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Rasa 8 opinions

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#26 jprideaux

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Posted 12 February 2019 - 11:43 AM

FWHM,  Thanks for the mount info.  It is good to know that some people are having success with the Rasa with an alt-az mount and particularly that it can be mounted on the Evolution mount (with dovetail converter).  I will factor this in for my own decisions on what to get.

I take it that you already had the evolution mount for your 9.25? 



#27 FWHM

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Posted 12 February 2019 - 12:00 PM

Yes the mount came with the sct Celestron evolution 9.25.I use the 9.25 sct for imaging  planets, smaller galaxies and nebulae (like m57). I use the Rasa 8 with greater FOV for larger emission nebulae ( like ngc 7000). 

 

 


Edited by FWHM, 12 February 2019 - 02:59 PM.


#28 lollywater

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Posted 12 February 2019 - 05:08 PM

I had the Rasa  mounted on an EQ6-R which has a Losmandy and Vixen slot.

It has synscan wifi and sits in a skyshed pod and doesnt move . That saves having to do a polar align or a 2 star align each time.( I was told that not long ago and it works well)

I also purchased a ADM Dual saddle Upgrade for the HEQ5 which is mobile and sits in a shed. I havent tried the Rasa on the HEQ5 yet.

My alt/az mount would not cope with the weight of the Rasa 8. I like the convenience of alt/az mounts and I suspect the Rasa on an alt/az mount is well suited to EAA

 

I already had some spacers which I measured to be within about 1mm of the  suggested 25mm

 

FWMH ,I have no doubt I will like the Rasa. It will do EAA the way I want to do it.

I hope I havent been too negative about it. Mine happened to arrive with a problem and Celestron are onto it. The added frustration of a broken wrist and only able to use my lighter scopes for the last 6 weeks has left me pretty grumpy.

cheers

Paul

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

a



#29 FWHM

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Posted 12 February 2019 - 05:33 PM

Hi Paul,

i hope you get your Rasa soon, and in one piece.

 

Rasa 8 Fan power plug

One question for you. Did you purchase a 12v DC 100 mA 5.5mm/2.1mm positive tip power plug?

if you did, i would like to know where?

i am using too many batteries and need to purchase the 100 mA plug.

thanks



#30 lollywater

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Posted 12 February 2019 - 06:46 PM

No, havent even thought about the fan and plug

 

Give it a couple of weeks.I will have 2 hands, I will have the Rasa ,will be very pleased with it,and will be a happy little chappy again

 

cheers

Paul



#31 jprideaux

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Posted 13 February 2019 - 09:38 AM

FWHM,  Can you comment a bit when you have time your thought process on getting the Rasa 8 for wide-field imaging as opposed to getting the hyperstar for your existing 9.25?  One of the options I'm considering is getting the evolution 9.25 along with the hyperstar.  The Rasa 8 with Evolution mount does look good for EAA but in my case there is the problem of getting the evolution mount since I don't already have one.  Celestron doesn't sell the evolution mount separately and does not bundle the Rasa 8 with the evolution.  That is why I was also considering the iOptron Az Pro with the Rasa 8 but I don't know if the iOptron Az pro would be comparable to the Evolution mount for getting up to 30 seconds exposures.  Another combination I'm considering is the evolution 8 HD along with hyperstar.  At this time in my hobby, I place a high premium on ease-of-use and fast-set-up so prefer to avoid equitorial mounts.  I may go down the equitorial path in the future but not to start off.  I've also considered one of those combination equitorial/AltAz mounts for both options but those start to get kind-of heavy for portability.



#32 descott12

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Posted 13 February 2019 - 10:06 AM

The evolution mount is really nice. Very stable, built in battery and wifi. And quite portable. Really hard to beat. If "ease-of-use and fast-set-up" are important, I don't think you could go wrong with the Evo 8 or 9.25. I also really love the flexibility of the hyperstar and it is nice to be able to take if off when you want to go f10.


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#33 jprideaux

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Posted 13 February 2019 - 01:01 PM

FWHM & descott12,

 

Thanks!   I appreciate both of your comments about using fast systems for EAA.  Anyone who wants to do EAA will have to think through all these issues and as you say, come up with the best solution for themselves.  One of the hardest things is "future-proofing" but if we worry about that too much, we will hesitate to do anything.

