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12" versus 16" for galaxy structure

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#1 azure1961p

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Posted 05 February 2013 - 06:17 PM

I'm in the middle of choosing between a 12" or 16" reflector. I love dust lanes in galaxies and such but the 16" is twice the cost of the 12". I've looked through both but for galaxies would u guys strongly recommend the 16 or is it a shrug? 20" is huge but simply out of my budget.

Thanks in advance. My sky's are generally 6.2-6.5v where I'd use it.

Pete

#2 Achernar

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Posted 05 February 2013 - 07:15 PM

A 16-inch will always show more than a 12-inch from the same site, but a 12-inch will also show a lot on galaxies from a good dark site. Are you looking for a mass-produced or a premium Dob? If you can afford a 16-inch without breaking the bank, I would take that route if you are looking for a telescope that will be more than enough aperture for observing galaxies. A 16-inch takes in about 80 percent more light than a 12-inch, a signifcant difference that will show through the eyepiece.

Taras

#3 Dave Mitsky

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Posted 05 February 2013 - 07:47 PM

I recommend getting the 16-inch for galaxy and other DSO observing.

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#4 IVM

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Posted 05 February 2013 - 09:37 PM

Galaxy structure is probably my primary observing interest. Although I never did a side-by-side comparison, it seems that my 12" driven SCT is roughly equal to my undriven 16" "Dob" in how much they reveal in practice, and my undriven 12" "Dob" is not very far behind. When the magnification used with a 12" is 3/4 of the magnification used with a 16", the surface brightness in the two scopes is the same. This theoretical relationship fits with my impressions. I use the 16" most of the time only partly because of its aperture (due to its design it happens to be the most portable of my 3 large scopes).

#5 JayinUT

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Posted 05 February 2013 - 11:32 PM

Aperture wins, always. Having said that I have a 14 inch I use and am content on that. The 16 will show you more. Weight is a concern.

#6 HellsKitchen

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Posted 06 February 2013 - 05:49 AM

My 12" can show galaxies as faint as visual magnitude 14 from my outer suburban location. A good 12" is a very nice thing to have, but for faint fuzzies, aperture is king.

If weight, size, portability and transport are not an issue, get a 16". The mass produced asian ones (e.g GSO, Lightbridge) have large and heavy bases which may be an problem for handling it, but are quite affordable and you really do get a lot for your money (especially compared to even a decade ago). Alternatively there are several flavours of premium models that may be lighter and incorporate a variety of features that facilitate portability and are often customizable to your needs. But you'll be paying at least triple the price of a mass produced model.

#7 Feidb

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Posted 06 February 2013 - 10:04 AM

I'd recommend the 16-inch also. I've been using one exclusively for 25+ years. First one I made, a 16-inch f/6.4 but it just got too cumbersome to lug around. I now use a 16-inch f/4.5 commercially made and you just can't beat the aperture for not only catching the faint fuzzies, but for structure. I've compared my commerical one side by side with 12 inchers from standard to premium optics and there is still a noticeable difference.

#8 coutleef

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Posted 06 February 2013 - 10:11 AM

i would say get the biggest aperture you can transport to a dark site. looking through briefly a 16 inch i had the impression that the jump from 12 to 16 was similar to the jump from 8 to 12. so quite worthwhile. galaxies or details you could see with averted vision on the 12 were seen with direct vision on the 16.

it will be significant especially on galaxies. but the 16 was just too big a scope for me to handle.

#9 Madratter

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Posted 06 February 2013 - 10:12 AM

As others have said, bigger is better. But even with a 20" f/5 (my scope) you can only see so much in terms of structure.

The bigger reason I like the larger scope is I really enjoy groups of galaxies. And the more light you can pull in, the better.

If you are interested in galaxies, get the biggest scope you will use on a regular basis. Big scopes are MUCH heavier.

#10 GlennLeDrew

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Posted 06 February 2013 - 11:43 AM

It's not so much that the bigger aperture collects more light, but that extra light collected supports the 33% larger image scale at given exit pupil. For the observation of object structure, image scale is the primary consideration, and the larger aperture provides higher surface brightness at any particular magnification.

Unlike bright object observation, the dim fuzzies have your eye working at rather low resolving power. You need the extra light to afford every advantage in eaking out those subtle details you strive to see.

#11 killdabuddha

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Posted 06 February 2013 - 11:49 AM


Wow Glenn, you do have a way. Thanks for stating it so succinctly.

#12 Tony Flanders

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Posted 06 February 2013 - 12:17 PM

I'm in the middle of choosing between a 12" or 16" reflector. I love dust lanes in galaxies and such but the 16" is twice the cost of the 12".


