RC vs Newtonian

[dannat]

I am curious about the advantages of the RC scopes vs a newtonian - ignoring the focal length difference..they are
1. coma greatly reduced
2. larger secondary providing better illumination of large chips

am i missing something?

[tlgerdes]

Balancability (if that is word). The newt puts the camera on the side, RC centre rear.

You can get different sized secondaries for the newt to better illuminate your CCD

F ratio is the main diff though as you said, which implies focal length.
Either way coma correctors, reducers, barlows, powermates etc are needed to make one more like the other.

[The_Cat]

Hi Daniel,

One difference between the RC and the Newtonian is that the RC is from the class of Cassegrain designs.

RC scopes are mainly for photographic work and is about the best one can do with 2 mirrors.

The two mirrors are hyperboloidal and must be matched exactly in shape.

When properly aligned there will be no coma or spherical aberration (The Newtonian has this in abundance off axis !!)

Their main call to fame is that the stars are are round but the scope suffers from astigmatism off axis - so aligning one of these scopes is not one of life's better experiences.

Jeremy

[multiweb]

My understanding is that the RCOS have no glass in the imaging train so they are suited for UV/IR imaging also and come with various enhanced mirror coating including gold. So they're pretty much the only top notch 'pure' reflecting system available on the market I'd say. :question: On any newt you'll need a corrector. Balance can be sorted out by placing the focuser facing up or down do you don't have any camera overhang.

[robz]

What's the cheapest type of corrector for a short F ratio newtonian?

Rob.

[Paul Haese]

There are pro's and cons to any design. The fact that the RC design is used in nearly all the large observatories across the planet is testimony to the optical design.

That said, Newtonians are a lot faster optically and that means shorter exposure times.

Issues such as image coma and optical alignment can mean the difference to some people.

Personally I prefer the RC design to the Newtonian. The lever arm effect on an Newtonian is just too much to deal with; and that is not to mention which system I think produces sharper images (but that is a matter of opinion).

[mldee]

My view is that aperture is a factor. I love my 8" f5 Newt with MPCC, good crisp images, it's fast, collimates, handles and balances easily on my EQ6.

At 10" or larger, I would choose an RC, from both the size and setup viewpoints, although you pay a premium for that convenience.

My next scope will probably be an RC10 for deeper DSO work.

My hobby-level 2c.

[dannat]

thanks for the replies guys

to answer robz -cheapest corrector new is the GSo sold at andrews for 179, or astrotech US version is 139 (if you already bought something add it to the package)

for those with the GSO RC"S - do they hold collimation better than a newtonian? (eg do you have to check /adjust each time you use one -

[apaulo]

Hi Daniel

I own a 8" F/4 newtonian and the mpcc I use is a Baader coma corrector.
I get clean sharp images, so for value for money I am very pleased.
I do collimate the scope very time I use it and it only takes about 5 minutes. The scope holds it's collimation well. Another thing to consider is the Dew affecting the primary mirror, I think dew would be more of a problem with RC's compared to Newts.

Best regards Paul.

[solissydney]

The attached info might be useful to you all.
Ken

[robz]

Thanks Daniel:thumbsup:

Rob.

[dannat]

anyone have an answer? "for those with the GSO RC"S - do they hold collimation better than a newtonian? (eg do you have to check /adjust each time you use one -"

[multiweb]

MPCC baader ~$250. Then it goes up quickly with the Keller, Wynne correctors, etc...

[pjphilli]

Hi Daniel
In the absence of comments on you query about GSO RCs holding collimation, I can only give you my comment on very limited experience on using my RC in view of consistently lousy viewing here in Sydney.
I have checked my collimation several times and it is still spot on as taken out of the original packing. I guess if it can hold its factory collimation with all the shaking around it must have got in transporting it, I would take an optomisitic view of its longer term stability.
Cheers Peter

[strongmanmike]

Hi Dan

Although a little more answer than necessary for the brief question in your post :D..perhaps there is something for you in the following :question:.

