RASA 8 or Sharpstar 15028HNT f/2.8 Hyperboloid Newtonian

[Clarkey]

Due to the very limited times that I can image, I am seriously considering one of the two scopes above to allow much higher data collection rates. In terms of FL, both are pretty similar at 400 and 420mm respectively. The RASA is quicker at f2 compared with f2.8 for the Sharpstar. Obviously, one is a dew magnet and the other gives diffraction spikes. The Sharpstar also claims to work with a full frame sensor which is much larger than the RASA, it can also work with a mono camera (if you can afford the filters).

However, what I would really like is some real world experiences. I know the RASA can be difficult to set up, with tilt being an issue and similarly high-speed Newtonians can be tricky to collimate. I also know the early HNT scopes had issues with the mirrors moving - but I believe this has been fixed. For anyone who owns or has used either of these scopes, I would like some opinions.

Thanks in advance,

Ian

[fwm891]

Ian, I use a RASA 8 (this is my second I foolishly sold the first one and regretted it. Couldn’t wait to replace it).

Like you I have a RASA to collect data when the skies permit. I have not experienced a problem with tilt on either of the RASA 8’s I’ve owned. The biggest single problem is the choice of camera you put on the front end. Specifically the back focus of that camera. If you chose a camera like the ASi533 with a 6.5mm back focus then you will have the option of fitting a filter drawer and use various filters with your RASA. If you chose a camera with a 17.5mm back focus you can’t fit a filter drawer and you are limited to finding a way of attaching individual filters and fitting them to the camera before you attach the camera to the RASA each time. Changing filters and realigning the camera being somewhat awkward!

(I’m currently modifying my RASA 8 so I can get a filter slide with a 17.5mm back focus camera).

I used an ASi533 with my first RASA and they make a great pairing. The ASi294 is also a good match but does have the amp glow problem which some seem to get round easily others not so (me)!

I can’t comment on the short focus Newt’s but I have owned and still own a 10 inch f4 newt . If as you appear to have an obsy the your not going to move the scope around too much it will hold collimation quite well. If your going to do much transporting with fast optics like that then collimation/tilt may become an issue.

 

Edited October 10, 2023 by fwm891

[tooth_dr]

I have a 180mm F2.8 Newtonian and it took a bit of work to get it aligned. Depends how fussy you are but nothing beats the resolution and light gathering capabilities of a big fast scope.  I prefer the glass like stars and diffraction spikes to the (imo) slightly fuzzy stars from the RASA, although AI takes care of things now so not really a bit issue. 

[tomato]

I image with a RASA8 when the mood takes me, I went the IMX 571 OSC camera route and use the IDAS NBZ filter for NB targets so I don’t need to bother with a filter draw set up. The 8” version holds focus very well, which I think is consistent with experience of other owners of this scope. 
 

Tilt can be an issue, but it is relatively easy to fix but I confess I am not as particular about this as some imagers are.

Be warned Celestron appear to have had an issue with mirror quality on a recent run of 8” scopes, it is documented on SGL. This may be fixed now, best to consult with your supplier if you decide to go with this scope.

[Elp]

Personally I've never had an issue with F2 or faster, this is down to the individual and how nit picky they are.

If it were my choice (and I've been umming it over for a while) would be the RASA 8 but as mentioned there is a quality issue with them, they were withdrawn from sale at the time I believe and sent back to Celestron, so exercise caution with any out there new or used. As far as I'm aware the issue still isn't resolved, was supposed to be sorted out earlier this year but most retailers are still not stocking them.

The alternative option would be the Edge HD 8 and Hyperstar, benefit is you'll have a multiple focal length imaging scope (native, focal reduced (with additional reducer) and HS) and also be able to use it for visual. It'll image planets very well, small nebulae and galaxies and fast wide targets. It's why I went with the C6 setup for the versatility and little space it occupies when not in use.

From the little experience I do have, I'd say aperture makes the difference well and above f ratio for speed and of course the resolution boost if seeing and tracking supports it.

Edited October 10, 2023 by Elp

[ollypenrice]

Obviously a dew magnet? Certainly not. The RASA 8 is the least dew -prone imaging optic I've ever used. We have never, ever, had any dewing and we have no dew heater. With a short dew sheild (I made this one) the camera warms the air around it and its fan also keeps it circulating. The fact that it is a zero-dew rig is a selling point and, for me, a real luxury. This is not an SCT!

At a given focal length F2 is a hell of a lot faster than F2.8. 4 minutes versus 7.84 minutes. Nearly twice as fast. That's a lot.

46 minutes ago, tooth_dr said:

Depends how fussy you are but nothing beats the resolution and light gathering capabilities of a big fast scope.

