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Celestron RASA 8" or Edge HD 8" with a HyperStar?

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#1 danielgallo

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Posted 06 September 2018 - 12:56 AM

I currently have a Celestron EdgeHD 8" scope and CGEM II mount, and recently been researching the HyperStar by Starizona as I want to do faster, wider-field, astrophotography.  For those not familiar, the HyperStar would turn my existing f/10 EdgeHD scope in to a faster f/2 imaging system.

 

However, I just saw the announcement today of Celestron's new 8" f/2 RASA: https://www.celestro...trograph-rasa-8.

 

So I'm wondering whether I should still go with the HyperStar for my EdgeHD scope (about $1,000), or wait and go for the more expensive Celestron 8" RASA ($1,699) that's due to be released in the coming months.

 

I have a ZWO ASI1600MM-Cool camera which I intend to continue using. Does anyone have experience with the Celestron RASA system, specifically in using monochrome cameras along with LRGB & narrowband filters?  Would that setup work?  Is there some kind of filter drawer system for the RASA?

 

And would the RASA in theory deliver significantly better images than a HyperStar and EdgeHD 8"?  Has anyone here had experience with both the HyperStar system and the existing larger Celestron RASA scopes?


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#2 james7ca

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Posted 06 September 2018 - 02:40 AM

I'd guess that this would be a wonderful system for one-shot-color imaging under a dark sky. However, the short backfocus (25mm) for the camera suggests that this would be kind of a "bear" to operate for narrow band or RGB filtered imaging. But, maybe not any more so than your EdgeHD with a HyperStar (you'd have to look to see what kind of backfocus is available on the HyperStar). I guess if you wanted full automation you could limit yourself to one filter per night (and use the built-in internal filter location that is provided on the RASA, but that's assuming that any narrow band filters become available that fit that internal filter location).

 

Otherwise, the thinnest filter draw that I current own is 15mm thick (from Teleskop Service) and if you assume that the stated 25mm backfocus is "real" and includes the proper and short t-threading then that leaves only about 10mm for the spacing between the filter drawer and the camera sensor. I guess that would work on the ASI1600 but that would also require a very short female-to-female t-thread adapter to do the gender change between the filter drawer and the male threads on the ASI1600 camera body. Unfortunately, the shortest such adapter that I know about is 10mm thick and given the minimum 6.5mm backfocus available on the camera body that would exceed the remaining 10mm (since you'd have 6.5mm on the camera body itself and an additional 10mm for the gender changer, so 16.5mm of total spacing). Actually, I don't think you could make that work since you probably need at least 6mm of threading on the gender changer to allow you to thread both of the male ends into the adapter. So, 6.5mm from the camera and 6mm for the gender changer and you're already beyond the available 10mm. 

 

In any case, at some limit you'd probably get better performance from the RASA and Celestron claims just that on their website.

 

 


When compared with “Hyperstar SCT” systems, the RASA 8 provides better optical performance and field illumination.

 

The focuser could be much, much better on the RASA and it would be nice to have the built-in fan for cooling.

 

[UPDATE]

It just occurred to me that you wouldn't have to use a filter drawer to mount your filters (at least for a camera like the ASI1600), you could just use one of the thin t-adapters that take an internal 1.25" mounted filter. Of course, that would mean that you'd have to unscrew everything to change your filter but it could work.

 

In fact, ZWO makes just such a product that you could use with the ASI1600 camera.

 

  https://astronomy-im...-filter-adapter

 

What you'd do is use the 10mm t-thread nose piece that comes with the camera (basically the thick gender changer I mentioned earlier) and attach that to the male threads on the camera body and then you'd screw the above filter adapter into the female threads of the nose piece. That would mean you'd use 6.5mm + 10mm ≈ 16.5mm of the available 25mm of backfocus.

[/UPDATE]


Edited by james7ca, 06 September 2018 - 03:46 AM.

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#3 einarin

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Posted 06 September 2018 - 03:02 AM

RASA has wider imaging circle and maybe a better focuser.

