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Thinking of getting a RASA 8 and have some questions...

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#1 The Enginear

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Posted 20 July 2021 - 06:54 PM

Like the thread title says, I'm thinking about getting a RASA 8 - specifically, the bundle with the AVX mount. I've been using an Orion XT8g and a Skywatcher Evostar 72ED on a Celestron SLT mount along with SharpCap for EAA. The AVX would be my first GEM mount. My main camera is a ASI294MC with a DIY cooler. I also have a RI224. 

 

Will the ASI294MC work with the adapters that come with the RASA 8, or is there something else I'd need to get? 

 

Will an Artesky 2" filter adapter allow me to use some of my current 2" filters with the RASA 8? For example, my Optolong UHC? 

 

How hard is it to add a finder shoe / finder to the RASA? At bare minimum, I'd want a red dot finder for star alignments.

 

Does the RASA 8 come with a cap for the end, and if not, what do people use to keep the dust off the corrector lens when they're not using the scope? 

 

I have a good view of Polaris from my Bortle 5 back yard. Do I also need a polar scope to go along with the RASA and AVX to do polar alignments? Speaking of polar alignment, how hard is that to do with the AVX? 

 

How hard is the AVX to use in general? Is it a heavy beast to move around? How many pieces does it break down to if I want to take the whole rig out to a Bortle 2 site about 2 hours from my house? 

 

The RASA would give me a similar FOV to what I have with the 72ED and I'm sure it would be outstanding for larger nebulas/targets, and it would be a lot faster, even with the .80x focal reducer / flattener I use with the 72ED, but for smaller targets like galaxies and globular clusters, I'm wondering how it would fare vs the XT8g - thoughts? 

 

$2,500 is a lot of money - talk me out of spending it, or give me some good reasons why spending that much on a RASA 8 would be a good addition to my gear list. 

 

Thanks!


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#2 Medic002

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Posted 20 July 2021 - 07:24 PM

I do not own a rasa but I have been thinking of purchasing one also. So far from what I have learned is that it will weigh somewhere around 17 lbs I believe and that is on the heavier side of the total weight that an avx mount can hold which is around 30 lbs max. It could give you a few probs with guiding once you have your camera and everything else mounted but maybe not if u have a really good avx. I have an avx myself and it works ok for guding no major issues so far. I think most of your filters u will not be able to use with the rasa. You will have to buy special high speed filters to use with the rasa. Most filters are made for f4 or higher and since the rasa is f2 u will need the highspeed filters. The avx mount is sturdy though and not real heavy to move around without the counter weight on it. I like the avx with the 2 inch tripod legs it supports my edge hd 800 well. You should be able to mount a finder scope or guide scope on the rasa with a basic finder shoe bracket with no problems. You should be able to mount a finder or red dot or telrad with no problems. Most everything mounts just like on a SCT telescope. A camera specific adapter has to be mounted on the front to attach your camera and most of them cost around the 50 dollar range. The only downside of the rasa is that it can only be used for astrophotography but from what I have seen it takes great pictures. Another issue is that a lot of the other items you may want to add on are more expensive like the filters. The polar alignment on the avx mount is not very hard to do. You can do your basic 2 star alignment and get stars centered and add calibration stars and then do the all star polar alignment routine in the hand control or use cpwi on the laptop. The all star polar alignment can be done on any star even if u can not see polaris. I would buy the Rasa myself if I had the extra money but I want to upgrade my mount to a cgx before I would buy one.


Edited by Medic002, 20 July 2021 - 07:30 PM.

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#3 DSO_Viewer

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Posted 21 July 2021 - 11:15 AM

The RASA 8 will be way too under sampled when using the MC294MC and a much better match would be the MC183MC.

 

Steve


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#4 SanjeevJoshi

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Posted 21 July 2021 - 08:42 PM

Hi, I own the exact combo you are considering -- RASA 8 + AVX.

 

First the simple part.  AVX is awesome for EAA purposes with RASA 8.  It is light and gets the job done.  It is my go to.  You will need 3 after market changes to the AVX to get the most out of it.

 

1.  With RASA 8, you will need the additional Orion 7.5 - 8 lb weight UNLESS it was purchased as a combo and the extra weight is included.

 

2.  The Starizona bubble just sticks to the front of it and helps you quickly get the AVX roughly level

 

3.  And the most crucial part -- replace the OEM saddle with the ADM dual saddle.  The OEM saddle is terrible... ADM dual saddle is a first class upgrade from center seat economy smile.gif

 

 

Now the slightly more complicated part:

 

1.  check the CCD suitability tool (and the FOV tool) on astronomy.tools.   As Steve points out above, 294 under samples for RASA 8 substantially, and this will not be good especially in a Bortle 5 type setting -- I found it problematic even in Bortle 7/8.

 

Given the amount of data RASA can quickly collect - with current CMOS camera options, you are left with three choices to achieve different results:

 

The best resolution (which will be fine for "average" seeing and almost adequate for "good seeing") comes from something like ZWO 183 MC Pro.   I use this to focus on single objects or clusters that are roughly up to 70 arc minutes.   Compare the 183 with the 294 and you will see what i mean -- 183 gives you roughly 1.24 arc seconds per pixel, where as 294 gives you roughly 2.4 arc seconds per pixel.   ZWO 533 MC Pro and 2600 MC Pro give you about 1.94 arc seconds per pixel.