I don't agree on resolution. The RASA 8 is not diffraction limited and speed tends to work against resolution anyway. It does not resolve at the level of a slower 8 inch instrument. However, I think the relevant and meaningful comparison is how it resolves against other possibilities offering the same focal length and, in this comparison, it's fine by me.  I maintain that it does better on non-stellar detail than stellar. Vlad insists that this is impossible. Hey-ho. I think the OP can decide by looking at RASA images and seeing if he'd be happy with that resolution.

Diffraction spikes in a FL of 400mm means... a  lot of diffraction spikes.

Tomato is right to flag up the QC issue. I would only buy from a retailer who acknowledged this risk and would offer no quibble returns.

Tilt is fixable, the stars could be better, but I've never had as much fun as I'm having with the RASA, despite having at least 250 clear nights a year.

Olly

Edited October 10, 2023 by ollypenrice
typo

[tooth_dr]

36 minutes ago, ollypenrice said:

does not resolve at the level of a slower 8 inch instrument. However, I think the relevant and meaningful comparison is how it resolves against other possibilities offering the same focal length and, in this comparison, it's fine by me.

Sorry yes, I ran it side by side with my 105mm APM, both the same focal length and the detail in the 180mm was quite a bit more striking in my eyes.

[Elp]

In freezing or humid temperatures the Celestron dew heater ring works wonders. Incredibly simple to install. With the dew shield it's just there for backup unlike the refractors which need them a lot when it's cold.

Edited October 10, 2023 by Elp

[saac]

I tried to buy a RAS8 at the start of the year and had no joy and I don't think the supply issue has resolved itself yet.  I believe they are attempting to resolve the QC issues they had with the mirrors on some early models.  I don't think the 11 inch variant is similarly affected by supply issues but there is a significant cost difference of course. I also think people hang on to these scopes, haven't seen many for sale second hand. 

Jim 

[Clarkey]

Thanks for the replies - really useful information. 

1 hour ago, fwm891 said:

. If you chose a camera like the ASi533 with a 6.5mm back focus then you will have the option of fitting a filter drawer and use various filters with your RASA. If you chose a camera with a 17.5mm back focus you can’t fit a filter drawer and you are limited to finding a way of attaching individual filters and fitting them to the camera before you attach the camera to the RASA each time. Changing filters and realigning the camera being somewhat awkward!

This is a bit of an issue. I was unaware there was no space with the longer back-focus. I have an IMX571 Risingcam so this could be a problem.

1 hour ago, Elp said:

The alternative option would be the Edge HD 8 and Hyperstar,

I think the overall cost rules this out - I had looked at it. Also, I already have a RC so I do not really need a longer FL

41 minutes ago, ollypenrice said:

Obviously a dew magnet? Certainly not

Good to know. I assumed given it had the front plate it would attract dew, but this is clearly not the case. Also, as @Elp says, there is the option of the Celestron dew heater - I assumed these only fitted the SCT's.

From the replies above, the general consensus seems to be the RASA is the better option. I was aware that the 8" version was out of supply for months, but I was unaware that this was a QC problem. I have noticed that FLO are now showing them as in stock soon which is the first time in months. I just hope the 'poor' mirrors are obvious and I don't spend ages trying to fix the unfixable.

1 minute ago, saac said:

I also think people hang on to these scopes, haven't seen many for sale second hand

No - I think you are right.  Much as I would like an 11" scope, my budget won't stretch to that.

 

Now I just need to get my 'financial planner' to give me the go-ahead. (Some grovelling may be needed🤣)

[saac]

3 minutes ago, Clarkey said:

Thanks for the replies - really useful information. 

This is a bit of an issue. I was unaware there was no space with the longer back-focus. I have an IMX571 Risingcam so this could be a problem.

I think the overall cost rules this out - I had looked at it. Also, I already have a RC so I do not really need a longer FL

Good to know. I assumed given it had the front plate it would attract dew, but this is clearly not the case. Also, as @Elp says, there is the option of the Celestron dew heater - I assumed these only fitted the SCT's.

From the replies above, the general consensus seems to be the RASA is the better option. I was aware that the 8" version was out of supply for months, but I was unaware that this was a QC problem. I have noticed that FLO are now showing them as in stock soon which is the first time in months. I just hope the 'poor' mirrors are obvious and I don't spend ages trying to fix the unfixable.

No - I think you are right.  Much as I would like an 11" scope, my budget won't stretch to that.

 

Now I just need to get my 'financial planner' to give me the go-ahead. (Some grovelling may be needed🤣)

That would be good news if they are back in supply - I'd drop FLO an email just to confirm the situation though.  Too late for me,  I got impatient and spent my funds on a nice refractor instead   The RASA will need to wait for another year or two for me but I think I'd equally settle for the Sharpstar, I had also like yourself considered that as an alternative. 

Jim 

 

[tomato]

I agree with Olly re the RASA not being prone to dew on the corrector plate, the mandatory warm air fan by virtue of the camera location does the trick.

