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Scope for planetary use


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Hello again,

I have searched through some older threads but I was wondering what the current thinking is on the best OTA for lunar/planetary use?.

Kind of tied between a SW 100ED (might also get away with some DSO imaging with the supplied focal reducer) or the SW Maksutov range (much longer focal length).

It will be mounted on a NEQ6 and will be for visual and webcam imaging.

The budget is ~£500-£700

As always, many thanks in advance

Dave

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If imaging comes into it the 100ED is the sensible option. The longer focla length of the Mak's will just mean longer exposure times and will amplify any tracking errors.

That's true for deep sky imaging, but it sounds like the OP wants to do some planetary imaging (judging by the webcam reference) in which case focal length is key and I would go for a Mak or an SCT.

Ian

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For planetary work (visual or imaging) the longest f/l you can get/afford is the way to go. Refractors are great for this but they are expensive.

Mak's and SCT's offer very long f/l and larger apertures.

My 200mm SCT has a f/l of just over 2000mm compared to 1000mm on my 90mm refractor.

I'm still trying to figure out how SCT's can be so short in length while having massive f/l's.

Edited by LukeSkywatcher
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For visual and webcam imaging of the Moon and planets go for the Maksutov - you will get the 180 at £15 over budget and you won't regret it. For DSO photography you will need a different scope - something like an ED80 PRO is a good starting point. Sorry but there is no such animal as the "does everything" scope.

If you did go for the 100ED then budget for something like a 2.5x Teleview Powermate for use on the Moon and planets. However the 100ED is generally considered too "slow" for Deep Sky Photography and doesn't really have enough focal length to be really useful for the Moon and planets.

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I'm still trying to figure out how SCT's can be so short in length while having massive f/l's.

Well, isn't that because in the SCT or MAK the light beam gets reflected multiple times back and forth? Place the beam sections behind each other as you would have in a refractor and their legths add up :)

post-27855-133877721349_thumb.png

Edited by assasincz
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Well, isn't that because in the SCT or MAK the light beam gets reflected multiple times back and forth? Place the beam sections behind each other as you would have in a refractor and their legths add up :)

Cheers for that. That diagram explains it well.

Just to clarify:

How many mirrors are there in an SCT?:

Primary

Secondary

and maybe a smaller one on each side behind the primary?

That would make 4 mirrors.

Nope: I am wrong.

According to Wiki:

In this Cassegrain configuration the convex secondary mirror acts as a field flattener and relays the image through the perforated primary mirror to a final focal plane located behind the primary.

So just what on earth is a perforated primary mirror?. I know "perforated" means it has holes in it. So is the primary in an SCT semi-translucent?

Sorry i dont mean to hijack this thread.

I do apologize.

But hey !!!!!!!!!!!!!

Its educational to all.

Edited by LukeSkywatcher
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I reckon that the "perforation" stands for the only hole in the middle of the primary mirror, through which the light beam gets to the eyepiece :) although on a similar note, I have heard that the LBT's primary mirrors (Large Binocular Telescope in Arizona) are performated at the back (not all the way through of course) and these holes serve the purpose of better coolant distribution to allow more stable mirror temperature.

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Yep, it's the term for a primary mirror with a hole in the middle of it. I guess if you include mirror diagonal then the SCT optical system has 1 lens (the corrector) and 3 mirrors.

OK now i am understanding it.

With a reflector................light comes in and collects on the primary,bounces up to the secondary and then onto the diagonal.

With an SCT.................light comes in and collects on the primary,bounces up to the secondary and then bounces back down to the "perforated" centre of the primary and then onto 2 smaller mirrors (prisms?) behind the primary and then up to the diagonal. So SCT's have 3 mirrors and an objective lens (as sorts).

Refracs just collect light from the objective lens and bounce it onto the diagonal and to your eye.

I must check my SCT out and see if i can see the hole in the primary. I just thought it was central spotted like a reflector.

Thanks peeps. I consider myself a bit more educated right now.

Edited by LukeSkywatcher
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No little mirrors behind the primary... the image is a cross-section, so if you sliced the OTA in half you'd see the primary is like a thick ring donut. Ok not so thick, but the hole is relatively small compared to the diameter of the primary mirror.

Just in through the objective, collected on the primary, bounces up to the secondary, then bounced back down the tube (as a narrower beam of light) and through the baffle tube - which occupies the hole in the middle of the OTA.

If you have one, the diagonal is then the third mirror. I guess the EP is really the second lens too.

HTH

Edited by Dunkster
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I guess the EP is really the second lens too.....

7 or 8 lenses if it's a Nagler eyepiece :)

It's amazing that 90% or more of the light from faint objects can get though all that lot when you think about it - all credit to modern optics and their coatings I guess.

Edited by John
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With an SCT.................light comes in and collects on the primary,bounces up to the secondary and then bounces back down to the "perforated" centre of the primary and then onto 2 smaller mirrors (prisms?) behind the primary and then up to the diagonal. So SCT's have 3 mirrors and an objective lens (as sorts).

There are no smaller mirrors or prisms behind the primary mirror. I think the poster was refering to the mirrors or prisms inside the diagonal when added to the set up.

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