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dpastern

Primer on telescope types

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Or the "watch with mother" translation - RC - Open tube catadioptric which uses a hyperbolic primary mirror to produce a flat field. The hyperbolic mirror is difficult to manufacture so they are expensive.

Characteristics - flat, well illuminated FOV. Relatively fast for long focal length scopes and can handle focal reduction well. Large central obstruction may effect contrast slightly.

Mainly sought after by astroimagers for their flat field and high quality optics. The weapon of choice for the worlds best imagers sited in their New Mexico observatories and operated robotically from Seattle. Also Hubble is an RC design.

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Thanks guys, wasn't thinking too straight last night as my dad's in ICU. George - usually Google *is* my friend.

I wonder why RC optics are so expensive to make...

I'm thinking of going something like this:

http://www.astronomics.com/main/product.asp/catalog_name/Astronomics/category_name/U5QNWB3RKWWL8N3EL99F9DX9A0/product_id/AT8RC

or do you think a Meade LX200 10" ACF OTA would be a better choice - more aperture, and supposedly RC like optics, and it's probably cheaper to boot. Planning my telescope investments for the next 12-24 months of expenditure ;-)

Dave

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Well if your going to the expense of a RC your obviously going down the imaging route :p if that's the case you need to be thinking about what mount your going to use.

The mount is actually more important than the OTA to a degree ie you can put an iffy OTA on a good mount and get reasonable images, but a good OTA on a wobbly mount is a no no :)

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For me, the focal length of that RC in the link would be too long, as would the LX200 ACF. The seeing, and the mount accuracy both conspire against using a long focal length for imaging, so I'd go for the SW Mak. Newt. option as its cheaper than the two 'scopes you refer to and probably as good for imaging. If you're a visual type, then get either a normal Newt. or an SCT as your eyes don't need the big flat field that a camera does.

For visual use you could do worse than a Tal maksutov, they're quite nice and cheap, but do tend to dew up a bit.

Kaptain Klevtsov

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i dont believe meade make a true ritchey chretien design, infact they were sued for stating 'advanced ritchey chretien'

my friend has a 6"f/9 from telescope house.

we havent used it on the sky (no stars), but he said it was flat over at least a DSLR sized chip, and gave a sharp image...at 1.8m focal length, thats pretty good.

the sky can probably handle a 1.8m focal length on most nights.,

but the mount needs to be good. Get a good mount.....

at 1.8m focal length, and f/9 you cant do unguided subs for DSO's, so you will have to guide, so make sure the mount has an autoguider port, most good mounts come with GOTO.

Look at the vixen sphinx series of mounts. They are good quality.

that 6" f/9 (astrotech?) on a sphinx mount would be very nice.

What camera are you planning on using...

paul

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I'd think that at f9, it'd need a reducer of some kind as it's a bit on the slow side.

Tony..

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yeah, fine, put on a reducer, get to about f6 or 7.

atmosphere gets more forgiving and you can reduce the sub length.

but which reducer have you in mind. it couldnt be a reducer/flattener, since the field is already flat.

i think AP used to do one for the AP Mak? but Im not sure, can someone correct me?

paul

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Yes AP do a plain reducer which gives 0.67 55mm from the chip. The Celestron 0.63 reducer does very little actual flattenning so can be used with flat field designs - I've checked this out using CCD inspector. Meade really boobed when they called their SCT system an advanced RC design. A real shame because it is an excellent design with a very flat well illuminated field.

Above 1000mm focal length resolution is going to be compromised by seeing unless you are using a camera with large pixels. Above 1500mm and the seeing will nearly always be affecting resolution.

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Also from what I've been reading elsewhere, there is a specific reducer being designed and maufactured for these scopes. Bringing one of these scopes down to about f6 (a reducer of about x0.7ish) or so would be superb considering the limited time we get over here in the UK.

Tony..

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A non flattening reducer with an SCT thread would certainly be handy. The AP reducer is great but the problem I have with it is that I use active optics which extends the image train to the point where it probably produces excessive reduction unline the Celestron reducer which is 0.63 at 105mm.

Sorry Dave we have started drifting away from the original topic. Have you RC questions been answered? Are there other telescope types you would like to know about - the catadioptrics are a pretty confusing area, Vixen make some interesting designs then there is the mak newt, schmidt newtonians, dall kirkhams etc etc. You are moving beyond the newcomer zone really with these designs

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Also from what I've been reading elsewhere, there is a specific reducer being designed and maufactured for these scopes. Bringing one of these scopes down to about f6 (a reducer of about x0.7ish) or so would be superb considering the limited time we get over here in the UK.

Tony..

I've read the same thing as well Tony.

The Astro-Tech OTAs are sourced from GSO. Australia's bintel.com.au also says that it's waiting on some Bintel branded RCs, which, looking at the images, will *almost* certainly be the GSO OTAs. Andrews communications has the GSO RC's in stock (both 6" and 8"), so looks like I'll be talking my dad into lending me a wee bit of money lol!!!

As to mount, I'm intending on getting a Celestron CGE. It compares well to the Losmandy G11 and is a fair bit cheaper. The G8 is about the same price, but has a small weight load of course, making it less value for money. From what I've been reading, the CGE is a very nice mount, very stable, and quite accurate.

As some others have deduced, I plan on doing mostly imaging, DSOs etc.

Whilst I'm relatively versed in Astronomy from a "theory" point of view, my practical experience with GE mounts is non existant, so I'm sure I'm up for a learning curve. I'd rather invest in quality gear than get a cheapie and waste money.

As to camera, I'll be using my Canon Mark IIn, and maybe a CCD imager down the track. I'll need to see whether the extra investment is worth it, etc.

I've been doing more reading on the Astro-Tech RC and it does sound very good, many people are seriously interested in it. I'm just worried that the Guan Sheng optics will be average etc.

I could go an alternative 10" Meade LX200 ACF tube, but the lure of a RC is very tempting for the obvious technical reasons.

Thanks for everyones posts, much appreciated.

Happy New Year to everyone as welll!!!

Dave

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I could go an alternative 10" Meade LX200 ACF tube, but the lure of a RC is very tempting for the obvious technical reasons.

Dave, what are the obvious technical reasons?

If you are new to imaging you can go for a long focal length scope like this but have you considered a shorter focal length refractor to start off with? These are easier to start off with but you never grow out of them.

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I had thought about it yes (a Newtonian short f stop scope), but I believe that from my research, and talking to some quite experienced guys over at photography-on-the.net/forum, that I'm best to consider several things:

1) A very good mount - cheaper mounts will really hamper astrophotography.

2) Furthermore, mounts within my financial grasp are going to limit the size, and therefore weight of the OTA that I can really effectively use. For the Celestron CGE, I've been advised to go no more than a 9.25" Celestron SCT in terms of size/weight, and stopping and thinking and doing the maths, I'm inclined to agree with their advice.

I'd given strong thought to the RC type of scope previously, but had said "no" to myself due to the extreme costs involved. They have several distinct advantages from my rabid research:

1) very flat field

2) less problematic in terms of collimation

3) far better coma performance than alternatives

4) far better in handling cool down than SCTs etc

True, #4 isn't really an issue with Newtonians either, but I believe fast Newtonians tend to suffer from coma and a not so flat field.

Of course, the RC design has some issues I believe:

1) slight loss of contrast to the larger than usual secondary mirror

2) Not always the greatest for visual observing

3) astigmatism issues on the edges

4) cost

5) cost

I look at it from the point of view - one can always use a focal reducer. One can always crop an image. I'm also probably not going to do a lot of visual observing - the imaging aspect interests me far more. There's no secret that most of the really good astro imagers tend to use RC's for their work, and for good reason.

A good Newtonian is not a bad choice, no, I agree. But, if these GSO RCs turn out to be optically good, then they're not going to be a lot more expensive than a similar quality Newtonian, but they'll have several key advantages as far as I understand.

I've done a fair bit of research, and given this quite a fair bit of thought, and unusually for me, asked several good astro-imagers over at POTN for advice. I want to make a solid investment.

I'd also be considering a 80mm refractor (I've been recommended a few, including the skywatcher equinox 80mm OTA, which apparently has been getting rave reviews), which will be used as a guidescope piggybacked on the main OTA, and probably for imaging as well.

Dave

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Just to add to the GSO RC's - I sent an EMail today to a major Australian telescope retailer and their reply was well...not encouraging. They said that they had tested the GSO RCs and provided a detailed test report back to the manufacturer and that the manufacturer was going to improve the focuser. They basically said that they are expecting another review unit, but would not commit to buying them in large numbers until they had tested it. Reading between the lines, that tells me that they weren't impressed with original test unit optically. They were rather curt (and rude) with me saying that the test results were between them and the manufacturer. I've politely pointed out that the GSO units are already in stock at another retailer in Australia, and in several in the UK. Also, the astro-tech is based on the GSO, so it's obviously entered the mass manufacturing and distribution phase.

From what I can see, the GSO RCs do not use pyrex glass for the mirrors, the spiders are plastic, not metal. The GSO focuser is a GSO brand, whereas the Astro Tech unit uses a Feathertouch crayford focuser (far better quality than the GSO equivalent I'd say). There are on real reviews on these scopes yet, and I'm really waiting to see what *real* users, and not retailers say before committing. As I said earlier, if need be, I'll consider the Meade LX 200 ACF unit (10"), although with Meade's financial troubles, and very poor customer service record, I'm reluctant to go with Meade to be honest. I do believe that the ACF units are optically better than the Celestron SCT OTAs (and by a reasonable amount), approaching RC levels of performance.

Australia has a very poor range of telescopes, most of the retailers either do a lot of the cheap stuff, or very expensive high end stuff, with little in between. I've seen only one retailer out of around 8 or 9 offering the Celestron CGE mount, which is pretty sad. We don't seem to have the variety that those in Europe, or especially, the US, seem to have.

I'm a quick learner, and would be joiner a local Astronomy club to further more knowledge etc.

Dave

PS Oh, and I'm tempted by the teasers that GSO will offer a 10" RC. With 50% more light (or thereabouts), it's very tempting. Weight wise it'll be lighter than a 10" SCT too, so bonus, as it'll be a lighter load on the mount for the same light gathering ability!!!

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Dave, this is esoteric gear you're looking at. The design of the scope is one thing, the quality of manufacture is another. The best imagers in the world do use RCs made by RCOS. They also site them in places like New Mexico at altitiude with amazing seeing and night after night of clear skies. They can make use of long focal lengths and have enough imaging time to mosaic their images together.

If you haven't already done it you I think the best thing for you to do is get on the phone to Ian King www.iankingimaging.com He sells the Astro Tech scopes and prior to this he was selling an Italian RC (which was a nightmare to collimate apparently), but he wont push you in that direction unless he feels it is right for you. He will be able to give you alternatives and a whole lot of other things to consider. You can trust the advice you get from Ian.

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Yeah, I realise that the *quality* of the manufacture is important, and that's why I'm hesitant about the GSO units. They're not cheap, AU 3k for the 8" OTA, but when compared to the alternatives, they're a bargain. Of course, one wonders why they are that cheap. As a rule, I'm not a fan of Chinese made products - they're usually of a very low quality standard from my personal experiences, and as a rule, I *try* and avoid buying Chinese made products as a result.

I'll consider calling Ian, although being in Australia, it might not be the best idea, simply because buying overseas means no local warranty should things go wrong, and it's very expensive to send large and heavy items overseas as you can imagine.

My mind is not set in concrete, I'm just leaning towards a RC because the design (or raison d'etre) is what I'm aiming for. I realise that RCOS is the brand that many of them use, but at 20k-30k, I cannot *justify* that sort of expenditure. I'm not well off, and I'm not on a highly paid job. I do value quality.

Other OTAs that I'm considering are a Celestron 9.25" SCT, the aforementioned Meade LX 200 ACF 10". I'd prefer a more compact design such as the SCT, rather than the bulk of a Newtonian. Someone mentioned a mak-newt, but I know little of this design, so I'm not so sure if it'll suit my needs/desires. I believe that most mak-newts are limited to 7.5", which sort of puts it out of my needs - I want at *least* 8", prefer 10" which offers another 50% or so light gathering capability. I don't want anything larger than 10" due to size/weight/cost, plus it would really require a far more expensive mount that can handle the weight/size of the larger OTA and track accurately for long image stacks.

I don't really want to spend more than AU $3k on an OTA anyways - that's really a maximum that I'm looking to spend. I'd really like to see some reviews on the GSO units, and some sample images, and I have requested some more information from GSO via EMail. Their lack of response makes me a bit nervous in regards to support and readiness for the units in the retail market, which sort of mirrors what this particular Australian retailer indicated if you can read between the lines of their EMail:

Yes they are made by GSO. We have one that was sent to us from GSO for testing and we sent him the results of those test with the full analysis of the scope.

He is looking at those results and will be making some changes to the scopes in particular to the focuser.

We will be getting another one sent out to test before we commit to buying any in great numbers.

If the tests results are to our liking we will order some.

We are not at liberty to release any of the results of the testing we did.

That is up to the manufacturer.

Like everyone else we are really looking forward to getting these scopes in to stock.

This is from Australia's largest retailer, who's been in the business for 30 years plus from memory.

I'm still in the early stages of pre-planning and investigation, so nothing is set in concrete. A week ago I was considering an Orion 11" SCT on an Atlas GO-TO mount :)

Dave

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Ah sorry Dave, it hadn't dawned on me that you are in Australia. Doesn't really make sense to discuss it with Ian then. Also changes the situation somewhat. Long focal lengths are much more of a challenge for imaging - any tracking errors show up, harder to find targets, critical focus etc. In Aus you will have much more opportunity to get to grips with the kit than here in the UK and your seeing conditions may also be better if you can get out to a rural area.

