[Chaos-l] Dob buying advice notacable

[Michael Hrivnak]

As a basic approximation of light-gathering power, we can easily calculate 
that a circle with 10" diameter has 56% more surface area than one with an 8" 
diameter.  So, a 10" scope really will gather quite a bit more light.  It is 
also quite a lot larger, as the tube for a 10" dob would probably be a good 
13" in diameter.

You're unlikely to do any photography through a dob, besides maybe the moon.  
No long exposures for sure.  One particular downside of shooting through a 
Newtonian scope is that unless you use a filter, your camera's sensor is 
exposed to the open air.

Eyepieces will depend on the focal length of the scope you get.  I'd 
prioritize getting a moderately-wide field eyepiece first.  Perhaps something 
that puts you around 35-50X magnification.  A 25mm or 32mm is likely to fit in 
that range.  Another factor to consider is the apparent field of view of these 
eyepieces.  A plossl usually has a 50 degree APOV, which is small.  When you 
compare the magnification of eyepieces, you have to consider the APOV.  For 
example, I have a 10mm Pentax that shows about the same amount of sky as a 9mm 
Nagler, because the Nagler has a larger APOV.

Budget is also a big concern with eyepieces.  One widefield eyepiece could 
easily exceed the cost of your scope, but there are lots of good alternatives 
that won't break the bank.  There are lots of eyepieces rattling around in 
most of our collections, so the best bet is to try some out.  Most dobs come 
with one or two eyepieces, so I'd just start with that.  Then get out 
observing with the club and borrow some eyepieces from those around you.  See 
what you like.

If you have a chance to buy a scope that does not include an eyepiece, I can 
certainly come up with a couple of loaners to get you started until you figure 
out what you want to buy.

Other tips- 

- You'll probably want a powered finder (like a GSO 8x50 Right Angle Correct 
Image) and something like a Telrad.

- Don't forget to buy at least one collimation tool.

- "Turn Left at Orion" is a great book for getting started with finding things 
in the sky.

- Try searching astromart.com and cloudynights.com for what you want.  I've 
bought nearly all of my astronomy equipment from people on those two sites.  
You can get really great deals, especially if you can be patient.

- Please make your purchase near a 50% waxing moon, so the punitive weather 
won't ruin any moonless nights.

Lastly, it's never too soon to starting thinking about the next scope after 
that.  For example, when you decide to get a bigger one, a 14" dob would 
collect about 3X the light of an 8", but only 2X as much as a 10".  If you get 
a 10" now, you might be stuck upgrading to a 16" later in order to get a 
justifiable improvement!

Michael

On Saturday 13 February 2010 05:19:43 pm Tony Rice wrote:
> I've outgrown my 4" go-to Meade telescope and am looking at buying a
> dobsonian, probably a push-to.   Something for backyard use and
> transporting out to darker sites. I dont mind the size as I've got a
> minivan and the dobsonians seem like the best bang for the buck. I'm
> mostly interested in observing clusters, nebula and some planetary
> viewing.  I do a little photography but nothing long exposure yet.
>
> The price difference between 8" and 10" isn't that great, how
> noticeable is the jump in light gathering and overall clarity.  Anyone
> have any experience with Orion's IntelliScope line?
>
> What eyepieces would you recommend, I'm thinking something around 25mm
> and 10mm and maybe one other (16mm or 32mm?)
>
> Any thoughts?
> _______________________________________________
> Chaos-l mailing list
> Chaos-l at rtpnet.org
> http://rtpnet.org/mailman/listinfo/chaos-l



-------------- next part --------------
HTML attachment scrubbed and removed
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 197 bytes
Desc: This is a digitally signed message part.
URL: <http://rtpnet.org/pipermail/chaos-l/attachments/20100213/d69bc8ee/attachment.pgp>