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

Why do most people image with a Schmidt Cassegrain and not a Dobsonian?

  • Please log in to reply
22 replies to this topic

#1 bluesilver

bluesilver

    Mariner 2

  • -----
  • topic starter
  • Posts: 281
  • Joined: 03 Jul 2020
  • Loc: Tasmania, Australia

Posted 01 August 2021 - 01:46 AM

Hi,  this is most likely a very basic question,  but would like to ask it anyway to help me understand things a little better.

 

I am talking about imaging the planets with a camera like the asi224mc

 

I see that most images of the planets are taken with scopes like a Celestron C11 , C14, C9.25

But i was interested to know why you don't use something like a 10" 12" or 16" Dobsonian that has goto like the ones Skywatcher offer.

 

I was thinking that with a Dobsonian, you would have the same sort of Aperture ( except for the larger 16" version ) 

You just don't have the focal length with a Dobsonian compared to the Celestrons

 

There are a few differances on the mounting,  Celestron are designed mainly for equatorial mounts, Dobsonians for azimuth mounts.

 

so the questions are:

1. In general why do most use Celestrons instead of Dobsonians for planetary imaging?

 

 

2. So if i were to be looking at a dedicated scope for planetary imaging with a asi224mc camera,  would i get better results ( imaging wise before processing ) from the Celestrons or Dobsonians?

 

I guess it is a broad questions,  but just trying to get the basic understanding here.

 

I have been lucky enough to pick up a good 16" Dobsonian that has goto ( Skywatcher )  and looking o have a dedicated planetary scope to image with.

 

 


Edited by bluesilver, 01 August 2021 - 01:56 AM.


#2 mayhem13

mayhem13

    Viking 1

  • -----
  • Posts: 616
  • Joined: 10 Jan 2021
  • Loc: New Jersey

Posted 01 August 2021 - 02:12 AM

Your 16” GoTo Dob will produce some pretty amazing images



#3 Tulloch

Tulloch

    Gemini

  • *****
  • Posts: 3,447
  • Joined: 02 Mar 2019
  • Loc: Melbourne, Australia

Posted 01 August 2021 - 02:32 AM

I don't know the reason, but Dobs are beginning to used more and more for planetary. Aperture rules with planetary (regardless of OTA type), but for best results (and ease of imaging) a tracking (goto) mount is preferred. Alt/az is fine (my SCT is mounted on an alt/az mount), no need for guiding and no need for polar alignment.

 

If you have a goto 16" Dob, you could get some amazing images of the planets, assuming good seeing. Capture at around f/20 with the ASI224MC, and watch these tutorial videos.

http://planetaryimagingtutorials.com/

 

This website (and the accompanying book) might also be of interest.

https://www.planetar...azimuth-dobson/

 

Andrew


  • CPellier likes this

#4 havasman

havasman

    Voyager 1

  • *****
  • Posts: 13,101
  • Joined: 04 Aug 2013
  • Loc: Dallas, Texas

Posted 01 August 2021 - 02:37 AM

I think more folks image with refractors than all others combined. But the SCT's bring the focal length with easier mounting requirements. From what I've seen, the best planetary images are got via C14's and even larger aperture STC's and like designs. A club member gets great results with his 20" Meade SCT but he's a pro.


Edited by havasman, 01 August 2021 - 02:40 AM.

  • peta62 likes this

#5 Tapio

Tapio

    Aurora

  • -----
  • Posts: 4,706
  • Joined: 24 Sep 2006
  • Loc: Tampere, Finland

Posted 01 August 2021 - 02:45 AM

There are plenty of imagers which use Newton scopes and dobson Ian.
I think the reasons why SC scopes might be more popular are more compact size, less demanding barlowing, and maybe field rotation in the longer sessions and animations (don't know if it's an issue).
  • CPellier likes this

#6 Supernova74

Supernova74

    Mercury-Atlas

  • -----
  • Posts: 2,761
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 01 August 2021 - 02:45 AM

Another reason could be is that the dobsonian wasn’t really designed as a dedicated imaging rig.its sole purpose was a large aperture cost effective visual scope.you never get the same kind of results with a equatorial mount and of course feild curvature etc which a large aperture dob would introduce.wrong tool for the job.


Edited by Supernova74, 01 August 2021 - 02:46 AM.

  • CPellier likes this

#7 AstroEthan

AstroEthan

    Vanguard

  • -----
  • Posts: 2,385
  • Joined: 16 Jun 2014
  • Loc: San Antonio area, TX, U.S.

Posted 01 August 2021 - 04:12 AM

4 years and 363 days ago, a younger Ethan asked a similar question. Might be worth checking out alongside the responses you get here.

 

Why are SCTs So Widely Used for Planetary Imaging?



