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Why buy a RASA when you already have a SCT w/ Fastar?

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#1 cmdrkeen

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Posted 24 September 2021 - 11:54 PM

When I first learned about RASA, I thought that it was a very outside the box idea. Then I realized that many of those semi-pro astrophotographers already had one or more decent SCTs w/ Fastar. Why they buy RASA when they already had SCTs w/ Fastar? Also, I haven't seen many people here in CN who actually utilized Fastar for astrophotography. Why not use them when you had the feature? 



#2 adosaj

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Posted 25 September 2021 - 12:09 AM

Also, I haven't seen many people here in CN who actually utilized Fastar for astrophotography. Why not use them when you had the feature? 

Simple, to do prime focus imaging I’d have to buy a $1000 accessory for my 8” EdgeHD. I don’t have the budget for it.



#3 DuncanM

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Posted 25 September 2021 - 12:55 AM

When I first learned about RASA, I thought that it was a very outside the box idea. Then I realized that many of those semi-pro astrophotographers already had one or more decent SCTs w/ Fastar. Why they buy RASA when they already had SCTs w/ Fastar? Also, I haven't seen many people here in CN who actually utilized Fastar for astrophotography. Why not use them when you had the feature? 

The RASA is designed as an astrograph, and has a larger corrected field than the Hyperstar. RASA V2 also has a superior focusing system. OTOH, if you have a Hyperstar capable SCT, already, then it might be the better choice as Hyperstar reducer is much cheaper than a RASA V2 OTA. 



#4 PABresler

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Posted 25 September 2021 - 02:12 AM

I had a Hyperstar for my Edge 1100; I took a couple of good shots with it. Then I sold the entire setup. I had an SV80 which provided the same field of view, although needed longer exposures. The problem with the Hyperstar was that it was a bit of production to set it up, and prevented other uses for the scope. After seeing some actually amazing results with a RASA, I just bought an 8 inch to use exclusively as a fast, wide field scope.

 

Peter

Attached Thumbnails

  • Horsehead & Flame Hyperstar small.jpg

Edited by PABresler, 25 September 2021 - 02:15 AM.

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#5 Fegato

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Posted 26 September 2021 - 03:07 AM

I had a Hyperstar with my 8 Edge HD. At around F/2 the tolerances on focus, tilt and the whole optical alignment are really miniscule. I loved the speed, but I could never sort out my alignment issues. I followed all the online advice (in particular see Jon Rista's article linked below), but I just couldn't sort things out and always had very poor comatic stars in the corners of my frames. In the end I sold the Hyperstar and bought a RASA 11 v2.

  

https://www.cloudyni...mize-perf-r3013

 

Some people advised that I was just replacing one difficult thing with another, but I've found it much easier to dial in a reasonably flat field with the RASA, and this with a full frame sensor. The key difference with the RASA is that the lens group at the front of the scope is specifically designed for and aligned to the rest of the system. So when collimation is good, you just have to focus on getting any issues with camera sensor tilt sorted out.

 

I should add that many people have success with Hyperstar. I suspect my 8 Edge and Hyperstar combo were just particularly unsuited to each other for some reason. Looking at my attempts now and with the experience of the RASA, I wonder if my backspacing needed to be increased well beyond the advised correct figure (I'd gone as far as 1.5mm extra - more than that).


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#6 JethroXP

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Posted 29 September 2021 - 06:02 PM

I had a Hyperstar with my 8 Edge HD. At around F/2 the tolerances on focus, tilt and the whole optical alignment are really miniscule. I loved the speed, but I could never sort out my alignment issues. I followed all the online advice (in particular see Jon Rista's article linked below), but I just couldn't sort things out and always had very poor comatic stars in the corners of my frames. In the end I sold the Hyperstar and bought a RASA 11 v2.

  

https://www.cloudyni...mize-perf-r3013

 

Some people advised that I was just replacing one difficult thing with another, but I've found it much easier to dial in a reasonably flat field with the RASA, and this with a full frame sensor. The key difference with the RASA is that the lens group at the front of the scope is specifically designed for and aligned to the rest of the system. So when collimation is good, you just have to focus on getting any issues with camera sensor tilt sorted out.

 

I should add that many people have success with Hyperstar. I suspect my 8 Edge and Hyperstar combo were just particularly unsuited to each other for some reason. Looking at my attempts now and with the experience of the RASA, I wonder if my backspacing needed to be increased well beyond the advised correct figure (I'd gone as far as 1.5mm extra - more than that).

I just did something similar, sold my C8v4 HyperStar and purchased a RASA 11 v2.  Question for any RASA 11 owners.  There was a small plastic bag containing two hex wrenches, which I know are for the collimation adjustment, but there were also two large thumbscrews and I have no idea what those are for?


Edited by JethroXP, 29 September 2021 - 06:02 PM.


#7 Fegato

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Posted 30 September 2021 - 02:00 AM

I just did something similar, sold my C8v4 HyperStar and purchased a RASA 11 v2.  Question for any RASA 11 owners.  There was a small plastic bag containing two hex wrenches, which I know are for the collimation adjustment, but there were also two large thumbscrews and I have no idea what those are for?

Yes, I had to think about those!   They screw into the losmandy bar underneath the RASA at each end - gives you security that it won't slip right off the saddle for any reason!


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