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Old 16-08-2019, 08:06 AM
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The_bluester (Paul)
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Celestron RASA 8 users

Anyone on the forum got their hands on one of these yet? I keep seeing the appeal of F2 imaging (Where my 5 minute subs at F4.9 would be more like 1 minute at F2) but the scope obviously has some designed in limitations. I have seen them described all the way up to being "Issues" or even flaws or defects, but to me they seem to just be design compromises and no matter what the configuration every telescope has compromises in one area or another.

My work usually produces a nice little annual bonus around March and in recent years a chunk of that has gone in to astro gear so I am looking ahead to next year.

At this point there are only a handful of images on the likes of Astrobin identified as being from an 8" RASA and they span the expected spectrum from pretty flash to pretty ordinary. I was hoping a member on here might have some insight on the real world performance of these scopes with their intended OSC camera configuration. They look like they could have been practically designed with my ASI294MC Pro in mind.

Another question is filters. I know for NB filters they need to have specifically designed filters for best results with the steep light cone at F2, but does that apply to a UV-IR filter? A handful of nm shift in the band pass at either end of a filter spanning around 400 to 700nm passband is not going to make much difference where a couple of NM shift on a 3 or 4nm notch filter is obviously going to change it's outcome significantly. I ask that as the 294 at least on my little ED72 benefited insanely from a UV-IR filter and examining subs from the SCT most of the same artifacts that were on the refractor images are visible there, so it seems likely to be a required filter in more than just a "cheap frac" configuration. The only image I have found that specifically couples the same camera as I have with a RASA 8" shows promise, but has a way to go and seems to me to show some of the filter-less issues I see with my ED72, I also have no idea about the users experience, how their night of data capture went and anything about their workflow on processing.

Anyone with any real world experience of these scopes on the forum?
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Old 16-08-2019, 09:02 AM
casstony
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Hope you don't mind me adding this to the thread Paul, just another option to consider: https://www.365astronomy.com/sharpst...scope-red.html

The advantage of the reflector is light weight, a larger illuminated field and you can use any camera, while the RASA has protected/enclosed optics and is faster again.
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Old 16-08-2019, 09:20 AM
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Lol. You are not making this any easier! Another obvious advantage is the ability to put an OAG in the train which the RASA lacks to be able to take much longer exposures without worrying about flexure. I imagine a sub F3 newt would be tricky to collimate well, though the RASA would be the same even it if does only have one adjustable component to do.

On that note and with the proliferation of tricky cameras I have been wondering why they don’t implement a “multi ROI” firmware where you can have more than one region of interest on the chip but read them off at different exposure times. Done like that you could use a chip slightly larger than your well corrected image circle and then define a little patch at the edge to be a live, online guider, reported in software as a second camera.

Last edited by The_bluester; 16-08-2019 at 09:37 AM.
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Old 16-08-2019, 06:48 PM
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I hope their secondary adjustments are better than the average fare as collimating at f/2 or f/2.8 must be a nightmare
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Old 16-08-2019, 07:19 PM
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The RASA looks to have a push pull screw system and really the only collimatable part is the tilt of the lens group in the middle of the corrector, apart from the F2 factor it should not be much harder to collimate than an SCT.

The Sharpstar looks to be a standard sort of newt arrangement, aside from having a one part CNC milled secondary holder. You would hope the threads on the secondary adjustment screws were a nice fine pitch.

Last edited by The_bluester; 16-08-2019 at 08:51 PM.
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Old 17-08-2019, 04:23 PM
casstony
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I was very curious about the WO Redcat and Sharpstar 61edph so I bought both Paul. In view of the lack of responses you may have to do the same here
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Old 17-08-2019, 05:01 PM
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Lol, my wife would kill me. The redcat got serious consideration before I bought my little ED72 actually. Easy ability to add autofocus tipped the scales on that one.

The only thing at the moment that is definitively in favour of the RASA is that I have seen images from one of those with no tilt or coma type issues at the corners and have not yet on the Sharpstar, though the only images I have found from the 150 so far have been with a bigger sensor than mine.

