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Dob vs Apo

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#201 Mitrovarr

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Posted 24 October 2020 - 12:52 PM

I haven't yet but maybe I should.  I have heard of people doing that but never really understood the theory since an insulated scope will take longer to cool down.  So you are saying that the insulation will reduce the tube currents even when the scope has not cooled down, giving a better view in an unacclimated scope.  Interesting.  I suppose the reflectrix is cheap enough that it would be fairly easy to try.

I think the theory with reflectix is that the scope cools so slowly you don't really get meaningful tube currents at all, so it's usable immediately and stays so. Having the scope be warmer than the outside air also apparently helps with dew/frost formation.


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#202 25585

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Posted 24 October 2020 - 03:53 PM

MN180 on an AZ EQ6? 



#203 Bomber Bob

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Posted 24 October 2020 - 04:11 PM

Are you adjusting for the magnitude of the stars?  And this thread is explicitly about a 4 inch refractor and an 8 inch Dob as per the moderator.  In my experience an 8 inch F/6 Dob shows tight stars.  

 

Jon

Both my 6" F8 & 8" F6 Newtonians have outstanding mirrors, but neither can present stars as the micro-dots I see in my 4" refractors, when all 3 scopes are at the same magnification -- say 30x (typical low power for me).  That's one advantage refractors have over reflectors.  Yes, I can split tight doubles with my Newts, but those individual stars won't be as tiny as in a refractor at the same magnification.

 

To be 100% fair (same apertures), stars in my 6" F8 ED were always smaller than in my 6" F8 Newt with both at the same magnifications.

 

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

 

Not picking on you OP, but why is it that most of these topics have the refractor at 1/2 the aperture of the reflector?

 

My APM 152ED was 3/4 the aperture of my Meade 826, and it routinely out-performed the Newtonian.  Neither scope is top-tier -- well-made mid-tier, IMO.  30+ years ago, my hand-made D&G 5" F10 refractor showed more planetary detail than the 8" Newts at star parties.  So, OP, you'd need at least a 5" refractor to tie / beat an 8" Newtonian on most objects -- though it would have to be an Outstanding 5" frac.

 

I sold my APM 152ED.  I'm keeping my 2 Newtonians -- and my 4" refractors.  Most nights, and most views, my 6" F8 Newt is like a 4" APO; and, my 8" F6 Newt is like a 6" APO.  Last night, in 8 / 10 seeing, my RV-6 was sharp on Mars & Saturn at 300x -- though I did back off to 240x for comfort.  So much detail on Mars, and at least 6 colors on Saturn.  Titan + 3 smaller moons.  I paid just $150 for this complete 1971 Classic kit (scope + EQ + 6 eyepieces & Barlow - all original).  I paid $600 for just the AT102ED OTA.

 

I know, reflectors almost always win the aperture per dollar argument.  But they can also win the performance comparisons -- just took me 4 decades to figure that out...


Edited by Bomber Bob, 24 October 2020 - 04:33 PM.


#204 pao

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Posted 24 October 2020 - 05:59 PM

 

Not picking on you OP, but why is it that most of these topics have the refractor at 1/2 the aperture of the reflector?

 

Just because (as already said) with the same amount of money you can buy a hand made 8" dob with premium optics or a 4" top apo refractor. So the question make sense. At least for me.

pao



#205 CHASLX200

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Posted 24 October 2020 - 06:14 PM

 A 4" APO can do many things better than a bigger Newt.  Like i keep saying, every kind of scope and size has it's good and bad points. No one scope is perfect for all kinds of viewing. Since i view planets 90% of the time a bigger Newt is my choice. But also sweeping with a much wider FOV with a faster 4" APO has it's good points.



#206 Echolight

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Posted 24 October 2020 - 06:19 PM

I have an 8” newt and an AT102ED. When I use them side by side, I use the AT102ED to pan around the Milky Way. When I see something I’m interested in, I point the 8” newt at it to see what the AT102ED can’t show me.

 

I don’t know all the physics and optics reasons for this, just what I see at the eyepiece. 
 

I use the AT102ED and my 12” dob more often than the 8” newt. They go really well together.

 

I see the planets better in the 8” than the AT102ED as well as most other things. The AT102ED shows wider fields and is easier to pan around with than the dob. I use the 8” when I want tracking on the AVX mount.

 

Jack

This is kind of the philosophy that I use and will use when I buy my eventual big scope.

 

My 6 inch achro, to my eye, puts up better images than my 8 inch SCT. But the C8 is a third or quarter the length and less than two-thirds the weight, is sturdier on the same mount with less eyepiece articulation, and probably goes a skosh deeper.

I kind of like using the yard cannon above all else despite it's many flaws though. Especially on a tall alt/az mount.

 

I can find stuff easier in the ED80 than either of the bigger scopes. And it's just so easy to use that I often don't mind that it's not as bright or can't go as deep. I don't use it much on planets. But it is the most relaxing astronomical tool that I have, and I appreciate it very much. Great widefield views.

 

When I eventually get a 10 or 12 inch dob, it'll be for better views of globs and galaxies. But I'll surely try it out on the moon and planets now and then.




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