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RASA Info Please

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#1 Lostone

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Posted 16 September 2020 - 07:32 PM

I have been tossing around in my head about adding a RASA 11 to my Scope collection.  But what I'm not sure about is the benefits of a RASA compared to the main scope that I'm using now other than it being an f:2.2 and my Esprit 120 is an f:7 without the reducer.  Are RASA's better for imaging Galaxy's or Nebula?  My 120 is good on both so this my be a tough nut to crack without some good intel please.  I'm gathering someone who is using both a RASA and a Refractor of similar size could help me decide.

 

Also If I do decide to add a RASA,  What auto focuser do/would you recommend.   I have a Primaluce Lab focuser sitting on the shelf that I took off my 120 when I got my Nitecrawler.

 

Oh and I did check field of view with Astronamy.tools and I didn't see much of a difference with my current camera's.

 

I would appreciate any insight or info you can to help me make an informed decision. 

 

Thanks,

Mark



#2 Lostone

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Posted 18 September 2020 - 09:32 PM

Is there nobody out there who can help make an informed decision????



#3 wxcloud

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Posted 18 September 2020 - 11:32 PM

What you are getting with a rasa 8 in my humble opinion is, a photon sucker which could use some specialized gear such as hard to find narrowband filters. It's also very bulky, but not horribly bad, but once again needs special attention to take flats and dark frames (I rarely took them), you almost need the dew shield for flats (I'm sure the camera could be unscrewed and capped for flats. I've good a little bit of tilt with mine. I do have the Celestron moto focuser I have yet to install on it. It's primarily a nebula scope, a couple larger galaxies could fit depending on your sensor size.

I've blown out m45 in one minute subs, well, the stars. Expect loads of subs, maybe a few less with narrowband and dimmer subjects.

I'm not sure if my post is helping. The f/2 is awesome but it's not a grab and go setup or something to set up on a whim. It's doable but sometimes I look at my rig and want something a little simpler... Should have started with a refractor as commonly suggested.

Oh, you'll likely need a rasa filter drawer and a couple extra drawers for it.

I'm not knocking the thing, I'd like to use it more (actually thinking dual rig) I originally got it for the f/2 and thinking I'd knock down objects quicker and this is partly right... It'll suck down photons quicker but don't keep the clouds at Bay any longer, and leaving it on automatic is a bit problematic unless you do OSC.

Just my thoughts :)

#4 George N

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Posted 19 September 2020 - 09:58 AM

Is there nobody out there who can help make an informed decision????

There have been many a long thread covering RASA right here on CN (search for them, and spend days reading) -- These threads are especially in the "Cats and Cass" group. You will find "it depends...." as the consensus opinion.  tongue2.gif

 

The 'third option' that you did not mention -- a normal or HD Celestron SCT and Fastar (Celestron name for it) system. Yet another option. While there are those who disagree - the Fastar systems work, but not a well as the RASA design. However, the SCT/Fastar is smaller, lighter, less expensive, and can be quickly converted back-n-forth between a fast wide-field to 'regular scope'. RASA is a 'one trick pony' - fast wide-field imaging only - but if that's the 'trick' you want -- it is very good at it. I've seen a presentation at NEAIC comparing one person's results with C-8 & Fastar vs RASA 8. The RASA 8 images were a little better, and would be a good solution IF you want to only do wide-field imaging. The C-8 with Fastar is 'close' for wide-field -- and can be used for prime focus, focal-reduced, and visual. It's all up to what you want.

 

You are interested in the RASA 11 -- note that there are two versions - the original available on the used market -- and the current (more expensive) option with improved focusing (other things??).

 

Compared to a similar small APO for wide-field imaging -- a RASA 11 will gobble up far more photons in the same time, and probably get closer to all the resolution the seeing provides. You will almost certainly have such short exposures with a RASA 11 that you won't need 'guiding' - but will need a mount that will handle the beast (bigger than a normal C-11). Beyond that -- the RASA 11 can be more "complicated" -- including handling all that mass and size, dew, collimation and camera-squaring, focus, and especially camera and filter selection (far more limited - with not a lot of info out there). Also -- if the threads on CN can be a guide - expect more 'fiddling around' with RASA compared to an APO (much more user friendly).

 

It's the classic case of -- is all the extra work worth it to produce a better result -- but how much better -- is a subject of much difference of opinion.


Edited by George N, 19 September 2020 - 10:00 AM.



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