What are my prospects?

[Whippy]

We have some 10" f/10 Meades here for the students. With off the shelf amateur CCDs on the back, binned 3x3, to give about 1" per pixel, I assure you that images taken with these will blow anything taken with small scopes at any f-ratio out of the water.

And that's fine, but mounting a 10" SCT plus making it guide accurately at that focal length proves it's own headaches does it not? What about cameras that don't have the ability to bin their pixels?

You've mentioned several times in various threads about aperture having importance in regard to imaging, yet I haven't seen any images or information from you to back this up.

Tony..

[mark7331]

I totally agree with Tony on this. Aperture just isn't important. The maths and the results back it up.

Mark

[dph1nm]

but mounting a 10" SCT plus making it guide accurately at that focal length proves it's own headaches does it not

Actually no - we don't use autoguiding, just the normal tracking. Its not the focal length that matters but the pixel size, because the tracking is accurate to a certain number of arcseconds.

NigelM

[dph1nm]

What about cameras that don't have the ability to bin their pixels?

Buy one with larger pixels! Actually I am not saying there are not PRACTICAL advantages to short focal lengths, there clearly are. For one thing, the average commercially available CCDs have pixels which are really too small for focal lengths over a meter or so, hence the need to bin.

However, there is no physics in this idea that there is some magic in having short f-ratios which overcomes the lack of photons coming from small aperture scopes.

Professional astronomers keep building bigger telescopes for one reason only, and that is to image fainter objects.

NigelM

[dph1nm]

You've mentioned several times in various threads about aperture having importance in regard to imaging, yet I haven't seen any images or information from you to back this up.

My point is simply that if you image with 1" pixels on an 8" scope and with 1"pixels on a 4" scope, then, irrespective of f-ratios, you will get the same image on the 8" in 1/4 of the time it takes on the 4".

NigelM

[RobH]

Although aperure isn't anywhere near as important for imaging as for visual work, to say it doesn't matter isn't really right.

There are important practical considerations that must be taken into account.

A 12 inch F8 scope will gather 4 times the light for a given period of time as a 6 inch F8 scope, or, get the same amount of light in 1/4 of the time.

I know this very well....it's why, in order to image very faint stuff, I end up doing really long sessions. With a bigger scope I could, in theory, get the same data as I already get in a lot less time for the same image scale, although, knowing me, I'd shoot for the same amount of time anyway and just go deeper

There is also the issue of resolution.

Although in this country resolution is usually limited by seeing, certainly up to a 10 inch scope, you'll get better detail the larger you go.

My 6 inch TMB has optics that, in terms of quality, blow my old 14 inch SCT out of the water, but the 14, in the right conditions, had better resolution.

I'd still take my TMB over the 14 though.....just a better scope

All this said though, an 80mm refractor as a first imaging scope would generally be a good starting point, especially as you won't need to spend a fortune on a big mount for it.

Cheers

Rob

[Whippy]

Buy one with larger pixels! Actually I am not saying there are not PRACTICAL advantages to short focal lengths, there clearly are. For one thing, the average commercially available CCDs have pixels which are really too small for focal lengths over a meter or so, hence the need to bin.

And in an ideal world, where budget isn't an issue I'm we all could have this choice. Unfortunately though, most of us are only able to buy the best kit we can afford, irrespective of what size of the pixels or the scope.

I've yet to see any article that manages to convince me that aperture matters other than it's relation to focal ratio. Anecdotal evidence I know, but I've certainly noticed the difference in terms of exposure time between my old FLT98and the ED80 I now own. The FLT is faster, noticeably so. Now is that because the FLT is bigger or because it has a lower 'f' number? I think it's because it's faster because the ED120 I also own has the same focal ratio as the ED80 and it struggles to pick up fainter detail on subs less than 10 minutes, like the ED80.

I'll be revisiting a fairly faint target I did last year with the ED80 when the weather plays ball but going for a wider field with my ZS66. I'm confident I won't need to spend quite as much time on it as I did last year to get the same level of depth because the 66 is faster. I guess the proof of the pudding will be in the eating.

Tony..

[Tom33]

Thanks for the advice Tony, all I'm trying to do is spend my money wisely. May main aim is imaging but it occurs to me that i will still want to look through the scope. The Equinox ED80 (looks nice in the shop!) but comes without a diagonal and eyepieces and I haven't a clue, so any recommendations would be appreciated. I don't want to go mad with these as i intend to spend more time with the camera but as a the novice there's a mind blowing amount opf choice.