And apparently it has some kind of filter drawer.

But IMO I would stick to EdgeHD as it's more versatile (can be used with different f/ratio combinations).

https://www.cloudyni...on-rasa-8-inch/


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#4 psandelle

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Posted 06 September 2018 - 09:54 AM

If you're strictly wide-field, I'd go with the RASA (built especially for that, seems like it has the better focuser, which at that speed, you need). Otherwise, the Edge has the more versatility. If I were starting out again, I'd be tempted by the 8" RASA, but I'm a wide-field junkie.

 

Paul


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#5 danielgallo

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Posted 06 September 2018 - 11:24 AM

Thanks for all the replies, and @james7ca - thanks for pointing out that ZWO adapter for mounting 1.25" filters inside the nose piece of the ZWO camera.

 

Decisions, decisions... :)  The more I read up on the RASA, the more tempting it is to go that route.


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#6 psandelle

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Posted 06 September 2018 - 11:29 AM

It's at such a great price, that maybe go with the RASA (not THAT much more for the RASA than for the full Hyperstar; I mean, what's $700, $800 amongst friends?) and keep the Edge for longer focal length. Less trouble in attaching and unattaching then having a Hyperstar and an Edge. Just dump a whole scope and camera on the mount and be done with it.

 

Paul


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#7 danielgallo

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Posted 06 September 2018 - 11:39 AM

Paul - Yes, that's what I'm thinking now; get the RASA for much faster wide field use (which, as pointed out above, it's designed specifically for this purpose), then use the EdgeHD for when I need something with a longer focal length.

 

As you pointed out, it's a pretty good price.  It's still a decent size investment, but I'm surprised it's being released at that price point.  Maybe it's a promo/intro price - I see that OPT has it available for pre-order on their site with a retail price of $2,799, and a pre-order price of $1,699.


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#8 mvas

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Posted 06 September 2018 - 11:48 AM

RASA

=====

"... Performs over a wider spectral range than most telescopes, from 390 - 800 nm.

So, more of the light passing through the astrograph is in sharp focus. ..."

 

Edge HD

=======

?

 

 

Q1) What is the wavelength range of a typical 8" Celestron Edge HD + Hyperstar ?

Q2) 390 nm is deep in the UV vs what for the 8" EHD + HS ?

Q3) 800 nm is deep in the IR vs what for the 8" EHD + HS ?

Q4) Will this be of any benefit for the typical astro-photographer?


Edited by mvas, 06 September 2018 - 11:49 AM.

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#9 suvowner

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Posted 06 September 2018 - 12:35 PM

wow, I was about to post that there is no such thing, but had recently been wishing there was !!!!!  so thanks for the heads up........

 

https://www.youtube....9Mc&frags=pl,wn

 

I think hyper star on my c8 works pretty good, but having some issues in the corners with not tightly focused stars, I know i can collimate the hyper star but that can be a pain, so I was thinking a rasa would be more forgiving in this way, and with its larger image circle an even flater field with aps-c or smaller sensors


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#10 psandelle

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Posted 06 September 2018 - 12:35 PM

Paul - Yes, that's what I'm thinking now; get the RASA for much faster wide field use (which, as pointed out above, it's designed specifically for this purpose), then use the EdgeHD for when I need something with a longer focal length.

 

As you pointed out, it's a pretty good price.  It's still a decent size investment, but I'm surprised it's being released at that price point.  Maybe it's a promo/intro price - I see that OPT has it available for pre-order on their site with a retail price of $2,799, and a pre-order price of $1,699.

Does seem like a darn good price.

 

Paul



#11 suvowner

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Posted 06 September 2018 - 02:51 PM

Does seem like a darn good price.

 

Paul

opt likes to list inflated retail prices..........

 

so most likely 1700 will be the long term price......

