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Celestron's new 8" f/2 RASA

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#1 RazvanUnderStars

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Posted 25 August 2018 - 04:48 PM

The back cover of Astronomy's October issue is a Celestron ad, announcing the new 8" f/2 version of their RASA (Rowe Ackermann Schmidt astrograph) family. There isn't much information online, not even their own page, https://celestron.com/rasa, mentions the 8" model, let alone the price.

 

At f/2, this could be an interesting alternative to the SCT + Hyperstar combination. I found a quick discussion in the video at https://youtu.be/uziAK_6PDsA?t=34m34s (the link jumps directly to that segment).

 

This forum had some earlier discussions about using the larger RASA scopes for EAA for those interested. 


Edited by RazvanUnderStars, 25 August 2018 - 04:53 PM.

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#2 OleCuss

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Posted 25 August 2018 - 09:08 PM

Your post caused me to look up the Celestron white paper for their RASA:  https://s3.amazonaws...e_paper_web.pdf

 

It sounds like they were interested in an 8" RASA from the beginning but did not pursue that aperture due to the obstruction expected from the DSLRs at the time.  The amateur community is now doing a huge amount of its work with dedicated astronomical cameras and when we now have a rather svelte dedicated astronomical cameras it is quite possible that the obstruction is not a significant issue for many of us.

 

I'm really looking forward to seeing the specifications of the 8" RASA.  It could be a great instrument for many of us!

 

BTW, that white paper has a very nice discussion of "etendue" and since that term comes up at times on the forum it could be useful.  There is also historical information which I find interesting.


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#3 RazvanUnderStars

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Posted 25 August 2018 - 10:16 PM

Thanks, OleCuss, that white paper looks a very interesting read. This will lead to interesting small talk on Monday: "So, how was your weekend?" "I read about high-étendue telescopes" "Ummm..." smile.gif

 

The new cameras are also a lot more symmetrical than a DSLR so the diffraction should be more pleasing as well.


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#4 Sigerson

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Posted 27 August 2018 - 08:05 PM

The RASA white paper also mentions EAA on page 18 - referencing "image live stacking" and uses the Atik Infinity camera as an example. 

 

I have been waiting for the main telescope manufacturers to sell a specifically designed EAA system that includes telescope, mount, camera, software and accessories to provide a complete, turnkey system.  The 8 inch RASA with an IMX294 based camera might be a good starting point for Celestron. 

 

It won't be cheap, but I am not looking for cheap.  I would seriously consider paying $5000+ for a dedicated, complete, hassle-free EAA system.  I intend to sit on the sidelines until such EAA systems become available from C, M, SW, V, iO and the rest.  My hat is off to those of you daring enough to invest time and money into designing your own EAA systems.  That is an investment I am currently not willing to make.


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#5 RazvanUnderStars

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Posted 27 August 2018 - 08:55 PM

I won't try to change your mind, of course, but one comment I have is that for me it didn't feel like "designing a system". I simply bought a camera which I used with the existing scopes. Then recently I upgraded the camera. I actually like the flexibility, I can upgrade any part independently or I can choose the right combination depending on the circumstances (eg smaller mount and tube for when I want everything to fit in one bag).

#6 OleCuss

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Posted 28 August 2018 - 04:24 AM

.

.

.

. . .I would seriously consider paying $5000+ for a dedicated, complete, hassle-free EAA system.  I intend to sit on the sidelines until such EAA systems become available from C, M, SW, V, iO and the rest.  My hat is off to those of you daring enough to invest time and money into designing your own EAA systems.  That is an investment I am currently not willing to make.

 

You may not have too long to wait.

 

The Stellina may already be sort of available - or you may have to wait until December?  https://store.moma.o...977-138977.html  They say it is back-ordered until the 7th of December.

 

In June of next year you are supposed to be able to get the Hiuni:  https://www.kickstar...ope/description

 

I cannot find the current estimate for when the Unistellar is supposed to be available, but I think the last I saw may have been November of next year?:  https://unistellaroptics.com/

 

I rather expect someone like Meade or Celestron to start selling this kind of system but I've not heard a peep from them.  Meade has already had much of the needed tech and it seems like an obvious move, especially because I think it could sell well and would pull people who have previously not considered buying astronomical equipment into looking at Meade.

 

But it is important to realize that each and every one of the offerings has significant technical limitations.  We really are not their market at this time and they are not optimizing their gear for the kind of use I think most of us want.  But if you are willing to pay $3,000 you can probably have a turnkey OAP system in your hands in 4 months.