Tom

[Whippy]

Thanks for the advice Tony, all I'm trying to do is spend my money wisely. May main aim is imaging but it occurs to me that i will still want to look through the scope. The Equinox ED80 (looks nice in the shop!) but comes without a diagonal and eyepieces and I haven't a clue, so any recommendations would be appreciated. I don't want to go mad with these as i intend to spend more time with the camera but as a the novice there's a mind blowing amount opf choice.

Tom

Especially in the current climate Tom, spending wisely weighs heavily on most people's minds right now! I own an ED80 and while I've only given it the briefest of looks through it (it normally has a camera on the end of it), you can see quite a few objects through it but it's not going to give you those 'wow' moments in quite the same way a larger scope can. Seconhand, these do go for less than £200 fairly regularly so it might be worth your while scouring the secondhand ads for kit. Places like Astro Buy & Sell are worth looking at. Maybe another way of looking at it is set yourself a budget and try to get the best you can for that amount.

Tony..

[Tom33]

Within reason budget is not a problem. I can run to an Eqinox ED80 on an EQ6 but am REALLY a complete novice at imaging. I've taken some half decent shots of the moon with my canon 40D and my 100-400L series lens and can process RAW images and use photoshop but thats about it. I'm reading Steve Richards book and trying to learn fast but thought I might treat myself to some gear before Christmas so I have time to play with it. That would also beat the VAT increase! I know I have a lot to learn and this forum will help. Does this sound a reasonable starting point or am I running before I can walk?

Tom

[Whippy]

Not at all Tom, I think most people's idea of a 'beginners' imaging scope would be something like an ED80. Good colour correction, short focal length (easier to handle) and not heavy so it wouldn't be so demanding on the mount.

I think there's two things you might want to be aware of if you're planning on using a DSLR.

Most DSLR IR filters unfortunately extend into the same wavelength as the ones primarily used by emission nebulae which in real terms drastically increases the time needed to spend on getting a decent image of one. What most people do is modify them in such a way either by removing the filter entirely or replacing it with a better one so they're then in the posession of a camera that's able to capture good images of these objects.

Secondly, you're almost certainly going to want to use a flattener because most scopes aren't able to produce a flat field across a big DSLR sensor, you're perfecly able to image without one, but you'll get abberations in the corners.

Just to add more thoughts in your head, have you thought about autoguiding? What this basically means is you have another camera (normally a webcam or something similar) effectively controlling the mount and making it track an object much more accurately. It sounds like withcraft, but with the right kit it's pretty simple and it enables you to use much longer exposures that helps you get a better image.

Tony..

[Tom33]

Tony, I am picking these things up as I read Steve's excelent book but had the idea that whatever I bought I could use for viewing as well as imaging. Please excuse my stupidity if that is not the case. When I first looked at the Altair 8" f4 newtonian, I figured this might be quite good for both and at £485.00 its only a smidge dearer than the SW ED 80. I appreciate your comments on reflectors but I have this scenario of one of my sons coming to visit and saying " lets have a look then". As for carving my beloved canon 40d up, thats a non starter, though I have seen some good stuff without modifications. Besides if I want to modify one I'll buy one of e-bay.

I hope you don't mind me rambling on but it helps me make my mind up.

I notice you have a Dob in your signature, do you use that for viewing ?

Tom

[Whippy]

Tom, imaging with a newtonian is possible and plenty of people use those newtonians and similar ones to great effect so it's entirely do-able but it is not as easy as doing the same with a refractor, especially if you haven't got a permanent setup (ie: an observatory) and something I wouldn't personally recommend to a beginner such as yourself. I really think that there's so many things that can go wrong in amongst all those pieces of equipment and wires that initially making it simple will give you less frustration and more enjoyment. You can always change stuff around to suit your particular style and needs once you get used to what you're doing.

About the Canon, you're right, people do achieve excellent results with non-modified cameras and I would imagine the effect is lessened on reflection nebulae (such as the pleades) and galaxies as they contain less of the emitting gases that cause the problem. I'm only assuming this by the way, I've never used a DSLR.

The dob, bless it, is a 110mm reflector. It's one of these: Tal - Tal 1 Newtonian Reflector with the tube taken off and put on a home made mount. I use it for viewing and it's a lot of fun but normally dobs come much bigger than that!

Tony..

[Tom33]

Tony, I think you have made your point. I am going to get an EQ6 and the SW 80ED and start the learning process. I appreciate all your comments which have helped enormously and know I have a lot to learn, as I've never owned a mount before, let alone a computer controlled one. I will discuss with the shop about an eyepiece or two and a guide scope, so at least I can have a look at what its supposed to be pointing at! I can then always upgrade at a later date.

Found M31 from my back garden with my bins last night, virtually straight up, so light pollution not that bad.

Thanks again for the help

Tom