Last edited by The_bluester; 17-08-2019 at 06:42 PM.
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Old 21-08-2019, 09:49 AM
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Hello Paul
I love my RASA 8 . There certainly were teething problems last December when it arrived from the US. It had mirror tilt . I sent it back to Celestron who were keen to fix up the problem and re collimate it. It was there and back in 10 days. You cannot collimate it yourself.
I have zwo 294, 290 and 224. I am purely videoastronomy and still learning the game .I keep stacked exposures to 3 minutes total and try to keep them at 1minute. The 294 gives a fairly wide FOV but the results are great compared to what I produce with my other scopes .I use the 290 mono for galaxies where I am just looking for structure and am very happy with that as well.
I have a number of other scopes but my RASA 8 is my pride and joy.
I am just waiting for clear skies and some decent weather.
I live in Bendigo which is not too faar up the road from you. Feel free to check it out if you wish.
cheers
Paul
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Old 21-08-2019, 10:28 AM
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I would love to come and check it out at some stage if it can work out time wise. There is nothing quite like actually seeing one in action.

Collimation wise they do provide the tilt adjustment for the lens cell within the corrector but I assume at F2 it would be quite sensitive, though maybe not RC territory. Did yours need the mirror tweaked? I would assume that would be quite user unfriendly and certainly warranty voiding to touch given the amount of dis-assembly that would be required.

Do you use any filters with the 294? Per what I posted above I found with my ED72 that a UV-IR filter made a spectacular difference, but the LPR filter that is available for the RASA 8" cuts off overall at roughly the same wavelengths as the filter I am using (Ignoring the various notches in transmission) so I assume would resolve the same sort of issues if they were present. I did some test shots with my C925 after getting the filter for the ED72 (Not one that would easily transfer to the SCT) and could see the same sorts of issues I was chasing on the refractor.
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Old 21-08-2019, 06:08 PM
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Celestron adjusted the collimation for me as well as the mirror tilt . There is still slight mirror tilt which doesnt bother me at all.I have a couple of sct's and am familiar with collimation, but I wouldnt touch the RASA.

I post on the Videoastronomy forum where the images are not post processed and thats how I prefer to do my hobby. I dont save my images very often after posting so most of my RASA 8 images are on that forum.

I have the Celestron Rasa 8 LPR and the Starizona filter slider which attaches to the zwo 294 and has the required backfocus of 6.5 +19.5 . However ,I am happy without them. There is no starbloat that bothers me with the 294. When using the ZWO 290mm and smaller FOV it is more obvious but I like to use it for galaxies where I am better off without a filter.
I have the ED72 and agree a UV/IR cut is necessary,but that is my grab and go scope and dont even bother with galaxies. That is the job of the RASA.
cheers
Paul
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Old 21-08-2019, 06:39 PM
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My only concern would be in buying something that I cant collimate myself with some patience and care (I used to work on a RADAR system where aligning the electronics on a particular "drawer" with about ten modules in it took a couple of days. Mirror I can understand, it is in a cell and should be set once in the factory and generally not touched again, just like my C925, but I would have to be happy to tweak the collimation screws in the lens cell in the corrector to buy one, I am a born "tweaker" though..

If you are doing EAA I assume you would be using fairly short exposures? Like everything else, the reflection issues I could see with my 294 (Is yours the cooled version?) were more and more prominent the longer the exposure and the brighter the star in question. I found the other forum so I will have a look at images on there.

Last edited by The_bluester; 21-08-2019 at 06:51 PM.
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Old 21-08-2019, 06:46 PM
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Celestron adjusted the collimation for me as well as the mirror tilt . There is still slight mirror tilt which doesnt bother me at all.I have a couple of sct's and am familiar with collimation, but I wouldnt touch the RASA.

I post on the Videoastronomy forum where the images are not post processed and thats how I prefer to do my hobby. I dont save my images very often after posting so most of my RASA 8 images are on that forum.

I have the Celestron Rasa 8 LPR and the Starizona filter slider which attaches to the zwo 294 and has the required backfocus of 6.5 +19.5 . However ,I am happy without them. There is no starbloat that bothers me with the 294. When using the ZWO 290mm and smaller FOV it is more obvious but I like to use it for galaxies where I am better off without a filter.
I have the ED72 and agree a UV/IR cut is necessary,but that is my grab and go scope and dont even bother with galaxies. That is the job of the RASA.
cheers
Paul
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Old 21-08-2019, 07:14 PM
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Have been reading a few posts about the RASA 8.
The Starizona Filter slider is just the right back focus for the ZWO 294, 1600 and 183. The 224 and others would be about 6mm too wide for this filter holder. I have the H alpha and O 111 but havent got into them yet.I need time and clear skies.