 

Personally, I think I'd wait until we have some good reviews from good astronomers.  I'd like to have the Stellina for public viewing events but without a review I trust and for $3,000 I think I'll be waiting a while (maybe forever).


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#7 Sigerson

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Posted 05 September 2018 - 02:48 PM

The 8 inch RASA is now on Celestron's website...

 

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



#8 mclewis1

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Posted 05 September 2018 - 03:15 PM

Pricing is now up on the OPT website ...

 

RASA 8 - $1699

 

and a really interesting deal - RASA 8 + CGX mount - $3699

 

Add an ASI294 camera for $800 and you've got a heck of a turnkey fast dedicated wide field setup. 


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#9 lucutes

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Posted 05 September 2018 - 08:43 PM

So would this RASA be only good for 4/3 camera's? 



#10 A. Viegas

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Posted 05 September 2018 - 09:16 PM

Not sure why you would get this rather than a vanilla C8 edge with hyperstar and probably even a reducer. So that would give you F10, F7 and F2. So same price and 3 F# vs just F2 all the time. No thanks

Al
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#11 OleCuss

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Posted 05 September 2018 - 09:22 PM

So would this RASA be only good for 4/3 camera's? 

I think that depends on just how picky you are.

 

The really pristine image circle seems to be 22mm but I think that you can image at up to 32mm - just with increased aberration.  It is possible that the aberration you acquire by going from 22mm to 32mm is mostly vignetting and that flats may mostly eliminate that.  Net effect is that I'd not be reluctant to use an APS-C sensor.

 

However, given that short focal length I'd expect something like the ASI294 to be quite significantly under-sampled.  Not a big problem for most of us when we are doing OAP but if you want to save your subs for later processing you may find that you are not too happy with the subs for that secondary purpose.

 

Might consider something like the ASI183?  It'd still be under-sampled at a 400mm focal length but it wouldn't be nearly as bad as the ASI294 or something like the ASi071.  Sensor size for the ASI183 would be smaller than I'd like for that 8" RASA.  I'm betting there is a better choice out there but I'm not sure what it is or if it is affordable. . .

 

I'm hoping someone else will point out a camera which might be a better match with good performance because I kinda want it all.  The 8" RASA should really fit into my interests very nicely with a wide FOV and lots of light-gathering.  I'm not too keen on that big obstruction for both light-gathering and diffraction reasons but it is possible I wouldn't find it overly obnoxious?

 

Ah, well.  I've not the money for it at this time anyway.



#12 OleCuss

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Posted 05 September 2018 - 09:29 PM

Not sure why you would get this rather than a vanilla C8 edge with hyperstar and probably even a reducer. So that would give you F10, F7 and F2. So same price and 3 F# vs just F2 all the time. No thanks

Al

You could use the 8" RASA with a nice big sensor at F/2.  I don't think the nearly fully-illuminated image circle of the 8" Hyperstar is nearly as generous as what you get with the RASA.  So bigger sensor and highly corrected optics would be the reason I'd want it.  Awesome for something like the ASI071 except for bad under-sampling.

 

But the 8" EdgeHD would appear to have less diffraction and better light-gathering than would the 8" RASA?  With the 8" EdgeHD you shouldn't have a significant problem with under-sampling at F/10 or F/7 with something like the ASI294 or the ASI071.

 

I think I'd have to agree that for the purposes of most of us the 8" EdgeHD just might be the better choice.



#13 Sigerson

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Posted 06 September 2018 - 04:20 PM

I would like to see one of the big guns get 'brave' and design an EAA-specific (no eyepieces) OTA (as part of a complete EAA system) in the Goldilocks Zone for bigger pixel sensors like the IMX294.  This would be a moderate aperture at a fast, but not extreme, f/3 - f/4 focal ratio.  This would get the image scale in the 1.5 arcsec/pixel range with nice, moderately large FOV.   And figure out how to make collimation rock-solid permanent.  And not cost an arm and a leg - maybe just a few fingers.  Not asking for much, am I?  wink.gif ​ 

 

BTW, the RASA 8 description sounds like it is using the new and improved focuser design discussed elsewhere on CN.  No image shift.



