll have to invest in some groovy eye pieces once i get it. I've not yet looked into it really, but Baader Hyperion 68 degree eyepiece looks quite decent from what i have heard.

i think i need to hit the books again.

Link to post
Share on other sites

You might have seen it mentioned but some people find the finderscopes on these skywatcher dobsonian scopes to be very uncomfortable to use. Theres a couple of threads discussing this and the recommendation is normally to replace it with a spotting scope that includes a diagonal and/or to get a red dot/telrad/rigel scope.

Something worth being aware of and perhaps budgeting for.

EDIT: See HERE and HERE

Edited by HappySachs
Link to post
Share on other sites
You might have seen it mentioned but some people find the finderscopes on these skywatcher dobsonian scopes to be very uncomfortable to use. Theres a couple of threads discussing this and the recommendation is normally to replace it with a spotting scope that includes a diagonal and/or to get a red dot/telrad/rigel scope.

Something worth being aware of and perhaps budgeting for.

EDIT: See HERE and HERE

Thank you for that. the telrad is a day one purchase for me! why is it not payday yet?

Link to post
Share on other sites
Some other things you might want to go for are red dot finders

Like these

Telrad Red Dot Finder

Finders - Rigel QuikFinder Compact Reflex Sight

A good book or two like these

Turn Left at Orion: A Hundred Night Sky Objects to See in a Small Telescope - and How to Find Them: Amazon.co.uk: Guy Consolmagno, Dan M. Davis: Books

http://www.amazon.co.uk/Deep-Sky-Companions-David-H-Levy

/dp/0521553326

A sub to the sky@night magazine. They have a guide to what to look for sky each month and usually features a deap sky challenge of some sort (has done for the last two issues anyway)

And a red light tourch to help you read at night while protecting your night vision!

These are all suggestions that were made to me about a month or two ago when I started on these forums. Very nice and helpful bunch here!:)

and download stellarium.....

I absolutely agree, I am a newbie and the wealth of knowledge and help that's on this forum is staggering! no shortage of finding anything out I am sure.. ;)

Link to post
Share on other sites
I may be wrong but the SW 250px dob is F/4.8 (F/4.7 on their web site) not F/5 although I really don't get the focal ratios etc so I don't know if this makes a great difference.

I think the SW 250PX is 254mm in aperture and 1200mm in focal length so 1200 / 254 = focal ratio of F/4.7 (rounded). My Orion Optics 10" newtonian is in reality "only" 247mm in aperture and 1186mm in focal length so it's an F/4.8.

Link to post
Share on other sites

The magnification is usually constant for all these scopes, because the focal length is usually the same.

It's the aperture that isn't. That's because some mirrors are made of metric 250mm blanks (minus a bevel) and others are made out of 10" (254mm) blanks, sometimes even a bit larger than that to allow for the unbevelled diameter to really be 10".

So the effective aperture is indeed anything from 245mm to 254mm, but they're all called "ten inch" scopes by the marketing people.

In fact there are slight differences in the mirror focal lengths from scope to scope too, so in that sense, aperture *and* magnification should all be taken to have a bit of variation.

Actually, the largest source of "error" is that focal lengths of eyepieces are usually rounded. A 4.2mm eyepiece labelled "4mm" magnifies 5% less than what crude arithmetic with the "official" focal length may make you believe.

Edited by sixela
Link to post
Share on other sites

Note to future readers of the thread looking for their first scope:

You really don't have to know all the technical details above to have a great time with your first telescope. Sure, some people love the theory, but trust me, when you get out under a dark clear sky, its the practical side that you will remember for a very long time.

Cheers

TJ

Link to post
Share on other sites

Of course; I was just answering the question (if your mirror is somewhat smaller that you think it is, it doesn't change the magnification, as that depends only on the focal length).

I do agree that whether your primary is 247mm or 254mm wide makes absolutely no practical difference.

Edited by sixela
Link to post
Share on other sites
Note to future readers of the thread looking for their first scope:

You really don't have to know all the technical details above to have a great time with your first telescope. Sure, some people love the theory, but trust me, when you get out under a dark clear sky, its the practical side that you will remember for a very long time.

Cheers

TJ

Amen!

I spent six months lurking here before I made a purchase. Whilst it's good to learn and read first, sometimes the pragmatic approach is whatever gets you to where you want to be in the straightest line.

Can you carry it? Do you know generally the name and shape of the main circumpolar constellations? (not hard...). Can you get somewhere dark with a view?

For me in a dark night alone in Devon, struggling with alignment (alt az!) and wet and cold and hadn't seen any exotic objects, I thought I'd just slew across to that bright orange dot I saw earlier. I caught a tremendous view of Jupiter and the Jovian moons that literally had mr holding my breath like a sniper or snooker player, waiting for the perfect moment to take the shot.

The view to me was better than any picture, magazine or image and felt like I was one of the few people observing this giant at that moment.

Get out there and enjoy whatever you've got!

Link to post
Share on other sites

Ah that is quite a relief!

although i do aspire to learn as much as Sixela one day in the distant future! but for now i suppose i'm going to go for the learn by doing approach.

I've been browsing so many threads, bending so many peoples ears and looking at so many videos i'm going a bit loopy! almost time to take the plunge and just do it.

Going for the 250px dob for now. But the 127 mak looks so good and portable, that it might make next years birthday prezzie.

Link to post
Share on other sites
....I've been browsing so many threads, bending so many peoples ears and looking at so many videos i'm going a bit loopy! almost time to take the plunge and just do it....

It's easy to get into a sort of "paralysis by analysis" with scope choices :)

I reckon you will be delighted with the 250PX - it's got enough potential for a lifetime of observing ;)

Link to post
Share on other sites
It's easy to get into a sort of "paralysis by analysis" with scope choices :)

I reckon you will be delighted with the 250PX - it's got enough potential for a lifetime of observing ;)

I agree John the choice is quite intimidating. Here's to many a good nights view with your choice of scope Dr Neb :D

regards

Kevin

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.