 

oh how I would like to see a test shot of a c8 hyper star vs the c8 rasa



#12 danielgallo

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Posted 06 September 2018 - 02:52 PM

oh how I would like to see a test shot of a c8 hyper star vs the c8 rasa

You and me both :)



#13 psandelle

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Posted 06 September 2018 - 02:52 PM

opt likes to list inflated retail prices..........

 

so most likely 1700 will be the long term price......

 

oh how I would like to see a test shot of a c8 hyper star vs the cI 

I was more suggesting it was a good price in general at $1700 for an 8" f2 telescope. My original 6" PowerNewt went for about that price, if I recall (maybe a little cheaper).

 

Paul



#14 HappySkies

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Posted 06 September 2018 - 03:35 PM

In defense of the HyperStar, for the new 8 it's specs are:

 

- F/1.9
- 390mm
- Optimized for 27mm image circle
- Can natively shoot with filters for all color and most mono cameras.

 

Source last page of their instructions: https://starizona.co...nstructions.pdf

 

When considering the larger obstruction of the RASA 8 and the speed difference, the HyperStar is 15% faster and would get the largest field of view when comparing the same camera on each scope.

 

Celestron's website says the RASA's image circle is 22mm. See the spec page and the last paragraph of the product page. "It is optimized for sensors with up to a 22mm diagonal, but performs well with sensors up to 32mm diagonal"

 

Reference: https://www.celestro...trograph-rasa-8

 

Having said all this it looks like the RASA will probably produce smaller stars, by how much not sure yet. I think the RASA seems like a good second scope since you're stuck at one focal length as long as you're not looking for a monochrome setup. But then again if you already have a C8/HD8 then I'd personally just stick with a HyperStar due to being able to use whatever filters you want and not needing custom adapters for your camera.

 

I'm currently shooting HyperStar w/ ASi071MC on my Edge HD 8


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#15 danielgallo

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Posted 06 September 2018 - 04:45 PM

I'm currently shooting HyperStar w/ ASi071MC on my Edge HD 8

How do you find the HyperStar overall?  Are you pleased with the quality, and are you able to get sharp enough images edge-to-edge?

 

Also one of the replies above pointed out the new and improved focuser on the RASA.  Is focusing more difficult when operating with the HyperStar at F/2 then, or just that the focus can shift during use?  Have you experienced any issues with focusing?

 

You're starting to sway me back toward HyperStar now!!



#16 HappySkies

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Posted 06 September 2018 - 05:21 PM

First off, there are HyperStar Bahtinov masks and auto-focusers. Those options always ensure basically perfect (or better than you could ever do) focus. 

 

When people say focusing is hard at f/2 or faster that's a bit misleading. The focus is no more difficult than focusing on any other scope. What's critical is the backfocus. Where on another system getting your spacers within 1mm is alright, with the HyperStar it needs to be almost exact. That's why it's nice that they already have the adapters pre-made for each camera.

 

I centered my corrector plate and collimated the HyperStar, once I got it in, it was tack-sharp edge to edge. With my camera (28mm) I'm getting 3.4 x 2.1 degree FOV.

 

I'm a big fan of the EdgeHD scope because I can get three f-ratios with the same scope. I shoot planets at f/10+, some EAA with the f/7 reducer, and the HyperStar for most everything else. There might be better scopes at each given focal length but nothing else is more versatile in my opinion. HyperStar is a bit expensive but cheaper than any APO with that FOV and much faster speed.


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#17 danielgallo

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Posted 06 September 2018 - 05:25 PM

First off, there are HyperStar Bahtinov masks and auto-focusers. Those options always ensure basically perfect (or better than you could ever do) focus. 

 

When people say focusing is hard at f/2 or faster that's a bit misleading. The focus is no more difficult than focusing on any other scope. What's critical is the backfocus. Where on another system getting your spacers within 1mm is alright, with the HyperStar it needs to be almost exact. That's why it's nice that they already have the adapters pre-made for each camera.

