Does size matter?

[leo82]

Hi all, 

In visual astronomy I understand aperture is king and in Astrophotography fast scopes are best, but what is best for video astronomy?

Many thanks.

[Peter Drew]

Aperture and fast is the best recipe for video astronomy. SCT's with 0.33 focal reducers or Fastar work very nicely.

[alan potts]

2 hours ago, Peter Drew said:

Aperture and fast is the best recipe for video astronomy. SCT's with 0.33 focal reducers or Fastar work very nicely.

Fully intend to couple my 12 inch with a F3.3 reducer, I have the F6.7. Bought a Mummut colour camera and have never seen a F3.3 since, the camera still has not been used and thats almost 5 years.

Alan

[Davey-T]

Tried the 3.3 f/r with my Meade and a webcam years ago, not a great success.

Dave

[leo82]

I guess a 120mm refractor or 130mm reflector is no good then? Would a skymax 127 also be no good?

thanks.

[HiloDon]

In EAA/Video Astronomy I find that speed and field of view are most important.  Larger aperture will allow for narrower FOV given the same focal ratio and sensor.  If you want a wide field shorter optics will work.  I use everything from a C11 at f/10 (2800mm focal length) down to a 50mm Orion Mini Guider scope at 162mm focal length.  With the longer slower optics, you will be limited to brighter DSO’s like PN’s and clusters.  Just to show that size doesn’t always matter, here’s some examples of using a 50mm scope at f3.2 to view larger emission nebulae with a narrow band Ha filter.  

Here are some closer views of Eta Carina taken with a C11 at f/5.  Same camera, different optics.  All images from an SX Ultrastar.

[Jim Smith]

I agree. I overlooked the importance of field of view. I used to have a C6/Hyperstar f/1.9 set-up. It was very fast and great for some things, but most objects I was interested in appeared unacceptably small. I have kept the C6 but I now use it at around f/4.0 to f/6.3. It's slower but I get the view that I like. If I really want a binocular-style field of view then my f/2 camera lens finderscope can give me that.

Also, don't forget that smaller scopes have benefits in terms of mount requirements, storage and quick set-up times. I think a 120mm f/6 refractor would give good results on brighter objects as long as you didn't mind waiting a bit longer to get them. However, a C6 captures more light and is overall more compact.

But, of course, it's always a compromise and we all have our own preferences.

This M27 view, using the C6 @ f/6.3 took 2 minutes to achieve but I think it was worth the wait...

[leo82]

Some nice pics there from the smaller scopes, sure gives me something to think about!

[arkosg]

I think Don & Jim have given some great examples and things to consider when picking a scope for EAA.

My experience has been that it really depends on the kinds of objects you want to view; sometimes I find I really want a larger FOV for larger objects (nebulae, etc.), other times I'd like detail (planetary nebulae, some galaxies, etc.).  I've enjoyed trying different scopes and reducer combinations, along with different cameras as well.  There isn't really a one-size-fits-all.  The SCT's certainly are flexible and offer varying focal ratios and fields of view, but I have to admit I've enjoyed the views using smaller refractors as well.  So it really just depends! :-)

Attached are a couple of examples - "tighter" shots are with SCT at various focal ratios (typically around f/4) and wider fields are with 5" refractor.  Of course, camera FOV matters too, but you get the idea!

Cheers,

Greg A

 

[RobertI]

I agree with the above comments, it's a combination of choosing the right f ratio for the brightness of the object and the right focal length to frame the object. You can't always get both! I have a variety of scopes and reducers for different objects, for example:

• The C8 at native F10 is good for small bright planetary nebulae like the Catseye Nebula
• The C8 at F3.3 is good for smaller, and usually faint galaxies and galaxy clusters
• The 66mm refractor at F3 is good for large faint nebulae like the North American nebula and Ha

The smaller refractors are so easy to use for EAA, as the wide field means stuff is easy to find and tracking does not have to be spot on. 

My most used setup is the RC6 (because I prefer the focusser over the C8) at F4.5 as this nicely frames hundreds of interesting galaxies and galaxy clusters which particularly interest me, and with exposures of 30 seconds or less. HTH.

[Robrj]

On 11/8/2017 at 03:57, leo82 said:

I guess a 120mm refractor or 130mm reflector is no good then? Would a skymax 127 also be no good?

thanks.

The 130 would be fine.  I have used a Celestron 130SLT and get good results.   Some of these are on the AVX mount and some are on the SLT alt/az mount (it's in the details on the image's page).

http://www.astrobin.com/users/Robrj/collections/171/

I have other sizes as well.  Mostly the difference is going to be the field of view and the detail you can see.  A bigger scope will allow more detail on a small object, but will require better tracking.  A smaller scope will be better for extended objects.

80mm f/5.9 APO Refractor (Orion ED80T-CF):

http://www.astrobin.com/users/Robrj/collections/424/

 

8" Skyview Pro f/5 newtonian:

http://www.astrobin.com/users/Robrj/collections/172/

 

12" Skywatcher Dob:

http://www.astrobin.com/users/Robrj/collections/173/

 

Here's a comparison of the same object with different scopes, same mount, same location:

M20 (80mm)

M20 (130mm on AVX mount):

 

[leo82]

Wow some great pics there! Thanks for sharing.