#14 mclewis1

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Posted 06 September 2018 - 05:18 PM

I would like to see one of the big guns get 'brave' and design an EAA-specific (no eyepieces) OTA (as part of a complete EAA system) in the Goldilocks Zone for bigger pixel sensors like the IMX294.  This would be a moderate aperture at a fast, but not extreme, f/3 - f/4 focal ratio.  This would get the image scale in the 1.5 arcsec/pixel range with nice, moderately large FOV.   And figure out how to make collimation rock-solid permanent.  And not cost an arm and a leg - maybe just a few fingers.  Not asking for much, am I?  wink.gif ​ 

8" AT imaging (f4) newtonian, AT field flattener, IMX294 camera, CGEM class mount ... done. Well ok except for the rock-solid collimation, but a nice little laser setup will take care of making it easy. Add a 2x 2" Barlow and you've got the sweet spot of focal lengths covered.


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#15 mrackerm

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Posted 23 September 2018 - 11:52 AM

Interesting thread.  

 

I designed the RASA-8 for Celestron.

 

Why would you buy the RASA-8 rather than one of the alternatives mentioned above?

The RASA-8 is a dedicated instrument.  It cannot be used at f/10 and is not as inexpensive

as an f/4 Newtonian or one of the other options one might consider.

 

The RASA-8 does one thing and does it extremely well.  It has a 4-lens, 2-group corrector.

The use of two doublets helps to improve image contrast and reduce ghost images.  The

corrector is designed to give exquisite image quality from 390nm to 800nm.  While the

HyperStar is a fine addition to any Celestron SCT, it has a more narrow field of view, lower

image quality and is corrected for a much more narrow range of wavelengths.  Simply put,

the RASA-8 really is a professional quality instrument available at prices that should be

attractive to most amateurs.  With the Zero-Shift focusing mechanism, the RASA-8 will

be a pleasure to use.

 

While the RASA-8 does one thing, and does it very well, in the hands of the right

astronomer, that one thing can be used for many observing purposes.  The RASA-8

could form the heart of an inexpensive tool for watching artificial earth satellites,

searching for comets and asteroids, searching for exoplanets, or just good old

wide-field imaging of the night sky.  

 

The fully illuminated image field is 22mm in diameter, but the design of the

corrector lenses provide very high image quality out to at least 32mm diameter,

with only a little light loss (vignetting) past the 22mm inner region.

 

If you wanted a RASA-11 but it was a bit too expensive, the RASA-8 might be

for you.  It will pull in many times more light that an APO refractor, and is 

much more affordable.  It has higher image quality than any Newtonian without

a coma corrector, and is much more compact, easy to use and really never

needs collimation.

 

Maybe it's the right instrument for you.  Maybe not.  Any optical design is a

compromise, but Celestron has really pushed the envelope with the RASA

family.  One really cannot come close to their performance for twice the

price with any alternative.

 

For those who would chose to pan me over this post, please note that

I have no financial interest in Celestron or in the success of the RASA family.

Celestron paid me to design various members of the RASA family and that

is the end of my financial contact with them.  I continue designing for

Celestron as they have proven to be an absolutely first rate company with

an exceptional leadership team.  They are dedicated to truly innovative,

high-quality instruments for the amateur and professional astronomical

community.  For those who do not have insight into the professional

astronomical community, I can tell you that many RASA-11s and RASA-14s

are in use by professionals all over the world.

 

Mark Ackermann


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#16 OleCuss

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Posted 23 September 2018 - 02:27 PM

That was very useful and appreciated.

 

Thank you!


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#17 lucutes

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Posted 23 September 2018 - 06:46 PM

Thank you for your hard work on the RASA 8".  I have ordered one and look forward to putting my ASi071MC-Pro on it. The biggest thing that interested me the most about this scope is its portability as a F2 wide field imaging system.  I have limited use with refractors of the same focal length but they don't come close to the power that this puppy will have.  



#18 RazvanUnderStars

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Posted 23 September 2018 - 07:08 PM

Thanks for your work, Mark and I hope you'll continue to design telescopes for them, looking forward to your future work!



#19 DonBoy

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Posted 24 September 2018 - 11:03 AM

Hi Mark,

 

Thanks for your input, much appreciated.  

 

Does this RASA accept standard M48 filters?

 

I'm wondering whether there will be a RASA 9.25?   

 

Don


Edited by DonBoy, 24 September 2018 - 01:46 PM.


#20 Benni123456

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Posted 27 September 2018 - 02:59 PM

Hi mrackerm If you are the RASA designer:

 

Well, I personally think an f4 newton with your ackermann reducer is simpler and cheaper for this.

 

But:

Why do you not design an f7 or even f:6 edge hd and then offer the corresponding teleconverters to f:10?

