Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Celstron 8” Rowe-Ackermann Schmidt Astrograph (RASA) alternative?

  • Please log in to reply
8 replies to this topic

#1 doc_cj

doc_cj

    Explorer 1

  • *****
  • topic starter
  • Posts: 52
  • Joined: 08 Jun 2017
  • Loc: Northern Colorado

Posted 16 April 2021 - 09:21 PM

I've been using a 127mm Newtonian on a basic mount system for the last year. I want to move up to bigger (and hopefully better) for both observation and imaging. My budget if $3,000 +/- $250. Here's what I'm considering and my thoughts. Any helpful comments are appreciated. If you want to tell me I'm an idiot, then keep it to yourself. My wife has already filled that job. :-) 

 

System 1) Celestron 8” Rowe-Ackermann Schmidt Astrograph (RASA) with the CGEM II Mount (40 lbs load capacity). I'll likely add something like a 55 to 80mm tracking scope and camera within a couple months. I have a Nikon D3300 for my photography work, and until I outgrow that I don't see buying a CCD or CMOS until I win the lottery or otherwise find a need rather than desire. I especially like that the RASA has a f/2 so that I can get objects brighter and tighter on the camera system. 

 

System 2) Celestron Advanced VX 11" Schmidt-Cassegrain Telescope with the new VX mount. I would add the guide scope/camera for this unit as well. While the RASA is f/2, the VX!! is f/10. Obviously, I would need a field reducer, which still puts me well within my budget. My reason for wanting the 11" is observation. Unless I'm suffering from aperture envy, I believe the larger aperture will mean more light gathering, but I'm not sure. 

 

ISSUES: There are three basic issues I'm hoping to get some help on. 

 

Issue 1) Assuming I use a field reducer on the 11" (taking it down to f/6.3 or so), how much difference on the size of the image will it make? I ask because, as I understand it, the lower the f-number the smaller the image will appear on the camera sensor. In other words, rather than getting a wider field, with lesser light on the sensor, how much different will the width and strength be between the two?

 

Issue 2) Filed of View. Assuming both units are using the same power lens, would the field of view be different for the RASA vs SCT?

 

Issue 3) I have some alternative choices as well, and I'll fund them by here for comment. In addition, if you have a different system then please let me know. I'm still in the research stage, so there's time to figure out what will be best for me. 

 

Alternative Choices:

 

Orion 10" Newtonian Astrograph mounted on the Orion Atlas mount with a nice guide scope/camera setup. This is actually about between $500 and $800 less, depending on the choice of other options. 

 

Meade LX90 Series 12" ACF (about $100 over my budget). 

 

Thanks in advance for your help.  



#2 dswtan

dswtan

    Viking 1

  • *****
  • Posts: 586
  • Joined: 29 Oct 2006
  • Loc: Morgan Hill, CA

Posted 16 April 2021 - 09:35 PM

Lots of potential comments here, but for the field of view question, I recommend you study the simulations available, e.g.: Telescopius or https://astronomy.to.../field_of_view/



#3 Huangdi

Huangdi

    Apollo

  • *****
  • Posts: 1,069
  • Joined: 24 Jul 2019

Posted 16 April 2021 - 09:44 PM

Well, I suggest you go to this page and check out the FOV's to get a better grasp of what you're getting into. https://telescopius....scope-simulator

I have a couple of comments

1. You can't use your Nikon D3300 with the RASA8. It doesn't have enough available backfocus so you'll never be able to reach focus.

2. A C11 is a large scope and selling it coupled with an AVX mount should literally be illegal. The minimum mount required for that telescope is a CGX/CEM60 kind of mount. I wouldn't be surprised if the AVX just imploded upon first usage.

3. The C11 has a very long focal length and will give you a massively different FOV from a RASA8. It will also be much more difficult to use and I would not recommend it as a photographic scope when starting out (or even when you're experienced, that's what EdgeHD scopes/ACF's exist for).

4. Same story goes fkr the 12" ACF. That's a huge scope and not really meant for beginners.

5. Decide what you want to image. A RASA8 is a widefield telescope. A Schmidt-Cassegrain is a very long focal length scope for planets and small targets like planetary nebulae and small galaxies.

I suggest starting with the former, but I wouldn't start with a RASA8 either.

I would consider purchasing a nice and future proof mount like the EQ6-R (unless you really want that big 11-1" SCT down the road, in that case consider a CEM70 or similar.

For the scope, I'd just go with an 80mm refractor to begin. It pairs nicely with your camera and if you buy it used, you can literally sell it without losing money. It'll be a lot more fun to start out with.
  • dswtan, kraegar, jdk and 1 other like this

#4 jdk

jdk

    Messenger

  • *****
  • Posts: 446
  • Joined: 06 Jul 2017

Posted 16 April 2021 - 10:16 PM

These are vastly different setups that you're considering. I've owned an 8" edge and an 11" rasa, and they really could not be more different in how they fit into an imaging workflow.

 

The rasa can work with a small DSLR* or mirrorless camera, but it is far from ideal. The protrusion into the light path will cause asymmetric diffraction spikes and eat into the f/2 a little bit. 