 

Quite simply, the 183 MC Pro is hands down the best camera for RASA 8 for single objects where you want to focus on details.   Do a search here and look for a view I captured of the Rosette -- you would think it was in an advanced astrophotography forum smile.gif

 

You will enjoy it all the way down to an object size of about 5 arc minutes, a bit smaller when seeing is better.

 

Go the other direction - looking for wide field stunning views -- say M31 and M32 in their full glory together -- you are looking for something like the 2600 MC Pro as the ideal camera.  It is very hard to match the image scale and quality of views you can capture.. both the 533 and 2600 have very little amp glow.

 

Finally the most complicated part -- is the RASA 8 worth it?

 

If you are looking for a very fast high quality light bucket you can use for EAA, albeit with a limitation on the resolution, very hard to beat RASA 8.

 

At the same time, its not for the casual observer who is starting out in the EAA hobby.  You need darks and flats to get the very best results with the RASA (not hard to do at all), you will need to tinker to optimize your workflow and results.

 

If one was getting a RASA as a bucket list item and could accept a higher weight (using a backyard observatory for example), I would get a RASA 11 in a heart beat.   All those concerns about being limited due to resolution go away.  You also get ample backfocus to set up your optical train perfectly.

 

RASA 8 is a compromise solution by comparison.   It is quite portable and a lot of fun.   In exchange you accept the resolution limit (1.24 arc seconds per pixel is the best current technology can do) and limits on optical train accessories.  You dont need much accessories to have fun for EAA.

 

Yes the Artesky or Starizona drawer or PreciseParts custom adapter all can help you tune the optical set up, add filters etc.   At Bortle 5, i am not sure the RASA will need any filters other than Nebula filter for max effect.  I use the Triad Quad and it is mindblowing for Nebulas given how much data the RASA can quickly collect.

 

You will not need a finder of any kind with RASA smile.gif   Just plate solve in a second or two, done!  But yes if you really need to for whatever reason, you can install one.

 

Accessories I would recommend with RASA -- a light box for proper flats, a filter adapter, and the Celestron focus motor.  Search this forum for tips I had posted on how to set up the Celestron focus motor correctly on RASA 8.

 

Software -- Sharpcap Pro, Carte Du Ciel (free) for planetarium, and CPWI (Celestron, free).

 

Good luck, and happy to address any other questions you have.


Edited by SanjeevJoshi, 21 July 2021 - 08:45 PM.

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#5 EmeraldHills

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Posted 21 July 2021 - 09:03 PM

Enginear, Sanjeev has done his typically thorough job of explaining everything. You asked about a cover for the corrector plate: Celestron does indeed provide one that is quite satisfactory, as long as you don't mind removing your camera whenever you have to pack up. (And that's what I think most everyone is doing.) Lately, I've been contemplating getting my camera set for backfocus and sensor tilt -- then just leaving it attached in that "perfect position." (We'll see if I give in to that temptation. It's probably not as a good for the Corrector plate to transport the scope with something heavy attached. But we'll see if, once I achieve proper sensor orientation, if I can KEEP it even when attaching and removing and re-attaching the camera.) The only 2 additional points I'd humbly ask you to consider would be:

 

a) You won't need any other help to polar align. The RASA 8 with SharpCap Pro's polar alignment routine is a quick winner. You're probably already familiar with that approach - so you already know, it's a bit like falling off a log. EAAsy.

 

b) I owned a previous 8" scope (a CPC 800). Wow. When I compare it to the RASA 8, it feels like I had a Chevy Vega and now, with the RASA 8, this feels a bit like a Corvette. It's really fast at light-gathering and really super-fun to use. Plus, plus plus. (But Sanjeev is, as usual, right - in that it's a bit of a compromise. Aren't they all? But it's a fun compromise.) To me, one of the most sobering compromises is -- as a super-fast focal ratio scope (f/2), the depth of field (or, as we say in THIS hobby, the critical focus zone) is so shallow it's insane. I've been told it's literally 1/2 the width of a typical human hair. Imagine how demanding that will be for proper focus, perfecting proper backfocus, and avoiding sensor tilt. It's ... a journey. But again, isn't it all a journey?  : )

 

Doug


Edited by EmeraldHills, 21 July 2021 - 09:06 PM.

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#6 SanjeevJoshi

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Posted 22 July 2021 - 07:37 AM

Hi, I forgot to mention one software that is useful for initial set up for back focus optimization.  I found CCD Inspector useful to run tests on backfocus adjustments with my equipment and settle on one value for each combo - a specific adapter (factory or aftermarket) with 183, 2600, then both with Quad filter (4 set up’s).  Baader spacer rings (0.3 mm, 0,5 mm 1 mm) and run tests to optimize.

 

With ZWO cams 183, 294, 2600 I find the sweet spot is typically 0.3 mm spacer added without filters.  It is simple math to add to that based on your filter.

 

it is amazing how little true tilt there is, but one-time calibration of backfocus per set up to within 0.3 mm makes the views so nice and crisp star shapes all across the wide field.  I have not touched the factory collimation at all.


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