#8 peta62

peta62

    Ranger 4

  • -----
  • Posts: 358
  • Joined: 24 Feb 2011

Posted 01 August 2021 - 06:04 AM

SCs have longer focal lengths, so you get magnification, which you need for small planets.

Aperture of course helps you with resolution, but you do not need a light buckets for shiny planets.



#9 Rac19

Rac19

    Vanguard

  • *****
  • Posts: 2,446
  • Joined: 05 Aug 2016

Posted 01 August 2021 - 06:30 AM

One reason could be that SCTs can provide a wide rang of focal ratios in with a single OTA.

 

f/2 using HyperStar

 

f/4 to f/8 using various reducer lenses

 

f/10 native

 

f/15 to f/25 (and beyond) various Barlow/Powermate lenses



#10 RedLionNJ

RedLionNJ

    Aurora

  • *****
  • Posts: 4,646
  • Joined: 29 Dec 2009
  • Loc: Red Lion, NJ, USA

Posted 01 August 2021 - 11:17 AM

Maybe it's just my personal interpretation, but as soon as you add something like computer-aided tracking to a Dobsonian, it's no longer a Dobsonian and should not be called that.  It's an altazimuth Newtonian.

 

A classical Cassegrain is the best optical design for a planetary telescope. There is a reasonable focal length in a relatively compact design and, when mounted equatorially, can have computer-aided tracking with no field rotation. It also has the benefit of no lens or corrector plate elements, so allows for maximum transmission (reflectivity) of UV wavelengths. Ideal for Venus, for example.

 

I think you need to be careful in differentiating between optical design, manufacturer and mount type, here.

 

There is nothing special about Celestron as a manufacturer. There are several other makers of SCTs and other compound optical designs. Some have argued even Meade could be superior, as Meades lack the extra lens components built into the Celestron "Edge" design.

 

You also appear to be confusing goto and tracking. Goto is only used to move the scope to an object initially. After that, tracking takes over. Guiding is a way to further refine tracking.

 

Altazimuth-mounted scopes suffer from field rotation. That's a great reason not to use one for serious imaging.


  • Jaimo! and peta62 like this

#11 wargrafix

wargrafix

    Fly Me to the Moon

  • *****
  • Posts: 5,399
  • Joined: 10 Apr 2013
  • Loc: Trinidad

Posted 01 August 2021 - 11:33 AM

Long dob and wind are not friends

#12 bluesilver

bluesilver

    Mariner 2

  • -----
  • topic starter
  • Posts: 281
  • Joined: 03 Jul 2020
  • Loc: Tasmania, Australia

Posted 01 August 2021 - 05:47 PM

Thanks for the replies,  some very good information there.

It seams to me and i could be wrong here,  that most use SCTs as they have a higher f number ( like f10 ) or there abouts.

Where as a Dob has something like f4 - f4.5 and therefore requiring something like a 5X barlow to get to f20 resulting in a large magnification .

To get this large magnification you need exceptional viewing conditions,  where a SCT with f 10 would only require a 2X barlow and maybe the seeing conditions don't have to be as good.

 

Therefore most go with a SCT for planetary imaging over a Dob.

 

Would that sound about right?



#13 Tulloch

Tulloch

    Gemini

  • *****
  • Posts: 3,447
  • Joined: 02 Mar 2019
  • Loc: Melbourne, Australia

Posted 01 August 2021 - 08:39 PM

Not really, the degree of "magnification" from a barlow has no impact on the results from good or poor seeing - the seeing is the seeing, no getting around that. The quality of the optics will have an impact, I'm guessing that a cheaper Dob will have poorer quality optics than an expensive SCT with the same aperture which may have an impact of the final result, but that's just a guess. The optimal focal ratio is dependent upon the pixel size of the camera, f/20 on a Dob = f/20 on an SCT.

 

My own point of view is that people never thought you could use Dobs for imaging - however with modern cameras, tracking alt/az mounts and dedicated software (for instance WinJupos can remove the field rotation from alt/az mounts) there's no reason why you can't use Dobs for planetary imaging, and in fact there are a number of fine examples on this forum from people that do.

 

Andrew


Edited by Tulloch, 01 August 2021 - 08:41 PM.


#14 bluesilver

bluesilver

    Mariner 2

  • -----
  • topic starter
  • Posts: 281
  • Joined: 03 Jul 2020
  • Loc: Tasmania, Australia

Posted 01 August 2021 - 11:01 PM

Appreciated,  makes fairly good sense.

Just a tad surprised to see a lot of planetary images are taken with the C14 and some with the C11.

Might have to have a bit more of a test with the 16"Dob and a few more barlows to see how things go

Camera is the ASI 224MC and Dob is on a goto mount.

Will take a look at WinJupos as well.



