Aperture, magnification, photography questions.

[SuperAli]

Hi

I'm working towards purchasing my first telescope and mount, but i'm very confused as to what i want/need. Please can someone help me understand the following:

If a were to line up a 6" telescope, a 10" telescope and a 14" telescope and set them all to, for example, 200x magnification using the same eyepiece, and point them all at, for example, jupiter, or saturn, or mars, or the moon, how much of a difference am i going to see between the view offered from each telescope?. I believe the planets would appear to be the same size throguh each telescope, only brighter and clearer through those with the higher aperture, is this correct? How much of a difference could i expect to see through each telescope, would the views be roughly the same, or will the 10" blow the 6" away, and will the 14" blow the 6" and 10" away?

As the planets are my key interest at the moment, and the earth's atmosphere prevents viewing the planets with high magnifications, i'm thinking there might be little point in me splashing out on a 10" if a 6" will give me just as good views. Please advise.

On another point, what sort of photography is possible with a telescope on a dobsonian mount? I believe DSO's are totally out of the question, but what about plantery photography? What is possible?

Thanks in advance

From a very confused first time buyer!

[rowan46]

It's not quite as simple as they would all be the same size and only different in brightness because a larger scope is capable of greater resolution analogous to megapixels on a camera the more mega pixels you can squeeze in the larger it can be blown up without the image breaking down. (I am not a very good photographer so take the analogy with a pinch of salt but its something like that) So a 14" scope can magnify more than a 6" giving it a larger and clearer view (subject to atmospheric conditons of course)

[ronin]

Same size, they are the same magnification. Brightness of the image would vary, the bigger scope giving the brightest.

Unfortunately the diameter can lead to an error called coma, but not a simple relationship.

With a dobsonian really forget imaging, for DSO's and for planetary you need a tracking mount in one form or another.

Could point it at the moon and get a shot but that is where in practical terms it starts and ends.

[rowan46]

As a first time buyer your first criteria should not be what can I see? but firstly where will I use it, How easy will it be to get it there then its what will I see. your final choice will then likely be a compromise between ease of use and what you can see. the best scope is the one you use. Which ideally is the biggest scope you can handle and transport comfortably in all the places you intend to use it.

[rowan46]

I have to agree with ronin that its possible to photograph using a manual dob but the only pics I have seen that are really good have used a tracking dob

[nmoushon]

Definately a +1 for a scope you are going to use. It doesn't matter how big it is if you hate dragging it out of the house to set up you'll soon stop using it.

The visual size of Jupiter (for instance) will very depending on two things. 1-your EP and its magnification and 2-the focal length of the telescope. The longer the focal length the more "zoom" you will start with and then you add the magnification power of your EP on top of that. i.e. A Dob with 1000mm FL combined with 200x mag will have less "zoom" than a Dob with 2000mm FL combined with 200x mag. The aperture, combined with the focal length to give you a focal ratio, will determine how bright an object appears. The larger the aperture the more light you collect which will help bring out dimmer objects.

Unless you get a tracking Dob (GOTO) I'm afraid you are very limited. Basically to very quick snap shots of the moon and planets. No DSOs. If you get a tracking Dob then you can do more but are still limited to planets and the moon. You can dabble a touch with some of the bright DSOs but really isn't worth the effort. IMHO.

[Northern Soul man]

Hi Ya SuperAli, that's the Million Dollar Question you've asked - I tnink the main thing you want to think about is what YOU want to get out of Astronomy for you - what's your sky conditions like where your going to do most of your observing, do you suffer from light pollution, have you got a really dark location - this is because there may be times when there are no Planets/Moon about - have you got unobstructed views of the Ecliptic (where the Planets move along) - are you going to be observing the Planets over any structures or have you a clear view from the horizon upwards - you have to take all this into account - Planets can be Photographed/imaged quite cheaply with a web cam and processed with software to bring out the best in them - but for the nights when there's no planets around - if you have a really good dark location - you can then hunt down the faint fuzzies - so you have the best of both worlds - that's even before you think of buying a scope - I use an SCT scope the most and have found that an 8" SCT will probably outperform a 12" SCT on more nights - these have short tubes and quite good mounts (Alt - AZ) but on only a few nights will the 12" SCT show better results and you can web can image, showing really good results - the only problem you have to put up with is the "Seeing" through the dirty atmosphere - this is where the 8" scope will show you maybe better steadier results - but on the fewer nights of good "seeing" then the 12" will be better - but to be honest - the software you use to process the Planetary images will "steady" the view for you.

Long exposures need really expensive mounts and short focal length scopes - the cost of the mounts probably start at around the price of an 8" SCT complete set up, then you have to think of the scope, guiding system, camera - the list goes on - a little more to think about SuperAli - I told you it was the Million Dollar Question!!!!. Paul.