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Expectations Using a Dob

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#1 Oscar56

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Posted 22 October 2015 - 11:58 AM

I have only been into this astronomy thing for the past 3 years and have been the owner of a dob for 2 years.  I am trying to get a little more formalized in my viewing by building observation lists based on RASC certificates and other lists.

 

I recently loaded the list of objects from "Turn Left at Orion" into SkyTools.  I use Skytools, and sketching, for documenting my observations.  Turn Left and some of the RASC certificates include double stars, something that I have viewed very infrequently.

 

In reviewing the posts on this forum it seems that most people use refractors for pursuing doubles and multiples.  Before I jump into another scope what should my expectations be regarding doubles when using a dob?  

 

In the same seeing conditions what aperture of refractor would equate to a 10" dob?  By knowing this I can more fully appreciate the posts on this forum from those using refractors.

 

Thanks in advance.



#2 Cotts

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Posted 22 October 2015 - 12:30 PM

Doubles are well viewed in any telescope.  Your 10-inch, well collimated and properly cooled, has the ability to observe tens of thousands of pairs. Your 130 mm will also have thousands of targets possible.  If you are using the RASC Handbook, check their list of doubles for starters.  There is an excellent variety of pairs there.

 

Once you are comfortable with the abilities of your eye-telescope combination then you can go searching for more challenging and/or obscure pairs.  There is no lack of lists, books and websites to carry you forward - see the 'stickies' at the top of this thread for starters.

 

I like to imagine the pairs  as dynamic things, with orbital motion and proper motion through space at the same time.  

 

As for 'what size scope type X is compared to scope type Y' I would recommend not to sweat this.  There are dozens of threads in the CN telescope forums, refractor, reflector and Cats 'n Casses amounting to thousands of posts all coming to two conclusions:  'It depends'  and   'It doesn't matter - use the scope you have'.

 

Let us know how you get on.

 

DAve


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#3 Oscar56

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Posted 22 October 2015 - 02:55 PM

Thanks for the feedback Dave.  I think I will build an observation list for the RASC Coloured Double Stars list. Looks like a good place to start. 

 

Is it my imagination or are most posters in this forum using refractors?



#4 drollere

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Posted 23 October 2015 - 12:29 PM

the principal problem with a dob is tracking. double stars require you to *look through* the seeing in a way that deep sky objects (with their very low scale of visual detail) do not often require. the usual dob "push and pull" tracking procedure, and the eyepiece drift that is continual, are not very friendly to double star astronomy — especially if you require high magnification to crack a very close double.

 

what you infer as a preference for refractors is actually a reliance on GEM or a tracking AltAz mount in order to put the system in the center of the field and then keep it there to watch it while the seeing does its usual veil dance of detail. yes, smaller aperture refractors are less sensitive to "aperture seeing" than much larger refractors, but you lose in both resolution and limit magnitude, and seeing is just fisherman weather that you learn to deal with (and to a limited extent overcome) with observing experience.

 

there is a *long* chapter by chris taylor in the argyle book "observing and measuring visual double stars" that documents the perfect equivalence of refractors and reflectors in double star observing. the type of star diffraction artifact produced by the two different optical designs is certainly different and, in my view and that of many others, the image in a refractor is just shucks gosh darn pretty. but we are at the point of sipping port wine when we have that discussion.

 

currently, i'm using my ƒ/20 royce dall kirkham reflector for double star astronomy. i have a TEC 140 refractor, but it's taking a rest.

 

the two essentials, really, are (1) GEM or tracking AltAz, and (2) GoTo pointing. the tube you use comes in a distant third.


Edited by drollere, 23 October 2015 - 12:33 PM.


#5 Oscar56

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Posted 23 October 2015 - 01:32 PM

Thanks drollere:  

 

The preference for tracking makes sense.



#6 havasman

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Posted 23 October 2015 - 05:14 PM

The XT10i works just great on doubles. It can split some really fine pairs. The 60mm refractor crowd talks of close splits at 2" but the Dob splits 0.8" doubles easily on good nights even from my White Zone driveway.

I think the biggest advantage the fine refractors may have is in color rendition. And the really beautifully etched definition of the objects. Same as with lunar.

But your XT10i still works wonderfully; I think better than a tiny antique refractor but that's just me. You should definitely give it a go.



#7 Oscar56

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Posted 24 October 2015 - 09:49 AM

Thanks Havasman.

 

I will carry on with the XT10 and halt my refractor window shopping.  We have had some clear nights this week but poor transparency.  I was not sure about my collimation at 4:00am until Venus and Jupiter rose a little later and looked like big fuzz balls. 

 

Although some folks have posted that doubles are a good target during the full moon I assume that you still need reasonably transparent skies?



#8 drollere

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Posted 25 October 2015 - 11:26 AM

grant, double stars are pretty robust to a wide range of observing conditions. if the seeing is bad, you just find wider pairs; if the transparency (or moon pollution) is bad, you just go for brighter pairs.

 

this again sets apart double star astronomy, because if the seeing is bad, planetary and lunar astronomers are pretty much shut out, and if the light pollution is bad, DSO astronomers can go to bed.

 

i wouldn't say that color rendition is better in a refractor: i think that's an artifact of the lower magnification at which modern low focal ratio and small aperture refractors are used. color is great in a binocular, but you can't separate many doubles at the typical binocular magnification.

 

the subtext here is "window shopping", and in that regard there are always two important criteria: know clearly what you want to do (or do next) with your astronomy, and look through other types of scopes. go to star parties in your area and try to get to larger astronomy events (stellaphane in the east, RTO in the west) to try out a large number of scopes. your personal preferences will quickly crystallize to specific types of instruments, and then the road forward is much easier.


Edited by drollere, 25 October 2015 - 11:27 AM.


#9 Oscar56

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Posted 25 October 2015 - 01:49 PM

Bruce:

 

Thanks for the pointer regarding transparency issues.  Pursuing doubles during the full moon or nights of poor transparency opens up a whole new avenue of viewing that I had never considered.  I wonder how many of my shortened nights that I packed up early due to poor seeing and transparency might have been extended if I had shifted to doubles?

 

I am a member of the RASC Okanagan Centre here in BC  http://www.ocrasc.ca/index.html  which gives me the opportunity to "test drive" a range of scopes if I get beyond the window shopping phase.

 

But first I will work with my dob. 

 

 

 

 




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