 

I mean, with the current reducers for the edge hd telescopes, one mostly gets out of collimation and then has a hard time correcting alignment and collimation in order to get an appropriate field.

 

I guess this is because the reducer is so far away from the secondary. Every small change in the direction of the secondary then has a huge effect at the positions where the lightrays fall into the reducer which then affects the field...

 

So it is clear that it would be more convenient, to start with an f:7 edge design and then offer a teleconverter to f:10. Since the teleconverter shows a subset of the full f:7 frame anyway, no collimation problems should appear...

 

For planets only, I do not need an edge hd but a usual sc should suffice... An edge would therefore make more sense at f:6 or f:7...

 

Also an 6'' edge hd with f:7 would probably be a nice small grab and go scope...

 

Finally: Cn one design these scopes such that they can be more easily opened and collimated?

 

And can one put in an option to close these venting filters and to add dessiccant tablets there if necessary.

 

In some regions the climate is moist and one should make sure that the air getting into the scope is dry and that no air gets into the scope in certain situations.. When the scope has cooled down, dessiccant tablets should make the air dry. And then I want to close these slits altogether. I do not want the entire moisture of the morning air to get unloaded onto the primary mirror...

 

Since moisture can come into the scope through these filters anyway, there should be an easy way how to open the scope, clean it from inside and collimate it (for which Celestron may provide manuals and collimation tools). Opening, cleaning of the mirrors, alignment of the Schmidt plate, and collimation of the secondary with a ccd should at least be explained in a white paper. With these filters and slits, Celestron has opened these scopes to the environment in some way, so a Customer should be able to clean them.


Edited by Benni123456, 27 September 2018 - 03:04 PM.

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#21 mclewis1

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Posted 27 September 2018 - 05:41 PM

Benni,

 

Mark Ackerman does not work for Celestron. He was contracted by Celestron to help develop the RASA series of scopes. He has nothing to do with the development of other existing Celestron scopes. So your questions are probably better asked of Celestron directly.


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#22 Benni123456

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Posted 27 September 2018 - 08:37 PM

They probably will contract him again on some scopes or he knows people there and can forward them a design he finds imteresting...

 

Probably Celestron would listen more to ackermann than to me... So when ackermann says to celestron

 

hello i have here a cheap design for a small grab and go  6" edge hd f:7 and a deep sky 8" edge hd with f6 and teleconverters to get to f10 and even f12 or higher for planetary view...

I guess Celestron would be interested...

 

When I tell them something, then I am just one of thousands of customers who want a different product...



#23 OleCuss

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Posted 27 September 2018 - 08:48 PM

Please consider the following to be something other than a lecture.  I'm not the expert on all the optical systems, design imperatives, markets, or cameras.  This is sort of my way of trying to work through why the RASA-8 exists and what it would take for it to be a good purchase for me (if I had the money right now).  If I were convinced it would fulfill most or all of my own wants/needs I might sell some of my current gear in order to get the RASA and a matching camera.  So I'd appreciate it if anyone chooses to correct errors in my thinking.

 

 

Maybe I have this all wrong (wouldn't be the first time), but I am not sure that comparing the RASA with a good Corrected SCT like the EdgeHD or the ACF is entirely apt.

 

Humor me and let me call both the ACF and EdgeHD CSCTs even though I know there are some design differences and CSCT is not at all a standard term.

 

The CSCT is a good all-purpose instrument.  Technically usable at multiple focal lengths and can make us happy for a lot of reasons.

 

The problem is that you just cannot do with any CSCT what you can do with a RASA.  It cannot be done and won't be done if you make the CSCT an F/6 or F/7 instrument.

 

The RASA may not really be aimed primarily at the amateur AP market.  We have access to it but that doesn't mean we are the primary demographic.

 

The RASA is designed to give a highly corrected and big image circle.  What is more, it will give you corrected access to more of the electromagnetic spectrum at the same time.  Even with the Hyperstar you are going to get what Starizona calls a maximum image circle of no more than 28mm.  The RASA will get you 32mm with apparently only mild vignetting.  If you have the camera for the image circle you just cannot get the same FOV with the Hyperstar.

 

Apparently the quality of the optics of the RASA is just plain better than our current CSCTs.

