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Joel Shepherd

Big Dobs, Bright Lights

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We live under Bortle 6 skies (18.6 on the SQM). With a 4-6” reflector or Newt, we can see open and globular clusters, the Ring and Dumbbell, Andromeda, etc. I’m wondering how much difference we’d see with a 14-16” Dobsonian. Would there be noticeably more detail in the clusters. Could we see some of the smaller, brighter galaxies (M51, M63 etc.) Does the aperture make that much of a difference under a fair amount of LP?

Interested to hear others’ experiences. 

Thanks — Joel. 

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Experiences will vary but here's mine... I do most of my observing from light polluted skies with apertures from 60mm to 350mm.  The brightest stars I can see naked eye vary from around mag 4 to 4.7 or so. The apertures I use most often are 120/127mm and 350mm.

Open clusters look very much better with more aperture, despite the light pollution. I can see more stars and more colour.

Globular clusters are also much better. They may be grey fuzzies with smaller aperture but can resolve into innumerable stars with 350mm despite the light pollution.

Where more aperture starts to struggle to have an advantage if there is light pollution is on lower contrast objects like nebulae and galaxies.

From home I've never seen structure within a galaxy even with 350mm. I can see size and shape, and condensation but that's it. I find more aperture just makes for a brighter fuzzy.

The very bright nebulae with high natural contrast  (e.g. M42 and M57) benefit from more aperture but most nebulae are not like that  and don't benefit so much. However more aperture does allow higher magnifications with deep sky filters before the image becomes too dim.

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I agree with Paz. 

LP is a major filter unfortunately. My TV60 on the dolomites (21.2) showed much more detail on anything but planetary targets than my Skywatcher 8" F6 dobsonian used in a location affected by severe light pollution (18.4).

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Taken by iPhone in my light polluted suburban backyard 40 mins ago 😀

 

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44 minutes ago, quaoar said:

Taken by iPhone in my light polluted suburban backyard 40 mins ago 😀

 

Magnificent!

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2 hours ago, quaoar said:

Taken by iPhone in my light polluted suburban backyard 40 mins ago 😀

 

Wow 👍

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Thanks guys. Even in light pollution, with the really big dobs you can still get quite good views of planets and the brighter dsos, especially planets. Admittedly a backyard with a few well placed trees and bushes helps.

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Thanks @Paz and @Piero: I appreciate the first-hand experiences. It'd be cool to have a big scope ... but I'm not it can really be justified given our sky conditions. Good food for thought.

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Some experiments with EVAA have shown me that with a 4" refractor and a camera I can 'see' faint galaxies in the suburban, reportedly Bortle 6 skies here just as well as with a 8" SCT.  It even challenges what I saw with the SCT at a dark skies Devon site.

From here I generally  can't see M110, a companion of M31, but the EVAA image shows it clearly.

Something to think about before you buy a big, bulky scope.

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I’m interested in learning about EVAA for outreach, but as an astrophotographer ... it’s not as interesting to me personally. It’d be more of a thrill to see a couple gray blobs and know it’s M51’s photons impinging on my own eyeballs. Whether that thrill is worth the cost of a big Dob ... hmm. 

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My back garden is a Bortle 5, I started observing with an 8 inch scope and used it for several years but then bought a 14 inch about 18 months ago and never regretted it.

Almost everything looks much better unless the seeing is really terrible. Clusters are really punchy (especially in bino viewers), the detail on brighter nebulae like the Ring and Orion is stunning. With fainter nebulas, like the Veil it's got enough oomph to push through a tight Oiii filter (8nm)- last weekend I was able to see almost every part of the Veil complex. Back in April I was able to galaxy hop all of Markarians chain. I've made a 160mm aperture mask and with this it turns into a tight double star splitter and planet killer.

The downsides are that it's a truss design and takes 10-15 mins to build and collimate, plus once it's set up it's so heavy that unless someone else is about it ain't moving! Oh yeah- and for the health of my bank account I try to avoid all observing reports for scopes 18 inches and up...😬

I guess this is a fairly biased account and your mileage, as they say, may vary. Especially if you're an astro photographer, a camera will always see more.

But for that intimate and instant 'at the eyepiece' experience a big dob is a wonder- don't go for the myth that all they pick up is more light pollution...

 

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