Have the ASIair also. For videoastronomy, you cant live stack which is a big disadvantage.I use it out in the field with grab and go and an ipad where you can only preview single exposures. In my case it is ED72 on an alt/az and 30 secs max. At home ,I cant see it competing at all with hard wiring to a laptop with your software of choice.

Paul,
My posts in Jan re RASA 8 http://www.iceinspace.com.au/forum/s...d.php?t=172533
cheers
Paul
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Old 21-08-2019, 08:11 PM
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I did see that thread, just looking at the Horsehead image the amount of data captured in the 45 second subs (Despite the tilt issue) compares with 5 minute subs I have taken with the ED72, which is what you might expect, about 1 minute at F2 being more or less equivalent to 5 minutes at F4.9.


It does not make my choices any easier! But it would be great to be able to go as deep as I am now with under 2 minute subs and for really faint stuff maybe even run to 5 min subs to see what is there.
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Old 21-08-2019, 09:23 PM
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Here are some images with zwo 290 on the Rasa 8,no filters or post processing ,5 or 10 sec subs ,total 1 minute
Attached Thumbnails
Click for full-size image (m104-10x6-c.jpg)
130.3 KB42 views
Click for full-size image (ngc-2070-5-x-12-c.jpg)
144.3 KB50 views
Click for full-size image (ngc-5128-10x-6-c.jpg)
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Old 22-08-2019, 07:41 AM
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A surprising amount of detail there for very short subs and a 400mm focal length. The smaller pixels of the 290 compared to the 294 would be a help there I imagine. I wonder how much of that I could recover with drizzle on multiple deep exposures with my ASI294, I don't have any plans to replace that in the immediate future.

You have mentioned it elsewhere (Perhaps on the video astronomy forum) about tracking issues which you can see to some degree in those, doing unguided imaging, how are you dealing with polar alignment? I messed around for a long time with PA and ended up buying a Polemaster camera, best money I have spent on gear not directly related to capturing photons since I started. Though it would depend on being able to see the pole. Particularly for such short focal lengths, so long as the pole and Octans are visible you can be aligned in 5 minutes.
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Old 22-08-2019, 08:37 AM
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Here are a couple of images taken with the 294 0n the Rasa8. A lot of the galaxies are a bit small with the larger FOV of the 294 compared with the 290 but the nebulae generally fit very well.
These are both 10sec sub x6 stack so a total of 1 minute.
I use Sharpcap and the images are auto saved at 1 or 3 minutes. I rarely go any longer . It is all about what I am viewing at the time.

I polar align the EQ6 and HEQ5 with sharpcap and it is very straight forward.
My tracking problems have been with the C6 nexstar alt/az .The C6 has hyperstar . There is a big quality difference if I put the C6 on the HEQ5 compared to its alt/az mount. They are short subs so it is not a rotational problem..more of a tracking problem.

The Meade LX90 SCT is a great visual scope,but I have produced some terrible images which I posted on the Videoastronomy forum mainly to get some feedback on what I was doing wrong..The tracking on the LX90 appears to be ok.
cheers
Paul

Last edited by lollywater; 22-08-2019 at 08:39 AM. Reason: omitted the images
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Old 22-08-2019, 08:41 AM
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sorry, I didnt upload the images
cheers
Paul
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Click for full-size image (1365 rasa 10x6.jpg)
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Old 22-08-2019, 08:57 AM
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That would be about the FOV I would expect for the 294, more or less the same as I have now using the ED72 (350mm focal length with the reducer I am using) Being ten second subs it certainly shows the photon sucking element of F2 compared to even F4.9

The biggest tracking issue I found when using my mount Alt-Az (Orion Atlas Pro) was that you absolutely had to level the base with a spirit level before putting the mount head on, ignoring the built in level. If you did not do that the goto and tracking accuracy suffered right from the start, get it right and both were good all night.

I have never used Sharpcap for polar alignment so I cant comment on that.
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