#15 Tulloch

Tulloch

    Gemini

  • *****
  • Posts: 3,447
  • Joined: 02 Mar 2019
  • Loc: Melbourne, Australia

Posted 01 August 2021 - 11:14 PM

Here's a few images taken with a 16" goto Dob...

 

https://www.cloudyni...ne-c-ii-camera/

https://www.cloudyni...nd-white-spots/

https://www.cloudyni...g-on-july-12th/

https://www.cloudyni...-april-22-2021/



#16 bluesilver

bluesilver

    Mariner 2

  • -----
  • topic starter
  • Posts: 281
  • Joined: 03 Jul 2020
  • Loc: Tasmania, Australia

Posted 01 August 2021 - 11:26 PM

Some very decent images taken from a Dob there.

Some have been taken with a Mono camera and filters,  but in general,  the images are pretty impressive.

I think the key is a decent Barlow like the TeleVue line and a ZWO ADC would also help things out.

 

When all this wet weather finally decides to go away i am keen to try imaging the planets with the Dob.



#17 CPellier

CPellier

    Surveyor 1

  • -----
  • Posts: 1,566
  • Joined: 07 Aug 2010

Posted 02 August 2021 - 05:17 AM

Hi, a lot of good answers here. I'm a planetary imager with a 12" Flextube since 2018 (and very happy with it)

There can be a lot of answers to the question, and comparing the two is relevant when one wants to chose a telescope, but the first clear answer as to why very few people use altaz dobs to image planets is that those instruments are designed in first place to make visual observations, so imagers are just not used to consider them. Also, maybe some people think that Newtonians are for beginners. Last, the field rotation has long been thought to be a tough problem for imaging but this is now perfectly corrected with free softwares (Autostakkert and WinJupos)

But anyway the current altaz (motorized) dobs are really good for planetary imaging and you can expect the same level of performance than with a SCT's. Choosing between the two is now more a question of:

- Camera fitting (starting from the different F/D ratios)

- Price 

- Weight and transportability

- Whenever you want to image in UV or if you don't care

- Diameter...

PS i'm the author of the comparison which is kindly pointed by Andrew/Tulloch in #13.


  • gfstallin and Tulloch like this

#18 matt_astro_tx

matt_astro_tx

    Surveyor 1

  • -----
  • Posts: 1,510
  • Joined: 19 Jan 2021
  • Loc: Dallas, Texas

Posted 02 August 2021 - 10:04 AM

I do planetary with an 8" f/8 newtonian.  It's not the biggest F/L of the bunch, but it gets the job done.



#19 unimatrix0

unimatrix0

    Viking 1

  • -----
  • Posts: 809
  • Joined: 03 Jan 2021

Posted 02 August 2021 - 12:29 PM

Portability plays a lot in also. Some people- to keep it enjoyable and easy - rather move around a short SCT tube on a goto mount  than a 10" or larger Dobsonian/Netwonian. Even if they stay at home and don't have a stationary observatory or anything. 
I have a quick rig I set up and finally able to track with it without a lot of errors  for deep sky imaging.

I also have a larger setup with an EQ6R and with equipment it weighs at least 50lbs, while the small AZGTI with the Z61 is a grab/n- go thing. The mount has its own power supply - a cheap power tank that lasts many days running the mount while, with the EQ6R, I drag out an extension cord. 

I can see myself buying a Schmidt Cassegrain , before I get one of those planetary defense -laser flak- cannon looking Dobsonians.  Although the price of a large Dobsonian is very very inviting. 


Edited by unimatrix0, 02 August 2021 - 12:30 PM.


#20 MalVeauX

MalVeauX

    Cosmos

  • *****
  • Posts: 9,464
  • Joined: 25 Feb 2016
  • Loc: Florida

Posted 02 August 2021 - 01:03 PM

Thanks for the replies,  some very good information there.

It seams to me and i could be wrong here,  that most use SCTs as they have a higher f number ( like f10 ) or there abouts.

Where as a Dob has something like f4 - f4.5 and therefore requiring something like a 5X barlow to get to f20 resulting in a large magnification .

To get this large magnification you need exceptional viewing conditions,  where a SCT with f 10 would only require a 2X barlow and maybe the seeing conditions don't have to be as good.

 

Therefore most go with a SCT for planetary imaging over a Dob.

 

Would that sound about right?

Not really.... focal ratio can be controlled in many ways. And it's deceptive with an SCT/Mak anyways because the primary mirror is not F10. That's just the result of the other optics involved. Newtonians have longer focal-ratio primary mirrors. Also corrector plates can be a problem depending on what you're into. Especially if you're interested in near UV. In theory, I think a long focal-ratio newtonian mirror is going to be the better instrument (especially a premium mirror) for this (but it comes with challenges of mounting).