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Still Martin, advice is welcomed and taken on board. Nothing is set in concrete at the moment, still pondering what setup to follow. We've had a wild and wet Summer in Brisbane with some major storms (we got flooded 7 weeks ago with 3" of water, I nearly lost all my computers, which sit on the floor in my room). I'm being very cautious about my choices!

Dave

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That's our problem Dave, we don't get weather here. We have rain with cloud, and we have cloud with no rain. It can be windy with cloud or calm with cloud. Cloudy and cold or cloudy and warm. Sometimes we have spells when we see no clouds - we call that foggy.

Right now we are sat under a high pressure system and at about 9.30am I spotted something blue overhead. Tonight it's a reasuringly cloudy dull orange sky illuminated by the light pollution which is pretty well universal across England.

Not that I'm bitter, we get lots of chance to practice processing images, normally other peoples.

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That would drive me crazy Martin :) We've had a higher than usual cloud cover this Summer, I wonder if it's worsening due to global warming?

I've just been talking to a local retailer via EMail and they're telling me that the Astro-Tech unit, when used with the feathertouch crayford focuser will have collimation issues due to it using a dovetail mounting system. I'm not sure why that should be an issue, do you have any idea? I probably should have this thread moved to the general help & question section, it sort of morphed from my original post unintentionally!!!

Dave

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Dave did you place your order yet I've just placed mine for the 8" at $2500 from Andrews.

The Astronomics version is exactly the same except thiers comes with an extra Losmandy dovetail. The focuser is not a Feathertouch is just advertised as such. A true Feathertouch focuer alone costs nearly 1/2 as much as the scope is US.

Check out the pcitures on Astronomics site, it's the standard GSO focuser.

The proof is in the pudding so too speak and as actual photo's taken by these RC's are rare and reviews scarce (although primarily reports are varied). I thought stuff it I'll buy it and do my own review.

It's an imaging scope and thats what it'll be used for.

Cheers

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I'd given strong thought to the RC type of scope previously, but had said "no" to myself due to the extreme costs involved. They have several distinct advantages from my rabid research:

1) very flat field

2) less problematic in terms of collimation

3) far better coma performance than alternatives

4) far better in handling cool down than SCTs etc

True, #4 isn't really an issue with Newtonians either, but I believe fast Newtonians tend to suffer from coma and a not so flat field.

I would like to comment on this.

1) The RC does not have a flat field unless the radius of curvature

of both mirrors are near to equal. If so, the secondary has to be

very big if the image "plane" lies behind the primary mirror.

Please have a look into the book "Telescope Optics" from the authors

Rutten/van Venrooij, chapter 7.

2) the RC is much more problematic in terms of collimation than a

Dall-Kirkham Cassegrain and more problematic than a clasical Cassegrain.

3) The RC corrects for off-axis coma with only two mirrors.

But it suffers from off-axis astigmatism and there will be field curvature

unless the secondary is very big. So you will need to correct for both

if you want to use the linked RC for wide fields.

But if you use an additional correcting lens arrangement you could use

other types of telescopes as well.

For example a Dall-Kirkham with additional lenses like the Orion UK ODK.

Or a Newtonian with coma corrector. Or a Hyperboloidal primary mirror

with a 3-lens correcting/field flattening lens arrangement like in the

Takahashi Epsilon.

I think this design has been invented by the german optical designer

August Sonnefeld.

4) the open cassegrain-type telecopes cool down much faster than

their closed brothers SCT and full-aperture maksutov casegrain.

But this is true for some other open designes too. Like Orion ODK,

Takahashi Epsilon, Takahashi Baker-Ritchie-Chretien, Newtonian with

comacorrector, sub-aperture Maksutov-casegrain...

But all of them will need a spider to suspend the secondary mirror

or secondary mirror plus corrector, wich causes diffractin spikes.

True, #4 isn't really an issue with Newtonians either, but I believe fast Newtonians tend to suffer from coma and a not so flat field.
Pure Newtonians do have a quite mildly curved field of view.

The radius of curvature is determined by the primary mirror radius.

Generally, in a Newtonian, the aperture stop is placed in contact

with the primary mirror.

The sagittal image surface is flat, and that the tangential image surface

is curved. The radius is the focal length devided by 2.

So there is astigmatsim, It is astigmatic difference between the flat sagittal

and the curved tangential image surface. The medial surface between

the flat sagittal and curved tangential image surface is curved with

a radius equal to the focal length of the miror.

So the field is relatively mildly curved, but suffers from off-axis astigmatism,

and most disturbing, off-axis coma.

Clear skies, Karsten

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