Celestron 11" f/2.2 Rowe-Ackermann Schmidt Astrograph (RASA) optical tube

[alpal]

11" f/2.2 Rowe-Ackermann Schmidt Astrograph (RASA) optical tube.

see link:

https://www.astronomics.com/11-f22-rowe-ackermann-schmidt-astrograph-rasa-optical-tube_p20206.aspx


What do people think about this new scope from Celestron?


F2.2 is very fast.
I wonder if you can use an OAG with it?

It would be nice if they published a large 4000 pixel wide frame of an image.

[RickS]

They quote a 70mm image circle then only show vignetting and spot diagrams for a 42mm circle :) It will need a compact camera anyway.

Back focus is going to be very limited. Shame they don't give us the data in the specs. I doubt it will be possible to fit an OAG but at 620mm focal length a guide scope will be fine if the mirror doesn't slop around.

Will be interesting to see how the results compare to a Hyperstar set up.

Cheers,
Rick.

[gregbradley]

Looks interesting. Mirror flop may be something to know if its a problem or not.

The posted images are very small so its hard to evaluate them. The usual problem with very fast scopes is they are often hard to collimate and very susceptible to any flex. So that would be the thing I would want to see, how rigid is it, does it suffer from tilt/flex with bigger cameras?

Correctors tend to dew up. But it could be interesting. I'd like to see more sample images. F2.2 potentially sounds good and 11 inches of scope is a good size.

Greg.

[alpal]

Hi Rick,
Yes strange how they don't specify the back focus.
It seems like an alternative to the Hyperstar.
You're right - they don't show the spot diagram at 70mm.

As the premium end of the market has no such beast for sale at below f3
I have to seriously doubt the credibility of this telescope design.
Still - I have an open mind & I'm interested to see the results- one day.

cheers
Allan

[alpal]

Hi Greg,
someone will buy one & we'll all get to see the results.

cheers
Allan

[Camelopardalis]

A chap over on CN already has one, so we might not need to wait too long to hear initial feedback.

The RASA has mirror locks like the Edge HD series, so once you've got good focus you just lock it up...bye bye mirror flop.

[Satchmo]

Alan - Dave Rowe is the inventor of the Corrected Dall Kirkham cassegrain design ( CDK ) which is currently having tremendous success for its relatively fast speed and and wide flat coma free field. You can be sure that the design is exceptional - on paper at least . Whether it can be pulled off mechanically is another question , but as modern imaging can pull off modest length subs and refocus when necessary and the image scale is fairly modest I can't see how it would not work well.

[alpal]

I can't wait to see the results.

[alpal]

Celestron could also be using his good name for marketing purposes.

[gregbradley]

It looks like the camera is meant to be at the corrector end. That would limit the size of camera to be used. So the 70mm circle would be hard to
use. Perhaps a FLI Microline 16803 may work as its very compact.

Greg.

[alpal]

Do you know under what name he posts at Cloudy Nights?

[Camelopardalis]

Coastal IIRC...have a look in the Cats and Casses section.

[alpal]

Thanks - here is the link & he's posted a cropped photo of M13.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6579302/page/2/view/collapsed/sb/5/o/all/fpart/1


cheers
Allan

[raymo]

It does tell you the back focus. It says the back focus from the camera
adaptor is 55mm.
raymo

[RickS]

Silly me. I thought they might have listed it in the specs :) No room for any fancy accessories...

[alpal]

Gee wizz - what camera , filter wheel & OAG can fit on that?

[gregbradley]

That's next to impossible. SX Trius, Mini SX filter wheel with built in OAG + Adapters is about 54mm that may work.

The Trius is 17mm to the chip from the lip of the camera.
The Mini USB filter wheel with OAG is 28mm
The 2 adapters on either side are about 9mm.
Then any adapter on the scope itself is an unknown. A Trius 814 would be good with this scope 9mp and high QE low read noise and small although the filter wheel is a bit large.

Probably best with a one shot colour and OAG.

Greg.

[Camelopardalis]

55mm happens to be normal for DSLR....

[MrB]

Yeah, DSLR without an OAG...
55mm is rediculous.

[rustigsmed]

yep camera at the corrector plate end, dslr, OSC or mono without filter wheel.
guiding? probably not - but do you need it?

[Rod771]

With the camera fitted to the corrector end of the OTA, its not intended to be use with a an OAG or filter wheel. As Rick has mentioned at 620mm focal length a guide scope will sufficient. My Hyperstar set up has a FL of 560mm, all
I use to guide is the Orion mini guide scope. Works perfectly.

The fact that the aperture is 11 inches and has 55mm of back focus suggests they have covered the DSLR market with the intention of making it "plug and play". You could still use a mono CCD using Starizona's (http://starizona.com/acb/Starizona-Filter-Slider---Complete-System-P3518C91.aspx) Filter slider.

Greg is spot on, the SX 814 would be a great camera for this scope. I'm looking at getting one for my Hyperstar set up. Just need the cash :rolleyes:

[alpal]

But why would Dave Rowe and Mark Ackermann design a telescope
corrector with so little back focus?
I like using an OAG - I don't want to throw away any subs from differential flexure.


My RCC coma corrector designed by Dave Rowe gives me 91.5mm of back focus for my Newt.
see here:
http://www.optcorp.com/ba-rcc-i-rowe-coma-corrector.html

That is what allowed me to use an OAG , filter & camera.
It also allowed me to use a Baader Varilock spacer to adjust for coma correction.
see here:
https://www.flickr.com/photos/24719437@N03/8029429824/in/photostream

What is it about this new design that does not allow for more back focus?
Surely Dave Rowe would have made a design to give more back focus if he could have?
That was the whole point of his Baader RCC corrector.