I must admit that I've never compared a 12-inch and a 16-inch side by side. But it seems to me that galaxies really start to open up in the 16- to 18-inch aperture class.

There are certainly plenty of galaxies that show a great deal of structure through a 12-inch scope. But the number of such galaxies really blossoms when you hit the next aperture bracket up.

It's sort of like globular clusters. An 8-inch resolves many of them, but also fails to resolve many. With a 12-inch, it's always a bit of a surprise when you find a globular cluster that doesn't show some degree of resolution.

#13 Ed D

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Posted 06 February 2013 - 09:32 PM

Pete, for what you want to do the more aperture the better. So, it boils down to getting what you can afford, what you can lift and handle, and what fits in your vehicle. Also, keep in mind that you will probably want a good quality laser collimator, adding to the expenses if you don't already have one.

Also, Glenn hit the nail on the head about thinking in terms of exit pupil and image scale. An observing friend and I both find a 3mm exit pupil to hit the sweet spot for observing galaxies, if that helps.

Ed D

#14 azure1961p

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Posted 06 February 2013 - 10:21 PM

Thanks guys - a lot of wisdom there and its appreciated. I love long focus but I can't entertain even medium f/6 in 16" . I'm going to build the thing out of carbon fiber cloth and epoxy with some wood laminate.

Ill go with Agenas 16" .

Again, gratis.

Pete
Ps: short term goal is having spectrum put 98% reflectivity on my 8" for spring galaxies and such.

#15 jgraham

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Posted 07 February 2013 - 01:11 PM

I have two biggo scopes; a homebuilt 16.5" f/6.5 and a Lightbridge 16 which is f/4.5. While I dearly love my homebuilt monster, the shorter, lighter Lightbridge is a joy to use. I made the jump from a homebuilt 10" f/6.7 to my homebuilt 16.5" and when you get up into the 16" range the sky just starts to open up. Having said that, there's no substitute for dark, transparent skies. To this day the best galaxy observing that I've ever done was with a homebuilt 6" f/4.5 from a very, very dark site (35 years ago from Cades Cove in the Great Smoky Mountains National Park). Alas, these days I do all of my observing from my red zone back yard. A bonus for me with the larger scopes is that they give you enough light that filters start to become effective.

#16 k9yr

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Posted 07 February 2013 - 11:03 PM

I have a 13" and built a 16" 2 years ago. The best thing I can say is the 16" has wow factor on basically everything. I took the 16" to dark skies in MN last year and M51 spooked me out. Go with a 16" and a gym membership

#17 Jeff Morgan

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Posted 07 February 2013 - 11:49 PM

It's not so much that the bigger aperture collects more light, but that extra light collected supports the 33% larger image scale at given exit pupil. For the observation of object structure, image scale is the primary consideration, and the larger aperture provides higher surface brightness at any particular magnification.

Unlike bright object observation, the dim fuzzies have your eye working at rather low resolving power. You need the extra light to afford every advantage in eaking out those subtle details you strive to see.


Yep, my reasoning in moving from a 12.5" to a 16".

With the 12.5 I felt that most galaxies didn't have much to offer, I was just checking off items from a list. Of course given the vast number of faint galaxies, that might be nearly as true with the 16!

Another big improvement for me in moving up in aperture is the addition of tracking. Based on past experience I suspect this will be as big a jump in performance as the added aperture.

#18 azure1961p

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Posted 08 February 2013 - 09:44 AM

Thanks for the input.

I'm going with the 16. I've got a lot of fiberglass/composite experience so Ill make the whole thing out of carbon fiber with aluminum or steel where needed. I m looking at Kriegs ultralight dobs as the way to go. I want the lines as clean and low profile as possible for ease of transport and storage. It's going to have a minimilast thing about it. I was going to buy the optics first THEN build it but ill probably do it the other way around

Has anyone got dob parts suppliers for truss tubes, mirror cell, etc. I'm going with Novak for the diagonal.

Jgraham, ill bet that longer focus 16 is a beaut but alas the size of it all. I'm 51 and back in the day when I was 30 or younger I had distant dreams of a 16" reflector at f/7 and the glorious climb up the ladder to contrasts views bare none. That was then lol. Now even the short step ladder for my 8" f/9 is about all I want to bother with. I've gotta believe though the big 16 turns out some spectacular sights given great seeing.