I have recently been deliberating on an OTA to up grade too and yes it was a difficult set of considerations to narrow down. Considering I am virtually a 100% portable imager and and such imaging time is very precious... in the end after considering a number of options I narrowed it down to a 12" F3.8 highly corrected Newtonian in a CF tube and as far as the particular design goes, this was why:

1) Decent aperture for bright images = less exposure
2) Super fast focal ratio for faster recording of faint extended objects
3) Decent focal length (1140mm in this case), providing between 1.2" - 1.6"/pixel image scale with the typical pixel size in the popular cameras on the market (more than adequate to reveal fine details in any object) but not too long that guiding in the open field becomes more of an issue (I think the sweet spot is between 1000mm and 1500mm).
4) Following from this - a large field of view to put plenty of objects within reach for framing (1.85deg X 1.85deg with 16803 chip) and allow for cropping into various individual shots all taken at once.
5) Large well illuminated and highly corrected image circle with exceptionally small stellar spot sizes off axis.
6) Relatively easy to collimate
7) Still compact and easy to transport
8) CF tube to hold focus well over temperature changes
9) Robust structure to minimise flexing

In the end for me, I wanted everything :lol: ie aperture, photographically fast, adequate focal length, adequate image scale, portability and focus stability = all things paramount to efficient portable high quality imaging :thumbsup:

Of course there are a number of potential pit falls with the fast corrected Newtonian design, particularly related to flexing and heavy imaging eqipment and appropriate measures need to be taken in the design to combat these...time will tell if the particular OTA I have on the way from Orion Optics UK has done this properly...I have my fingers crossed :prey:...the small number of existing images done with this OTA already and those taken with other fast higly corrected Newts at least suggest this is very achievable.

NB** While the exact scope I settled on isn't manadory to address all the criteria mentioned by any means, I think these desired outcomes (or similar) are more achievable with a corrected Newtonian design than an RC.

Mike

[dannat]

thanks mike -great answer

[strongmanmike]

Cool Daniel, glad it had some usful ideas/facts for you.

I am trying to accomodate a 37mm X 37mm chip in a large camera with filter wheel and digital focuser hanging off the side of the OTA - all of which is a large image circle and pretty heavy kit, so if you are only trying to cover a smaller chip in a smaller and lighter camera (like a DSLR) then things can be even easier, simpler and cheaper with a fast corrected Newt :thumbsup:

Mike

[Archy]

Get a quote for a 16" RC and mount and compare with a 16" Dob.

[gregbradley]

Mike has summed it up.

Cost is a big factor. RC designs typically prior to the GSO scopes are mega expensive and also require a beefy mount as they are heavy.

Newts have short focal length for their size and have bad coma unless you have a good corrector. If you want to use a Newt with a large camera like Mike's then you are back to mega bucks.

GSO RCs judging from images posted in this forum would be hard to beat for imaging by a Newt. But there are also lots of nice Newt images as well. But they will be more widefield generally. The RC gets up close.

If you are new to imaging the RCs long focal length will be a liability as all the tracking errors get magnified and it will prove to be very difficult.

Shorter focal length is always the best way to start so you can have some fun without getting frustrated by the tracking errors.

Of course if you spend megabucks on an excellent mount then this is less of a problem.

Mount is more important than the scope.

A really good mount will make a really cheap scope take good images.

A really good scope on a crap mount will not produce good images.

I take it you already have a mount? If not ask the many experienced people here about what is a good mount and your price range.

Greg.

[dannat]

greg have just bought a secondhand em10:thumbsup:

[gregbradley]

Good move. I am sure it will be a source of a lot of fun in imaging.
The thing I like about the Tak mounts is they are really user friendly and simple to operate plus very accurate.

Greg.

[dannat]

i know this goes outside my original question ..........but

why would you want to blow $2000+ on a 4" refractor when you can get more light gathering for less?

[strongmanmike]

The Orion Optics AG12 OTA is only about AU$11 000 at the moment compared to a 12.5" RCOS at AU$23 000. It's still a lot of money but cheaper than all but the GSO RC's - although I don't know what the price of the GSO 12"RC will be? If you go with a smaller cheaper camera like an FLI Microline, QSI, Starlightxpress or even a smaller SBIG then the standard OTA should be perfect ie good bang for your buck I recon..? Assuming it works as advertised...time will tell, I'm still just a little nervous :scared2:

Mike

[TrevorW]

Actually it's not all about LGP when imaging, Craig Stark has written a very good article on this and suggests that a good refractor can be better than a larger RC/Newt when it comes down to resolution

http://www.cloudynights.com/item.php?item_id=2394

[dannat]

thanks trevor - maybe i should get a 90mm tak:thumbsup:

[gregbradley]

There are hundreds of super images taken with 4 inch refractors. You don't have to know anything about the theory just look at the final results.

The multitude of images speaks for themselves.

An FSQ106 whilst not $2000, is in a league of its own and many would consider it the king of widefield imaging scopes.

If you want close up galaxy images though you can't get away from the need for largish aperture.

Greg.