In fact I have experienced another problem, on a warm night the dewshield can cause the warm exhaust air to recirculate through the camera and reduce the efficiency of the cooler, so low set pt temperatures (below -10 deg C) cannot be achieved. Fortunately I haven’t found this to be an issue with modern CMOS cameras.

[symmetal]

1 hour ago, Clarkey said:

I just hope the 'poor' mirrors are obvious and I don't spend ages trying to fix the unfixable.

Yes, it's fairly obvious as indicated in this thread when I had the issue on a new scope in August 2022.  Twice, FLO kindly sent me a replacement scope, but they all had the same issue which is when it seems Celestron withdrew them from sale and sent them back to China. It was the latest RASA 8 scopes, (at the time) which were affected and not the earlier ones which were fine.

Alan

[gorann]

RASA is the way to go in my opinion (running a dual RASA8 rig now for quite a while). No dew or frost issue whatsoever even at -15°C in the middle of the Swedish winter. The camera works as a dew heater, which it would not do on a Newton. The only advantage I can think of with the Newton is the possibility to have a filter drawer/wheel for a 17.5 mm camera. Will be very interesting to hear how @fwm891 is abut to solve that - keep us RASArians posted!

[gorann]

PS. And if you pixel peep at images posted from those Sharpstar Newtoneans you may notice that they do not only give star spikes on bright stars, but that the smaller stars are squares and not round.

[tomato]

A thread on Cloudy Nights indicates that Celestron fixed the problem and replacement RASA8s were being shipped from April/May of this year but I would definitely check with your supplier. 
FLO have due in 30-40 working days on their website, but this read “Contact us” previously.

[Clarkey]

10 hours ago, gorann said:

but that the smaller stars are squares and not round

Probably a dumb question, but why? The only reason I can see it due to the resolution, but that would be the same on any 400mm scope.

[ollypenrice]

44 minutes ago, Clarkey said:

Probably a dumb question, but why? The only reason I can see it due to the resolution, but that would be the same on any 400mm scope.

I think it's simply that the spikes are reduced to vestigial extensions close to the star and don't extend into full spikes. We just see the four spike 'bases,' if you like, and these tend to form a square. I haven't knowingly seen any Sharpstar images but I've seen the phenomenon on Tak Epsilon images and also on images from the Vixen Cassegrain which had thick vanes and no corrector plate. I'm absolutely not a pixel peeper and found them pretty obvious.

Olly

[Clarkey]

@ollypenriceand @gorann - I see what you mean about the square stars. Just checked out some images on Astrobin.

From all the above information, it looks like the general advice is towards the RASA. I have contacted FLO regarding the mirror issue and it would appear to be a limited number of poor examples. I will have quite a bit of time to think about it - the new ones are not expected until Dec / Jan. Having read lots of threads on here and CN, there seem to be fans and supporters of both options. However, as there is a knowledgeable crowd on SGL regarding the foibles of the RASA - best stick with what YOU know. I will undoubtedly be back with more questions in the new year. (SWMBO permitting of course)...

[ollypenrice]

9 hours ago, Clarkey said:

@ollypenriceand @gorann - I see what you mean about the square stars. Just checked out some images on Astrobin.

From all the above information, it looks like the general advice is towards the RASA. I have contacted FLO regarding the mirror issue and it would appear to be a limited number of poor examples. I will have quite a bit of time to think about it - the new ones are not expected until Dec / Jan. Having read lots of threads on here and CN, there seem to be fans and supporters of both options. However, as there is a knowledgeable crowd on SGL regarding the foibles of the RASA - best stick with what YOU know. I will undoubtedly be back with more questions in the new year. (SWMBO permitting of course)...

Imaging at this speed is simply a different world, as you'll see!

Olly

[CCD-Freak]

F2.8 speed is where it is at.

This is a single 5 minute exposure of B33, IC434 and NGC2024 with a SS15028HNT with an ASI533MMp with a Baader 7nm Ha filter with Astroart 8.

Edited October 20, 2023 by CCD-Freak

[Clarkey]

16 hours ago, CCD-Freak said:

F2.8 speed is where it is at

I do still like the look of the Sharpstar- in particular using mono and filters. Feedback from FLO seems to be that most of the earlier issues with the primary mirror.

[GalaxyGael]

For what it is worth, there is a newer version that just slipped in without fanfare since they released the smaller 13028 version. The three copper studs under the mirror in original 15028hnt version (that I had to remove and replace) now have cork pads instead. The entire cell now has 6 holding screws in the base rather than relying on the push pull system to hold it in place. collimation is with grub screws like the epsilon instead of those large and easily loosened bolts on the original version

Hope they fixed the need for ED glass in the corrector too, but overall is a version with changes implemented by the cries on forums and other forms of contact I believe.

Here is the back of the current version, and sharpstar's diagram on the screws when I asked for it, just fyi if anyone finds this.

I've found it to match the epsilon 130D, slightly better stars (if you dont get a turned down edge) since the epsilon has mirror clips rather than a retaining ring.