Help me choose an 11" plus setup please

[GazOC]

Beat you

Post edited...

[GazOC]

It uses a two-element corrector behind the secondary.

http://tinyurl.com/jatbq (about half-way down)

..and thinking about it, that also rules out most of other Cass designs....

[gurney]

But the spectroscopy guru (Maurice Gavin) reckoned the C11 would be ideal!

[FLO]

Well I know very little about spectroscopy, mostly only what you have posted here, so do check with him before acting on anything I say

[GazOC]

But the spectroscopy guru (Maurice Gavin) reckoned the C11 would be ideal!

Spectroscopy is a pretty small niche area in amateur astonomy, if you've got it on good authority that an SCT will be OK from an expert on the subject then you should be OK. People here are just trying to cover all the bases and warning of potential issues you should look into before buying.

[gurney]

Yeah - it's ok guys - I know you're all doing your best. I'm not going to be holding any of you accountable.

Why is spectroscopy such a niche area? I just don't get it. surely it's in the spectra that you actually learn about what you're seeing - chemical composition, redshift, and in some cases proper motion. Just taking images is pretty and awe inspiring - but it's in the spectroscopy that you can pull an image apart and dig beneath the surface.

So why don't more amateurs do it? All seems very odd to me.

[FLO]

Why is spectroscopy such a niche area? I just don't get it. surely it's in the spectra that you actually learn about what you're seeing - chemical composition, redshift, and in some cases proper motion. Just taking images is pretty and awe inspiring - but it's in the spectroscopy that you can pull an image apart and dig beneath the surface.

So why don't more amateurs do it? All seems very odd to me.

When you put it like that, it sounds fascinating 8)

Now we have you onboard perhaps we'll see more people giving it a try.

[ringz]

When I was a member of the Stratford upon Avon Astro. Soc. we had a loaner spectrograph and no-one ever borrowed it.

I guess it's because you need to have a really expensive piece of kit to be able to produce decent spectra with anything other than the brightest stars.

--

Martyn

[Jeff RV]

Yeah - it's ok guys - I know you're all doing your best. I'm not going to be holding any of you accountable.

Why is spectroscopy such a niche area? I just don't get it. surely it's in the spectra that you actually learn about what you're seeing - chemical composition, redshift, and in some cases proper motion. Just taking images is pretty and awe inspiring - but it's in the spectroscopy that you can pull an image apart and dig beneath the surface.

So why don't more amateurs do it? All seems very odd to me.

All a bit to high tech, I'm still trying to find which end of the scope points to the clouds.

Wouldn't any multi coatings give false spectrum colour ? (think that's what i mean)

Jeff.

[Keagle]

This is something I would consider, if you tell me how you get on

[GazOC]

This is something I would consider, if you tell me how you get on

There are a few budget spectroscopy kits that you could use to get your feet wet.

[beamish]

Hard time getting to grips with a webcam :shock: Good luck with all Gurney, I'll await your results .

Karlo

[gurney]

Here's perhaps the cheapest and most useful option I've found so far. http://www.patonhawksley.co.uk/staranalyser.html

I'm thinking of collating my findings so far into some sort of primer on spectroscopy. But I think I better work out what I'm doing first!!!

[FLO]

Found this on their FAQ page:

What type of telescope can I use?

Just about any telescope or even a camera lens can be used provided the object is bright enough and is reasonably stellar in appearance. The simple arrangement of placing the grating in the converging beam of the telescope produces some aberrations. The advantage of the low dispersion used in the STAR ANALYSER is that these are kept to a minimum. There are some trade offs with focal length. Less aberration is introduced by using long focal lengths but the resulting larger size of the stellar image will tend to limit the resolution. In practise, the STAR ANALYSER performs well with the typical focal lengths found in amateur telescopes, though in poor seeing conditions or at focal lengths over 2m, a focal reducer will generally improve the spectrum sharpness by reducing the size of the star image.

Some additional care is needed when focusing using an achromatic refractor as with these instruments not all colours (particularly Violet and Infra red) focus at the same point. Particularly with a short focal length achromat, you may find that not all the spectrum can be brought to focus at the same time and you may need to concentrate on one segment at a time. (The spectrum will appear "fish tailed" at the violet and or red ends) Reflectors, including Cassegrain designs do not show this effect. Apochromats are affected less and, if corrected for imaging including the Infra-Red region, may show no chromatic aberration. Indeed the STAR ANALYSER is an interesting way of checking the chromatic correction of your telescope!

[gurney]

Aha nice find!

Thanks Steve - not sure why I hadn't spotted that. ooops

[FLO]

After reading that it looks like a C11 would be ok if used with a focal reducer to bring its 2800mm focal length down to less than 2000mm. It also suggests that the 8, 10 and 12" f5 Newtonians would all work well, unless other spectroscopy devices have different requirements.

[gurney]

HI guys,

I've had the following response from somone on a yahoo spectroscopy group:

As you say, the merits of different OTA designs is a BIG issue and always hotly debated. I have both Newtonian and catadiptric types and, for me, the latter is the clear winner when it comes to using instrumentation for observing, i.e. CCDs, filter wheels, focusers, rotator, flip-mirrors, focal reducres, etc. The Newtonian just doesn't have nearly enough back-focus available for more than the most basic setup. But before buying an OTA make sure you put more thought and cash into the best mount you can afford (unless only doing visual work), then worry about the OTA and instrumentation.

Can someone explain to me what the issue is with backfocus that he mentions (I've put it in bold and underlined it so you can spot the bit I mean)? I haven't the foggiest what that's about I'm afraid. Does he mean there won't be enough 'room' to add in bits of equipment at the focal point?

And if so, are there ways round it?

[Kaptain Klevtsov]

The back focus issue with a Newt. is because you can't wind the focuser in far enough. If you need to get "stuff" between the 'scope and CCD you need to be able to wind the focuser in enough to accomodate the added length in the light path.

Probably best to get a SCT then.

Kaptain Klevtsov

[Vega]

Looks like this is warming to SCT and I must admit if I had the money, I would have gone down the C11 route over a 10" Newt even just for observing purposes. However, we all have our financial limits! Hell, if I could have afforded a C11, I probably would have gone C14 :laughing3: . As for imaging/viewing planets... Large SCT's win over large Newts by a Country mile.

Matt

[gurney]

Bum! I was really hoping for the 300P! Looks like I'm going to have to go for the C11 now, which is a bit of a bummer budget wise to be honest, and means I'll have to get the C11 on a G5 mount, sell the mount, remount it on a EQ6 Pro and so on and so forth.

life is hard

anyone want to buy a brand new and unused G5 mount when I get one - if I get one!

Are there really no ways around the backfocus issue on the newt?

[GazOC]

You can use extension pieces but they can be a bit of pain in backside, another problem is that Newts don't have much forward focus either so may have trouble with focal reducers etc. and thats harder to remedy.

[gurney]

So all in all - newts rubbish, SCT's great.

Is that it?

why had I always thought that SCT's were a convenient compromise and newts were the cheaper but more perfect way to go? hmmmm. I must have been misled somewhere along the line.

[GazOC]

So all in all - newts rubbish, SCT's great.

Is that it?

why had I always thought that SCT's were a convenient compromise and newts were the cheaper but more perfect way to go? hmmmm. I must have been misled somewhere along the line.

I personally think Newts give better views than a SCT of the same aperture but for imaging SCTs are more flexible and above a certain size Newts just get a little too unwieldy when equatorally mounted.

Its not a case of SCTs being 'better' than Newts, every design of scope has compromises in some aspect of its design/ performance, thats why a lot of amateurs have more than one scope. Its just a matter of setting a budget, prioritising what you want to achieve with the scope and weighing the pros and cons based on that, its very rare you'll find a perfect solution.

[Kaptain Klevtsov]

I must have been misled somewhere along the line.

Didn't set out to misle you in any way, but I think very few of us are aware of the technicalities of astrospectrographisation, so we went on the information available. I'm sure that many others are waiting to see what can be done using affordable kit, I know I am.

Kaptain Klevtsov

[gurney]

Aha! Sorry, I wasn't implying anyone here misled me - more of a case of my own preconceived ideas, mostly from yesteryear and lack of understanding. The info and advice here has been nothing but useful and helpful. I've been greatly encouraged by the debate to be honest, that's it not just me who isn't completely sure of the answers!