Normally I'm skittish about an f/4 or f/5 system as I never liked my ten, but here's no other way for me to comfortably use 16" in anything longer and it'd be a custom pricey order so the overwhelming light gathering advantage is swamping my resistance on several points.

It'd be great to see eyes in the owl, dust lane on 891, even some obscure galaxies coughing up do e goodies now and again.


I question how good a figure Agena can turn out on a 16" glass. That's a chin rubbing factor I'm not settled on.

Probably start building at the end of feb.

Pete

#19 Doug Culbertson

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Posted 08 February 2013 - 11:54 AM

It's not so much that the bigger aperture collects more light, but that extra light collected supports the 33% larger image scale at given exit pupil. For the observation of object structure, image scale is the primary consideration, and the larger aperture provides higher surface brightness at any particular magnification.

Unlike bright object observation, the dim fuzzies have your eye working at rather low resolving power. You need the extra light to afford every advantage in eaking out those subtle details you strive to see.


I just thought that this needed saying again. Thanks for putting it in such a succinct manner.

#20 deepskydarrell

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Posted 08 February 2013 - 12:23 PM


Jgraham, ill bet that longer focus 16 is a beaut but alas the size of it all. I'm 51 and back in the day when I was 30 or younger I had distant dreams of a 16" reflector at f/7 and the glorious climb up the ladder to contrasts views bare none. That was then lol. Now even the short step ladder for my 8" f/9 is about all I want to bother with. I've gotta believe though the big 16 turns out some spectacular sights given great seeing.

Pete


Consider one thing before you write off a longer 16" -- An orchard ladder. I use a six foot, three legged, aluminum with a flared base. It grabs onto any uneven terrain and is the most solid, stable thing I've ever climbed. It cost under $100, is excedingly light but very solid and if it didn't fit in my vehicle it rode the bike rack on the back.

I'm 58 and can understand the age concern, but I hope I'm climbing those two or three wide steps for many years to come.

And the 2.6 inch secondary gives excellent contrast.

All the best on your building endeavour.

DSD.

#21 GlennLeDrew

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Posted 08 February 2013 - 01:52 PM

If the emphasis is on the observation of dimmer, low contrast, extended objects, a short f/ratio system is no impediment. The larger secondary will impart absolutely no detectable reduction in image contrast on galaxies and nebulae. Sure, the planets and crowded globular cluster cores will suffer some degradation, but in the regime of the faint, our eyes' resolving power is *very* much the weak link.

With this knowledge in mind, a squat system which keeps one's feet always on the ground is entirely justified. That's what I'd do. Indeed, an f/3 system is not at all off-putting, if it keeps me Earthbound.

#22 Dave Mitsky

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Posted 08 February 2013 - 02:09 PM

I've used a couple of the new f/3.3 and f/3.6 Dobs and the views were excellent. That includes the planetary views. Being able to stand with two feet on the ground while looking through a 22" Newtonian is wonderful.

Dave Mitsky

#23 Jeff Morgan

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Posted 08 February 2013 - 09:16 PM

Has anyone got dob parts suppliers for truss tubes, mirror cell, etc. I'm going with Novak for the diagonal.

Jgraham, ill bet that longer focus 16 is a beaut but alas the size of it all. I'm 51 and back in the day when I was 30 or younger I had distant dreams of a 16" reflector at f/7 and the glorious climb up the ladder to contrasts views bare none. That was then lol. Now even the short step ladder for my 8" f/9 is about all I want to bother with. I've gotta believe though the big 16 turns out some spectacular sights given great seeing.


Look at Aurora Precision for a mirror cell. I really like the design and construction on mine. Which BTW is a 16" f/7, Zambuto mirror. Here is a photo after daylight testing of the ServoCAT. Looking forward to First Light, weather is conspiring against me currently.

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#24 Jeff Morgan

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Posted 08 February 2013 - 09:21 PM

Consider one thing before you write off a longer 16" -- An orchard ladder. I use a six foot, three legged, aluminum with a flared base. It grabs onto any uneven terrain and is the most solid, stable thing I've ever climbed. It cost under $100, is excedingly light but very solid and if it didn't fit in my vehicle it rode the bike rack on the back.


For mine I'm going to use a rolling platform ladder. It has siderails, a large top platform, and it folds. It is surprisingly easy to handle, balances very nicely on the large wheels. Even on a reasonably uneven surface (that being, one level enough to set up a scope) there is zero - zero chance of a topple.

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#25 Jeff Morgan

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Posted 08 February 2013 - 09:23 PM

This is what it looks like next to the scope. Suitable for scopes up to about 120" of focal length. And they make bigger - much bigger - models.

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