 

The edge - even reduced - has a much more narrow FOV even with a relatively large sensor. It will make imaging more challenging. 

 

I think it would be helpful for you to tell us what you're not getting out of your current setup that you'd like to be able to achieve going forward. 

 

Edit: *apparently the 8" rasa cannot work with DSLRs...only the rasa 11 can.


Edited by jdk, 16 April 2021 - 10:20 PM.


#5 petert913

petert913

    Aurora

  • *****
  • Posts: 4,697
  • Joined: 27 May 2013
  • Loc: Silverton, OR

Posted 16 April 2021 - 10:16 PM

1.  RASA is strictly imaging only

 

2.  A C11 on an AV-X mount will never work. Add more weight with a guide scope and it is disaster.


  • jdk likes this

#6 jeffcrilly2

jeffcrilly2

    Explorer 1

  • -----
  • Posts: 61
  • Joined: 25 Jun 2011

Posted 18 April 2021 - 06:39 AM

 

... as I understand it, the lower the f-number the smaller the image will appear on the camera sensor. In other words, rather than getting a wider field, with lesser light on the sensor, how much different will the width and strength be between the two? 

 

(I believe the "f-number" you refer to is the "f-ratio"... i.e. the ratio of optic diameter to focal-length.)

 

The size of the image on the focal plane is determined by the focal length of the optics.

 

      Diameter * f-ratio = focal length

 

It is true that for the same focal length, an optic with a lower f-ratio will gather light faster.

Why?  because the diameter must be much larger to offset the smaller f-ratio .

 

Here's some examples:

 

8" RASA:  203mm diameter * f2 = 406mm focal length

11" C11 w/ reducer:   280mm diameter * f6.3 = 1764 focal length

 

From this, I believe objects with the C11 will be about 4 times larger than with the 8" RASA.

 

Also.. as the focal length increases , the precision of the mount becomes more important.

It is best if the imaging setup is "over mounted".

 

Honestly, I would not go with a C11 for imaging unless you also get a large mount.

A G11 might work -- my experience with the G11 is dated, but it was good to about 1000mm (mine did not have the high-precision worm).

 

Otoh the C11 would indeed be good for visual work.  (but as others noted, the VX mount may be taxed with the C11.)

 

I think you need to figure out how much visual vs imaging you want to do.

 

For imaging, I would suggest a decent refactor with a focal length between 500mm and 900mm, on an appropriate mount.

(The f-ratio is basically driven by how much you want to spend.  Lower f-ratio = bigger diameter lens = more glass = more $$, basically.)

 

For visual... honestly, there's nothing like a good dob, say 10" or so , which is reasonably priced (unless you want goto).

 

-jeff



#7 Phishin_phool

Phishin_phool

    Messenger

  • *****
  • Posts: 478
  • Joined: 31 May 2020
  • Loc: Ohio

Posted 18 April 2021 - 11:49 AM

. as I understand it, the lower the f-number the smaller the image will appear on the camera sensor. In other words, rather than getting a wider field, with lesser light on the sensor, how much different will the width and strength be between the two?

 

This is not correct the f stop number is simply the focal length divided by the aperture (in the case of a telescope the objective lens). This will give you a relative idea of the amount of light that gets through (smaller f stops mean greater ratios = wider opening and more light gets through). Faster f stops, meaning lower numbers, accumulate more light/photons quicker and require shorter exposure lengths. The smaller the f stop also the shallower the depth of field and the more critical your focus becomes. It does not however have anything to do with the image size that is controlled by other factors and then how wide your captured fov is controlled by your sensor size.



#8 TxStars

TxStars

    Mercury-Atlas

  • *****
  • Posts: 2,570
  • Joined: 01 Oct 2005
  • Loc: Lost In Space

Posted 18 April 2021 - 12:00 PM

Fast focal ratios do not gather light any faster than a slow focal ratio...8" is 8" does not matter the f/ratio

What the faster focal ratio (f/2) does is focus the light into a smaller area at the focal plane than the (f/10)slower focal ratio.


Edited by TxStars, 18 April 2021 - 12:03 PM.


#9 Phishin_phool

Phishin_phool

    Messenger

  • *****
  • Posts: 478
  • Joined: 31 May 2020
  • Loc: Ohio

Posted 18 April 2021 - 03:10 PM

Fast focal ratios do not gather light any faster than a slow focal ratio...8" is 8" does not matter the f/ratio

What the faster focal ratio (f/2) does is focus the light into a smaller area at the focal plane than the (f/10)slower focal ratio.

If  focal length is kept the same then yes faster f stops do indeed gather more light - exactly 2x as much per each f stop.  Now if you are talking about well the scope "opening" (objective lens / aperture) is 8" and 8" is the same no matter what and any 8" scope will collect the same amount of light  then 1 of two things must be true - 1) the scopes have exactly the same f/stop and focal length (they are identical scopes) or the scope with the larger f/stop has a larger focal length and will be less illuminated My statement above is based on two scopes with the same focal length and different f-stops.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics