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HELLO I am new and in need of advice!

eddie3000

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eddie3000

eddie3000

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Hi! I am quite new to astronomy. Don't know much either. But I am very enthusiastic about photography, and I've already started doing some astrophotography and I quite like the experience.

I already have a mount, a skywatcher heq5pro with goto, and a digital dlsr and a couple of telephoto lenses and I think it is about time I bought myself a telescope.

As like many amateurs, I cannot afford a RC scope. Pity. Wish I could! :( And refractors in the price range I'm looking at aren't really all that much better than my camera lenses, so I don't think it's worth getting one. So I think I will settle for a reflector.

As many of us learn with time, you tend to get what you pay for. That is why I want to try and get a good telescope, one that lasts and my children can use too when they grow, even though it hurts my wallet.

After quite some time doing some research on scopes, I think a good compromise would be a schmidt newtonian telescope like the lxd75 from meade. The problem is that I can't find anywhere where I can purchase just the tube as it seems to be sold bundled with a mount, which I don't need. Does anybody know of a telescope of this sort, maybe a different make, that can be purchased seperately? Doesn't anybody else make this type of scope?

Here's another question regarding coma. Is the coma on a standard newtonian really that bad? I have read that the coma distortion gets worse away from the center of the field of view, and with low f ratio numbers too. Does this mean that, even using large amplification, the center of the frame would be coma free? Are coma correctors any good?

I think maksutov, cassegrains and other common types aren't luminous enough for good astrophotography.

So, as you can see, I am quite doubtful. Newtonians seem to be the best regarding quality/aperture/money ratio. What should I do?:)

I have even thought about building my own scope. It would probably have to be newtonian though. Parabolic mirrors are available easily. I have even seen hyperbolic mirrors available for a couple thousand euros for making your own RC scope, but I think that is just a bit too ambitious to start off with.

At the moment, I think I like the lxd75 series or other similar one for the moment. Further research needs to be done.

Any advice and suggestions will be very helpful. THANK YOU AND ALL THE BEST!

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eddie3000

eddie3000

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Thanks guys for such a warm welcome.

I like the looks of the skywatcher mak-newt Richie pointed out. It seems to fit exactly what I had in mind. It's a bit pricey, but it may be worth it.

The 2nd hand meade DarkKnight suggested could be quite a bargain, all depends on how good a state it's in and how much the p&p would cost.

What would you get? The new Skywatcher or the 2nd hand Meade? Any other suggestions?

I'm going to keep an eye on these scopes, they both are quite tempting.

THANKS!

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eddie3000

eddie3000

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Thanks David o. There are many newtonians similar to that one, maybe better or worse. Are coma correctors good? I've seen that some are quite pricey. And how can a separetly sold coma corrector be as good as a specifically designed corrector lens in a sch-newt or a mak-newt?

And, by the way, what's the difference between a schm-newt and a mak-newt? Has one got a spherical mirror and the other is a parabola? Is a normal newt with a coma corrector ep as good and cheaper than a schm-newt or mak-newt? :):confused:

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david o

david o

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Hi eddie3000,

Schmidt-Newtonians and Maksutov-Newtonians both use spherical primary mirrors, thus they suffer from spherical aberration not coma - which is what you get with parabolic mirrors.

They differ in how they correct the spherical aberration. The SNT uses a thin, aspherical corrector plate and the MNT uses a thick, spherical meniscus lens.

Another difference is the secondary mirror. The SNT uses a convex spherical mirror mounted on a support in the middle of the corrector plate and the MNT relies on a silvered central spot on the back of the meniscus lens.

As for the merits and usefulness of coma correctors, or the pros & cons of Newts vs S/MNTs I really couldn't say. Maybe one of the forums' imagers could weigh in?

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david o

david o

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A quick google found these reviews:

Meade LXD55 SN-8, seems to get mixed/negative comments.

Skywatcher Explorer 190MN DS-PRO Mak-Newt Astrograph

Astro-Tech Altair 8 inch F4 Imaging Newtonian , there's even a comment about the Baader Multi Purpose Coma Corrector (MPCC), 4th post down.

Both the Sky-Watcher and Astro-Tech seem to be positively rated for astro-imaging.

HTH :)

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david o

david o

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I should only worry about coma in the astrophysics...

True if they use a parabolic primary, but I can't find an explicit mention of the mirror type. Mind you be a bit odd not using one in a fast imaging newt.

Skywatcher says their telescope is coma free. Suppose it is, if it uses a spherical mirror.

Ahhh. Seems I was at least partly wrong - found these snippets:

"Spherical mirrors have coma unless the field stop is at the center of curvature, like in Schmidt Camera..."

"Catadioptric Newtonians such as Schmidt-Newts and Mak-Newts still use spherical primary mirrors, but they also add a corrector lens over the full aperture at the front of the telescope to eliminate spherical aberration. This lens also has the effect of minimizing the coma inherent in Newtonian design."

The above link and this one (Compound Telescope Designs, Some Pros and Cons.) might help you with a decision.

Sorry if my inaccuracies have hindered more than helped. :)

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eddie3000

eddie3000

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Taking a decision is going to be difficult. Astrophotography preferably needs loads of light, so lo f ratios are preferable in that aspect, but low f ratios produce in newtonians more coma. So, correct me if I'm wrong, I think these are the possible solutions:

1) A newtonian scope with a f ratio of f8 or more to reduce coma and take longer exposures with all the hard work that implies. It is also the cheapest solution.

2) A maksutov cassegrain or similar, smaller and lighter, more expensive than option 1. They aren't very luminous so exposures would have to be long and that implies much more work than in option 1, and deep sky AP wouldn't be as good either.

3) A very luminous newtonion, and purchase a coma corrector. Probably more expensive than options 1 and 2.

4) A compound newt-schm or newt-mak. The most expensive option, but very luminous and reduced aberrations (coma, spherical distorion, or whatever). Less dirt gets into them too. Probably the most desirable option, except for the money part.

I'll have to think how to convince my wife about this.....:). Any Ideas??

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david o

david o

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Hi Eddie,

wouldn't you still need a coma corrector on a slow Newt for imaging? Couldn't you use a focal reducer to speed it up?

You can use a focal reducer with a Mak- or Schmidt-Cass though.

A fast, short focal length Newt (like the Astro-Tech Altair 8") would allow some very wide DS images.

A Mak- or Schmidt-Newt (like the SW Explorer 190MN DS Pro) would as well, and it is to die for I agree. :headbang:

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