 

Jeff



#34 OleCuss

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Posted 13 February 2019 - 01:18 PM

Personally, I'd contend that you get the fast exposures because you have a bunch of light-gathering, not because you have a lower focal ratio.  I learned early on in astronomy that it was the focal ratio that mattered and it took a while to get that beat out of me - and seeing some fast acquisitions of targets at rather high focal ratios was rather confusing.

 

Assuming the same aperture, if your target (presented in the image circle) fits on the sensor you should be getting exactly the same amount of light hitting the pixels of your sensor whether your focal ratio is 2 or 10.  You get the same amount of signal.  Read noise is diminished by using the faster focal ratio but with a modern (especially cooled) low-noise CMOS sensor your read noise is not high enough to be a real bother for most of us and could be considered negligible.

 

Historically a camera lens would change its effective aperture (using the old iris) when one changed the focal ratio.  It is very rare in DSO imaging that we stop down our optics so the choice of focal ratios does not involve changing the aperture and thus does not involve changing the amount of signal you are getting from the target.

 

With a system with a focal ratio of less than 4 you apparently actually start running into problems with the angle of the incident light on the microlenses of your sensor and start getting some reduction in the amount of light your sensor actually captures.  At F/4 I understand it really isn't significant but at F/2 it is more significant and how significant may depend on the sensor and I think most of us would consider it to be acceptable.

 

In a good and fast camera lens they actually add straightening elements so that you can have a very fast focal ratio without a big reduction in the amount of light being gathered (and an increase in the amount being scattered) but so far as I know, our astronomical OTAs don't have straightener elements.


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#35 DonBoy

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Posted 13 February 2019 - 01:27 PM

 

Celestron doesn't sell the evolution mount separately and does not bundle the Rasa 8 with the evolution.  That is why I was also considering the iOptron Az Pro with the Rasa 8 but I don't know if the iOptron Az pro would be comparable to the Evolution mount for getting up to 30 seconds exposures.

The evolution mount is available part #CEL-12091-MOUNT.  A better choice may be the iOptron AZ Mt Pro or the SW AZ/EQ 5.  All around the same price. 



#36 Don Rudny

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Posted 13 February 2019 - 01:42 PM

Personally, I'd contend that you get the fast exposures because you have a bunch of light-gathering, not because you have a lower focal ratio.  I learned early on in astronomy that it was the focal ratio that mattered and it took a while to get that beat out of me - and seeing some fast acquisitions of targets at rather high focal ratios was rather confusing.

 

Assuming the same aperture, if your target (presented in the image circle) fits on the sensor you should be getting exactly the same amount of light hitting the pixels of your sensor whether your focal ratio is 2 or 10.  You get the same amount of signal.  Read noise is diminished by using the faster focal ratio but with a modern (especially cooled) low-noise CMOS sensor your read noise is not high enough to be a real bother for most of us and could be considered negligible.

 

Historically a camera lens would change its effective aperture (using the old iris) when one changed the focal ratio.  It is very rare in DSO imaging that we stop down our optics so the choice of focal ratios does not involve changing the aperture and thus does not involve changing the amount of signal you are getting from the target.

 

With a system with a focal ratio of less than 4 you apparently actually start running into problems with the angle of the incident light on the microlenses of your sensor and start getting some reduction in the amount of light your sensor actually captures.  At F/4 I understand it really isn't significant but at F/2 it is more significant and how significant may depend on the sensor and I think most of us would consider it to be acceptable.

 

In a good and fast camera lens they actually add straightening elements so that you can have a very fast focal ratio without a big reduction in the amount of light being gathered (and an increase in the amount being scattered) but so far as I know, our astronomical OTAs don't have straightener elements.

Excellent points.  Here is a link to an article by Craig Stark that explains this in more detail.