 

It is interesting to remember the F/6.3 Meade SCTs.  They had a bigger central obstruction and a lot of them really didn't have very good optics.  I understand that at the end they were doing a better job on the optics, but the net effect is that it may be considerably more difficult to make F/6-7 SCT optics and I'm really not sure that you even can make a CSCT at F/6-7.  But even if it turns out that it is pretty easy to make a F/6-7 CSCT I'd probably not make it if I were Celestron because years ago Meade tried F/6.3 SCT and people didn't buy enough of them to make it sustainable.

 

I do have a lot of concerns about the RASA and suspect it really isn't an instrument for me any time very soon.

 

The concern about moisture is of real interest but wouldn't I have the same problem if I got an EdgeHD at whatever focal length?

 

Lots of under-sampling for me with my current sensors whether I get a Hyperstarred SCT or the RASA.  Slightly less under-sampling with the 8" RASA than with a Hyperstarred 8" SCT but not enough to make much difference IMHO.  If I get an IMX183 camera my under-sampling issue is much less but my diagonal size is now smaller than what I suspect is the highly corrected image circle of the Hyperstarred 8".  So if I try to minimize the under-sampling with the cameras I might afford and will fit within the 8" RASA central obstruction  - I'm going to a sensor which doesn't take advantage of the bigger image circle of the RASA anyway.

 

And if I'm grossly under-sampled I'm not entirely sure why I would want the more highly corrected optics of the RASA since my under-sampling is (I think) killing much of the benefit of the better optics.  Not sure my current cameras can benefit much from the wider spectrum anyway.

 

It leaves me frustrated.  I really like the idea of the RASA.  I like a wide FOV and I like great optics.

 

What I'd really like to do is to use the 8" RASA without under-sampling and with a sensor which is matched to the image circle.  This would mean that I can get that nice wide FOV and be able to zoom to any portion of the image that I want to better see - and see it in glorious detail.  Right now I don't know how to make that work.

 

The 11" RASA would give less under-sampling but I'd need at least a full-frame sensor to keep from losing too much of the FOV.  Price and weight go way up as well.

 

Even the RASA-8 is probably more than I should put on an AVX and that would mean that when I go to a dark site with it I'd have to bring the G-11.

 

Maybe some day?


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#24 Benni123456

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Posted 28 September 2018 - 10:56 AM

The rasa is for full frame cameras. A Newton with f 2.8 just has 22mm image..

 

If aps c suffices, a newton with f:2,8 will be cheaper..

 

If one wants a scope that fast one may also consider a very good telephoto lens with fluoride glass canon 300 2,8 etc..

 

But this is more expensive than the rasa...

 

Why should one need a scope this fast? for sources that change their lightning output... Trying to find the magnitude increase  in supernovae could be one example...

 

Saving image time is another...

ackermann says exoplanets... this could also work if one searches throuh the image with a computer program... But I guess for this, seeing is a severe problem. and you need to bring the rasa into a satellite or at least to a mountainous location (when you place it next to Mauna Kea, the indigeneous population may see the small rasa as a relief since it does not disturb their mountain so much as a thirty meter instrument..)

 

 

The concern about moisture is of real interest but wouldn't I have the same problem if I got an EdgeHD at whatever focal length?

Yes. If used in the field and not under a warm dome, the airvents tend to cool the mirror such that it gets fogged up...

 

Maybe in California you do not have this problem, in europe you do...

A professional observatory in the alps here would get problems I guess...

 

If I use my sc from the warm balcony and look into the scope with a flashlight in the morning, the primary is clean...

 

In the field, in the morning it will turn out to be fogged up

The airvents really do their job... cooling the primary beyond dew point and let moisture get in... This is annoying when the sope is difficult to open, clean, and center.... like the edges and rasas


Edited by Benni123456, 28 September 2018 - 11:10 AM.


#25 OleCuss

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Posted 28 September 2018 - 11:45 AM

I'd happily agree that the 11" and 14" RASAs are intended for use with full-frame cameras.

 

I'm pretty sure the 8" is not intended for use with a full-frame camera.  For the RASA-8 I'd expect an APSC-C or a 4/3rds sensor to be about right depending on priorities/likes.

 

That's one of the things which makes the RASA-8 so interesting.  I have the sensor sizes which I think should  be very nice with the RASA-8 but the under-sampling will be quite significant and paying more for the great optics then seems like a waste when I could get something like an EdgeHD and get results which may (in some cases) better than with the RASA-8.  But if I want to work at around a 400mm focal length I'd have to get the Hyperstar which is an added expense.

 

And yeah, humidity is not a big problem for me at this time.  It does happen, but infrequently.




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