 

I centered my corrector plate and collimated the HyperStar, once I got it in, it was tack-sharp edge to edge. With my camera (28mm) I'm getting 3.4 x 2.1 degree FOV.

 

I'm a big fan of the EdgeHD scope because I can get three f-ratios with the same scope. I shoot planets at f/10+, some EAA with the f/7 reducer, and the HyperStar for most everything else. There might be better scopes at each given focal length but nothing else is more versatile in my opinion. HyperStar is a bit expensive but cheaper than any APO with that FOV and much faster speed.

Thanks for the insights - very helpful!



#18 danielgallo

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Posted 06 September 2018 - 05:32 PM

@HappySkies - out of interest, do you have any of your HyperStar captures posted online somewhere, e.g. AstroBin?



#19 HappySkies

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Posted 06 September 2018 - 05:51 PM

I'll post some once I get home from work tonight :)



#20 danielgallo

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Posted 06 September 2018 - 05:51 PM

I'll post some once I get home from work tonight smile.gif

Awesome, thanks!



#21 james7ca

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Posted 07 September 2018 - 12:20 AM

Quick update, the female, t-thread nosepiece (gender changer) that is offered by ZWO has a length of 11mm, not the 10mm I wrote earlier. That still leaves a good amount of backfocus for use with the RASA and the ZWO ASI1600. So, 17.5mm added by the camera and the nosepiece with the capability of mounting a 1.25" filter internally to the nosepiece (with ZWO's adapter).

 

Thus, 25mm (backfocus on RASA) - 17.5mm ≈ 7.5mm to spare (without considering the effect of the filter, which could mean 7.5mm + 1mm ≈ 8.5mm).



#22 danielgallo

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Posted 07 September 2018 - 12:41 AM

Quick update, the female, t-thread nosepiece (gender changer) that is offered by ZWO has a length of 11mm, not the 10mm I wrote earlier. That still leaves a good amount of backfocus for use with the RASA and the ZWO ASI1600. So, 17.5mm added by the camera and the nosepiece with the capability of mounting a 1.25" filter internally to the nosepiece (with ZWO's adapter).

Thus, 25mm (backfocus on RASA) - 17.5mm ≈ 7.5mm to spare (without considering the effect of the filter, which could mean 7.5mm + 1mm ≈ 8.5mm).


Thanks for the update!

#23 cuivienor

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Posted 07 September 2018 - 01:39 AM

I'm in the same situation - have always been wary of the Hyperstar because of the gazillions of threads talking about bad stars, difficult to do collimation, corrector plate centering, and all sorts of stuff I don't want to do :)

 

That being said I'm in a highly light polluted area. So, would the RASA (or Hyperstar) work with a filter like the OPT Triad (the 3nm Ha bandpass makes me think no)? If so, I could use Hyperstar/RASA + OPT Triad + ASI294MC and have exposures of like 30 seconds !



#24 lucutes

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Posted 09 September 2018 - 01:44 PM

Going back to Bahtinov Mask. The only ones I can find are for SCT's with Hyperstar.  Assuming the same one will fit on it's respective sized RASA how do you take off the Bahtinov Mask without removing the camera? I plan on using the ASi071-Pro and eventually a ASi1600MM-Cool if they ever sort out the filter drawer for the 8" RASA.



#25 evan9162

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Posted 09 September 2018 - 02:01 PM

The 8" RASA is 24" long, whereas the 8" Edge is 17" long.  With a Schmidt corrector, the farther away from the primary mirror it is, the better corrected the FOV is (to a point).  The larger spacing between the corrector and mirror in the RASA probably leads to a better corrected FOV.

 

The larger spacing (or perhaps a re-worked corrector) likely leads to less spherochromatism, which may be why Celestron is emphasizing the wavelength range for the RASA.  A "standard" F/2+5x SCT is only well corrected at green wavelengths, and suffers from undercorrection in IR, and severe overcorrection in deep blue and violet wavelengths.  


Edited by evan9162, 09 September 2018 - 02:01 PM.



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