This major point is somehow hidden in the slick advertising here:
https://www.astronomics.com/11-f22-rowe-ackermann-schmidt-astrograph-rasa-optical-tube_p20206.aspx

[RickS]

The f/2.2 light cone is very steep. The front corrector would need to be larger and more expensive at longer back focus and it would cause a greater obstruction.

Cheers,
Rick.

[RickS]

A back of the envelope calculation shows that increasing back focus from 55mm to 110mm would add about an inch to the diameter of the front corrector lens assembly. I'd say that the 55mm back focus was an economic decision...

[alpal]

Thanks Rick,
I knew there had to be a good reason for it.

cheers
Allan

[alpal]

Yes - the camera goes on the front corrector plate:

http://www.celestron.com/browse-shop/astronomy/telescopes/rowe-ackermann-schmidt-astrograph-with-cgem-dx

How can you stop the fan from the peltier cooler from disrupting the air flow?
Also - the camera has to be connected by cables - so I take it that they
are placed across the aperture?

[Rod771]

You could just cut power to the fan if you dont want it to run during imaging.

Yes the cables run across the aperture just as with any Hyperstar set up. There is a trick to running the cables. If you run the cables in a curved configuration like in Gerald's picture here (http://www.werbeagentur.org/oldwexi/fotos/hyperstar_komplett.JPG) the result will be a nice image without odd diffraction spikes, which is what you're likely to get when running the cables off in a straight line.

[alpal]

Thanks - now I see how the Hyperstar works.

That's clever with the curves cables -
just like a curved spider in a Newt.

If you cut the power to the fan on the Peltier cooler the
temperature will rise.
You would increase the thermal noise.

Maybe at f2.2 you wouldn't notice the disturbed air in the light path?

Really - you need to have the camera attached to the rear as in a classical Cassegrain or RC scope.

I don't like the design of this new telescope.

[Camelopardalis]

Atik sell a range of cylindrical ccd with fans for Hyperstar use, so maybe they "solved" the turbulence issue or maybe it isn't an issue at that focal length. There are a lot of people imaging with Hyperstar, it's pretty popular over the big pond.

And with the evolution of mirrorless DSLR, the body is shrinking all the time, and some models already barely cause any obstruction to the light path. I'd say this is a product looking forward...

[Octane]

So, I take it, a 16803 chip with a 10-position filter wheel, is out of the question? :P

H

[gregbradley]

:lol: Works fine, just a bit of vignetting to sort out!

Greg.

[Rod771]

Well actually, even though the Hyperstar is designed for a 27mm chip diagonal, Andreas has proved here http://astrob.in/78743/B/ with the correct flats it will work with 45mm chip. He's using the Atik 11000. Granted its not the 16803 but its pushing the boundaries.

Narrowband imaging is also possible at F2 where bandwith shift is present. Andreas is also testing new "High speed " 10nm Baader filters with his Hyperstar set up. See here http://astrob.in/99398/0/

But a 10 position filter wheel :question: Might have to remove the secondary off Greg's 17'' :D

[Paul Haese]

Hmm some thoughts come to mind.

1. The front corrector is going to be the main problem. An 11" corrector plate is going to dew up like nobodies business without a dew heater and using a dew heater will mean tube currents.

2. This appears to be a metal tube. Not a grand idea either. This sucker is going to move around like a mad woman's custard until it gets cold.

3. A QSI583WSG would fit the back focus but it is a narrow field of view. You would get an OAG with that camera.

4. No room for a rotator or fine focusor.

5. I have never liked the idea of hanging a camera off a piece of glass.

In short I don't see how this could be a great system. It could not be effective as a remote scope. Nice idea but I am uncertain how it would work well in practice.

[alpal]

Yes - how do you stop the weight of the camera from distorting the glass?

Yet alone - the air current disturbance?

You can't use an OAG or a filter wheel.

I can't see how such a system can work.

[Camelopardalis]

I have no interest in buying one, but there is some info on Celstron's RASA webpage (http://www.celestron.com/browse-shop/astronomy/telescopes/rowe-ackermann-schmidt-astrograph-with-cgem-dx)...

Glass is pretty rigid and doesn't have a great tendency to bend...ask any hyperstar users how they feel about loading up their corrector plate. Besides that, the secondary mirror assemblies in large SCTs aren't exactly light.

The biggest issue with the corrector plate is dew, and that is climate dependent. A dew heater tape should do a decent job of keeping the corrector clear.

And then the "12V MagLev" reduces cool down time (quote), and should keep the internal air somewhat equilibrated.

I'm surprised they didn't discard the corrector entirely and use some nice curved spider vanes.

Fine focus is accounted for with included 10:1 FeatherTouch.

I'm not trying to be smart, just quoting other info that's out there :)

Whether it all adds up to something you pros want to buy....sounds like a "no", but time will tell how it works out for others when they get out in the wild.

Btw, the picture on their website shows their Nightscape CCD on the front...maybe that's what they envisage people using with it :confused2:

[multiweb]

This is like a built-in hyperstar. You wouldn't hang anything too heavy on the corrector, certainly not a filter wheel or a wide camera. Cylindrical Starlight Xpress would be ideal. Having said that the corrector would be quite thick to compensate the weight. I've never had any flexure or aberrations with my C11 at the front. It's always been primary flop originaly. I doubt very much the usable imaging circle would be very wide for a big sensor. Dew is less of an issue with these systems actually because the camera fan and heat tend to keep the moist out of the front if you have a dewshield long enough. That was my experience with the C11. Good pricing for what it is though.