 

The truly key reason people use SCT or similar is simply the compact nature of it. For the size/weight, you get a lot of aperture. Convenience. That directly means you can get away with a lot less mount. Mounting a C11 can be done on a pretty entry mount, quite well, for imaging planets. But mounting a 280mm F10 newtonian? You'd have to have a monstrous $10k+ mount to begin to think about that (plus the custom mirror to even make such a thing). And there are commercial 14" and 16" newtonains for imaging. They're affordable too. They're just monstrous and require serious mounts to handle. The cheap alternative is to use a tracking alt-az dob mount or a dob mounted newtonian on an Equitorial Wedge and these can produce great results and are affordable. But again, bottom line on SCT is size and compact nature for the aperture. It's far more friendly to budget and physical moving of the things when it comes to the mount (exception, tracking dobs; mostly referring to the idea of a newt on an EQ mount being crazy huge). Camera/filter train placement comes into play a bit, but more personal preference, but behind the SCT is a very simple way to deal with it, compared to a long newtonian on EQ mount. That camera/filter system on EQ mount is in a wonky place when mounted on newtonian and EQ mount. Just look at the differences. Anyhow, this concept holds pretty well even with smaller apertures. Just look at a 6" F8 newtonian compared to C6 SCT. The mounting needs, mobility and size is so different.

 

Here's an example with smaller scopes, like a C8 and a 8" F6 Newtonian. Look at where the eyepiece/focuser is. Which do you think is better mounted and more stable in light breeze? Which do you think will be easier to use the finder or put a planet in the FOV? Which do you think is easier to focus manually (both ideally would want mechanized focusers, but the newtonian pretty much requires it!). Which would you want to hoist up there or take down? Obviously this is a little extreme in terms of an example, but it helps highlight the major ergonomic and physical differences.

 

Lunarsetup_B_03172021.jpg

 

Lunarsetup_A_03172021.jpg

 

Very best,


Edited by MalVeauX, 02 August 2021 - 01:14 PM.


#21 CPellier

CPellier

    Surveyor 1

  • -----
  • Posts: 1,566
  • Joined: 07 Aug 2010

Posted 02 August 2021 - 02:57 PM

Portability and compacity is indeed an argument. But it's not that simple :) a 8" altaz dob is more portable than a 8" SCT and its equatorial mount... especially if the OTA is collapsible ! And the dob is more easy to set up. It is also much, much cheaper... even if motorized ! In general, a complete altaz dob costs less than the OTA of a SCT alone.

But many of those arguments may not be decisive for everyone. There are two, which, in my opinion, must be kept in mind more than the others:

- The tracking of the altaz dob will certainly be poorer than an equatorial telescope. That can be fixed easily by software, but some people would not bear that, which can be understood. I prefer my 12" Flextube to any SCT of the same aperture because I prefer Newtonians for optical reasons, and because it is incredibly cheaper. But having to keep an eye on planet so it does not go out of the field is really tiresome. Only high-end dobs could be better.

- The aperture is also critical. One of my argument in favor of altaz dobsonians is that they are going to give you acess to a larger telescope because they are cheaper. With the budget of a 8" SCT on its mount, you can get a 12" Newtonian! But also, altaz dobs will open the way to apertures larger than 16". A 18", a 20", or a 24" telescope is now within grasp, and the instrument will remain relatively easy to set up and even to move, because the mount is very simple and relatively light weight.


  • mayhem13 likes this

#22 mayhem13

mayhem13

    Viking 1

  • -----
  • Posts: 616
  • Joined: 10 Jan 2021
  • Loc: New Jersey

Posted 02 August 2021 - 03:31 PM

I’m currently using a 12” manual Dob and tracking has been by biggest obstacle…..but when the seeing is good, IMO aperture is a big deal as it allows for faster exposure and lower gain which increase frame rate and improves the signal to noise ratio………a 16” Dob on an equatorial platform should be a planetary killer!

 

My DIY eQ platform is almost dialed in so hopefully within the next 2 weeks or so I’ll have some images to share.



#23 Tulloch

Tulloch

    Gemini

  • *****
  • Posts: 3,447
  • Joined: 02 Mar 2019
  • Loc: Melbourne, Australia

Posted 02 August 2021 - 03:42 PM

I’m currently using a 12” manual Dob and tracking has been by biggest obstacle…..but when the seeing is good, IMO aperture is a big deal as it allows for faster exposure and lower gain which increase frame rate and improves the signal to noise ratio………a 16” Dob on an equatorial platform should be a planetary killer!

Focal ratio determines exposure and gain levels, not the aperture. If you are imaging with a 6" or a 16" at f/20, the gain settings on the camera will be the same - only the image will be bigger for the 16".




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