 

http://www.stark-lab...tioAperture.php


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#37 jprideaux

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Posted 13 February 2019 - 02:27 PM

DonBoy, thanks for the tip on the SW-azeq5 (pro?).  I have been considering that as well.  I'll be comparing the weight of that to the Evolution and iOptron AZ-pro to see how much they differ.  The SW-azeq5 would allow me to work in AZ mode for convenience and also dabble in AP for longer exposures if i want.  Of course, dabbling in AP will probably make we wish I got the SW-azeq6 instead...  Ha!    But everybody should probably have a good portable travel mount anyway even if they eventually get a big heavy monster mount.

 

Don Rudny,  thanks for the link to the nice article "Aperture, f-ratios, myths, etc." 

 

And OleCuss, thanks as always for your valued comments.  I appreciate that it is all about photons hitting sensor pixels.   



#38 OleCuss

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Posted 13 February 2019 - 02:58 PM

Re: same amount of light and camera well saturation time

 

Faster compared to what ?

At f2, one second exposure is equivalent to almost 9 seconds at f6.3

this is why using my sct at f/6.3 with reducer, i could not image faint nebulae, and now i can with the Rasa. As for the distortion you are talking about at f2, Celestron has engineered the Rasa for that.

 

with my alt-az mount, i can go up to 60 secs exposure which is the equivalent of 9 minutes exposure at f/6.3.

to achieve a 9 min exposure requires an equatorial mount. In the city, i can't see Polaris. It would be hard for me to get setup.

so a stack of 30 x 60 secs gives me something close to 270 minutes, 4.5 hours when compared to sct at f6.3.

 

this is from my experience  using the sct f/6.3 and the Rasa8. What was out of reach (faint nebula) with my sct at f6.3 is now accessible with the Rasa. Fast in the sense that the target is acquired in 1/9 of the required time.

 

Thanks for the link.

 

keep looking up!

The fast focal ratio issue is not about distortion of the image.  It is about the angle at which the photon hits the microlens.  If the angling is not within the necessary range the photon doesn't go into the pixel as it should and can be effectively scattered.

 

Understand that I am not arguing against the RASA.  I'd really like to have one.  The very good optics and construction make it very desirable from my point of view.  I view it as getting something a little like a compact and light-weight 7-8" apochromatic refractor - at an immensely lower cost and without the need for an immense (and very expensive) mount.

 

I would like to have the RASA!



#39 FWHM

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Posted 13 February 2019 - 06:18 PM

FWHM,

I realize that you are only trying to help other members and that is very commendable. However all such assistance cannot violate the forum rules.

The EAA Forum Rules state specifically that post processed images are not allowed for any reason. Please become familiar with all of the rules.

If you have any questions feel free to contact me or an administrator.

______________________

 

My last post,

I deleted what I could, please delete the remaining posts. Thank you.

rules are important. keep looking up !

 

fwhm out.


Edited by FWHM, 13 February 2019 - 06:42 PM.


#40 OleCuss

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Posted 13 February 2019 - 06:43 PM

FWHM:

 

If I might make an appeal to you?

 

I am sure you have found some/much of this to be frustrating and it sounds to me all too much like you may be planning to quit the forum.

 

I may not always agree with you either currently or into the future but I think you have a lot to offer to both the forum and to the sub-forum and I hope you will choose to participate in the future.

 

If you can, I'll appreciate it.  If you cannot, well, I guess I understand.


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#41 Don Rudny

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Posted 13 February 2019 - 08:45 PM

What the heck is going on here?  Now someone is questioning my captures posted as being post processed.  And, some wonder why regulars don’t post here as much anymore.

 

I’m with Ole Cuss, FWHM.  Please stay.


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#42 jprideaux

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Posted 21 February 2019 - 07:49 PM

I look forward to clear weather in enough places around the world for more people to get first-hand experience with the Rasa 8.  If one is to jump on the "new is better" band-wagon, there is a new mount by iOptron to be announced soon that might be a nice option for the Rasa 8.  The cem40.  There is some traffic about it on the CN mounts forum.  It is like an iOptron cem25 but with a larger payload capacity (and some other upgrades).  I just checked my preferred observing place in my front-yard and I do have a clear view of the North star so perhaps an equatorial mount would work fine for me giving me the future option to do longer exposures.  That mount probably won't be ready for a few months, though.   About the same time as the Stellina will becoming available.  It might be an interesting Spring and Summer for EAA!




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