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 Post subject: Telescope types
PostPosted: Tue May 14, 2013 9:17 pm 
Hi all,

I have just returned from an excellent talk on image processing, absolutely excellent and thank you.

I have about a squillion questions but I'm going to start with just this one and please bear in mind this is the "Absolute Beginners" section, so please be gentle.

In one of the opening slides there was a comparison on the types of scopes and what was best for imaging - top was apo refractor, then reflector, then SCT. I'd just like to know why this is the case. I don't doubt it for a second but would just like to know the rationale.

From my very limited research I'd learned that it was all about aperture, the bigger the better. But, having come home tonight I see 6" refractors for the same price as 12" SCTs not to mention the cost of reflectors for less than a quarter the price.

The talk was very clear - you get great results from small aperture refractors, I'd just like an explanation as to why this is so. Optics maybe?

Many thanks

and again well done on the presentation!


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 Post subject: Re: Telescope types
PostPosted: Tue May 14, 2013 9:39 pm 
The very basic and quick explanation to your question John is this ...... The more you magnify things ie, the longer your focal length, the more you magnify your errors.
By errors, I mean, polar alignment, mount movement etc etc etc.....
With smaller scopes, shorter focal lengths, you can get away with some if not most of these errors as you don't magnify them as much.
As an example, I have been using an 8 inch sct for a few years now, I, like many others would have bought a 20 inch scope if I could afford it not realising the torture they bring.....great for observing but when you use it for imaging its everything else you use that comes into play, the mount needs to track almost perfectly with zero movement in any other direction than where it's tracking, by zero movement I actually mean better than zero movement !! Any movement in the wrong direction shows up hugely on your images..... as you found in your latest orion image post, long stars etc....The smaller the scope the easier it becomes as you are no longer magnifying the mount errors etc as much....
I have had way more success with a much smaller 70mm cheap refractor I have had for years because of the reasons above...
I hope this helps, it's the non scientific answer to your question .....


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 Post subject: Re: Telescope types
PostPosted: Tue May 14, 2013 10:26 pm 
Dean,

Many thanks for the explanation.

So, on a slightly less newbie question, is it mainly about focal length? I mean, were it possible to get a SCT, refractor and reflector all of the same focal length, would they take images of a similar quality if build quality, environment, tracking etc were equal.

I can remember an earlier presentation at the club where one of the slides showed someone just imaging using a camera and nothing more - just a camera on a simple tracking mount (a 'barn door' if I remember the term correctly). So I'm guessing the answer is a sort of 'yes'.

It's a very difficult leap of faith to make for someone new :roll: but I suppose it is less about small objects and more about faint objects. One requires magnification, the other about long and tracked exposure - have a got it right?????

The reason for my asking is I will be looking to replace what I have and want to know what type to get. But... I work with computers and all I ever get asked is what PC / laptop / tablet should I get and all I ever can say is - "well, that depends....."

I suppose I want to run before I can walk.

Thanks again.

John


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 Post subject: Re: Telescope types
PostPosted: Wed May 15, 2013 5:46 am 
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jMcConnell wrote:
one of the slides showed someone just imaging using a camera and nothing more

Yes John, Keith's wide-field image of Orion (which was our first example last night) was taken with just a 135mm lens (a Pentax lens with a modified Canon camera on one of Brian's tracking platforms).
Then if you're imaging smaller objects, a longer focal-length is required: Dean's image of the Leo triplet of galaxies was taken with our StarWave 80 which has a focal length around 480mm (with the reducer/flattener fitted).
But if your target is a single DSO (e.g. M51 or M57), then you could use an SCT & focal reducer: I operate my C8 at around 670mm with an LX-webcam, or 1280mm with a DSLR.
However for planetary imaging, focal length is king: fitting my C8 with a Barlow lens and spacer takes it up to an effective focal length of over 7m - but this will only work with very short exposures (1/25-sec or less) of bright objects: the Moon or nearer planets.

And yes, a larger aperture will collect more light, so you'll build up the image more quickly. For instance, a C8 with a 0.63 reducer would collect light around 4x faster than a 102mm ED refractor with an equivalent focal length (but the image quality would be better with the refractor).

Before you go out and spend money on a new 'scope, do spend some time experimenting with your current system, exploring its capabilities and limitations - then you'll have a better idea of what to get next.
And try out some of the 'scopes and lenses from our loans pool too.

But remember that whatever 'scopes you have, a decent mount is a prerequisite, so this should be your most important investment, and something that will serve you well for many years


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 Post subject: Re: Telescope types
PostPosted: Wed May 15, 2013 6:55 am 
Gentlemen,

thank you


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 Post subject: Re: Telescope types
PostPosted: Wed May 15, 2013 7:31 am 
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Joined: Mon Oct 12, 2009 8:27 pm
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Location: Adlington
John,
I think everyone has already explained it well. The slide of telescope types was not which is best for imaging but which is easiest for imaging. All three are good but down the screen gets harder. If your interest is faint fuzzies then big is better. It is much much harder but not impossible.
David


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 Post subject: Re: Telescope types
PostPosted: Wed May 15, 2013 7:34 am 
i agree with both Dean & Ross, there has been a seismic shift from large aperture scopes of both SCT & reflectors towards the small apo refractors for all the reasons Dean has mentioned, errors, pain & failure.
As Dave was repeating in his talk last night - its not easy & Dave's opening slide said that taking the images is the easy bit. You need to make it as easy as it can possibly be.

The biggest sin you can commit is to rush out & buy something because someone else says so. Dean has always preached to me to use what you have until it cant take you any further then look for something else.

Its easy to burn through cash in this game & everyone has their view on the best way forward - learn to use what you've got properly - the acid test is polar align take pictures of 30-60 seconds if there's no (bad) trailing then you're well on the way to getting somewhere.
If you cant do that with your kit - then its down to you & buying more/different kit will not improve it. There is no magic button on a go faster eye in the sky mount or a piece of kit or software to get round the basics - that comes through nights of pain & failure.


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