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12" versus 16" for galaxy structure

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#26 azure1961p

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Posted 08 February 2013 - 09:37 PM

Wow you fellas have some phenomenal sixteens - and a Zambuto!!! Yeah your going to have heavy cloud cover for six weeks.

It's interesting how there's both extremes here - the testimonials of fine f/3 and the proponents of the long focus designs. It is amazing too that with super huge aperture that despite the fast focal ratio the secondary can remain startelingly small .

I appreciate the points made in that with the eyes resolution of dim images the contrast boost of a small secondary is not realized . I think in stars it would be but not on objects like the Veil.

To keep costs down Im not going with a custom focal ratio but I understand the points made - and the agricultural ladder was well taken.

Thanks guys.

Pete

#27 azure1961p

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Posted 08 February 2013 - 09:39 PM

Jeff,

That's amazing. You've got the whole thing worked out and in style. That is going to be a phenomenal performer. Please post your experiences. Even 5" planetaries are going to show well in that.

Pete

#28 IVM

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Posted 09 February 2013 - 03:57 PM

A real Serrurier, wow.

#29 Jeff Morgan

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Posted 10 February 2013 - 10:19 AM

So a question to current 16 owners from a new 16 owner:

When hunting down "the faint ones" how close do you go to the theoretical limit for the aperture?

I realize this will be dependent on weather, dark site quality, and a few other things, but on a "typical" night from your best site was what I was thinking.

The reason for the question is that I am using Astroplanner to generate observing plans (by constellation) for SkySafari and there are a couple of issues here:

1) Sheer mass. Going from magnitude 14 to 15 (the limit of SkySafari Plus) about doubles the number of galaxies. Should I upgrade to Pro and go down an extra 0.7 magnitudes I would expect a similar increase, and;

2) Catalog issues. I was seeing some "drop out" of list targets when going from Astroplanner to SkySafari. Bill at Southern Stars was right on top of it, and said the source data can have some conflicts depending upon the measurements were done in. The difference could be a couple of full magnitudes in cases.

So my concern was that pressing to the theoretical limit could end up in a lot of time wasted chasing ghosts, and I'm curious what the experienced Galaxy Hounds do about it.

Or perhaps I should set the limit at 14.0 and marvel at how bright they all look and how many show detail?

#30 azure1961p

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Posted 10 February 2013 - 10:44 AM

In my 8" the faintest galaxies I could see with standard coatings was 13.5 under 6.2v sky's and stellar limit of 15. Moving to 16 I would think should add at least a full magnitude onto that and maybe 1.5v extra? I would also think the galaxies extended surface brightness would have a bigger improvement than the stellar. Seeing where I am is lousy often and the 8" with small CO produces some fine tight stars that larger apertures and COs tend to bloat and soften. Great seeing is another matter.

Ok that's my view from 8" anyway.

Pete

#31 Madratter

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Posted 10 February 2013 - 04:03 PM

Or perhaps I should set the limit at 14.0 and marvel at how bright they all look and how many show detail?


I have a 20". The dimmest galaxy I have bothered to hunt down was listed at 16.0 (NGC 913). That was under good but not great skies (a little better than 6.0). I have bagged four others listed between 15-16. That said, a) I don't think the catalog values are all that accurate for visual work and b) personally, I don't go hunting for the really faint ones unless they are part of a group. They just aren't that interesting and there are plenty of brighter, more interesting ones that I still haven't seen.

#32 faackanders2

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Posted 10 February 2013 - 05:57 PM


Or perhaps I should set the limit at 14.0 and marvel at how bright they all look and how many show detail?


I have a 20". The dimmest galaxy I have bothered to hunt down was listed at 16.0 (NGC 913). That was under good but not great skies (a little better than 6.0). I have bagged four others listed between 15-16. That said, a) I don't think the catalog values are all that accurate for visual work and b) personally, I don't go hunting for the really faint ones unless they are part of a group. They just aren't that interesting and there are plenty of brighter, more interesting ones that I still haven't seen.


+1

#33 IVM

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Posted 10 February 2013 - 08:17 PM

This is not exactly galaxy structure anymore but it seems that the original poster does not object, so... As far as deliberately going after faint galaxies, I usually stick to those plotted in Uranometria. This means to visual mag. 15.0 (that's what Uranometria says) and photographic just under 16 (my guess/generalization). The real limit (hitting the 0 probability of detection) for me with the 16" seems to be another half-magnitude fainter at around 16.5, although freaky exceptions even with normal morphology may go beyond 17.0 and still be visible. I don't mind wasting time ;)

EDIT: I meant to say that my probability of detection at photographic mag. 16.0 is 50/50, and it is better than 50/50 for these faint, non-NGC galaxies in Uranometria that seem to concentrate in the photographic 15.5-16.0 range. Around photographic 15.5 the probability tends to 100% with my 16".

#34 george golitzin

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Posted 10 February 2013 - 10:09 PM

The dimmest galaxies I found in a 16-inch were between mag 15.5 and 16, in the Corona Cluster--that was definitely pushing the envelope for that aperture. Like Madratter, I don't hunt these down unless part of a galaxy cluster: it's a particular sort of game.

You bring up a topic of interest with regards to charting. Once you step up to 16 or 18 inches, ordinary charts cannot really go deep enough for certain situations--Uranometria only goes so far (although it does contain deeper maps of specific regions), and so one must pick and choose magnitude limits for specific targets: one size no longer fits all. Last night I had several different charts with very different parameters--the deepest was the Perseus cluster, where I was able to identify 36 galaxies within a 1-degree field. For that chart I put no effective mag limit. For wider maps, that's usually not possible.

-geo

#35 HellsKitchen

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Posted 10 February 2013 - 10:32 PM

I have a 12" Zambuto and where I live in the outer suburbs of a large metro area, the faintest galaxies I can detect are about V mag 14. The faintest stellar object I've seen is the Quasar HE 1015-1618 in Hydra which NED lists as V mag 15.7. My NELM is approx 5.5, I would say that at mag 13.5 for galaxies and mag 15 for stellar is where things start to get challenging.

#36 Jeff Morgan

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Posted 11 February 2013 - 12:07 AM

In my 8" the faintest galaxies I could see with standard coatings was 13.5 under 6.2v sky's and stellar limit of 15. Moving to 16 I would think should add at least a full magnitude onto that and maybe 1.5v extra? I would also think the galaxies extended surface brightness would have a bigger improvement than the stellar. Seeing where I am is lousy often and the 8" with small CO produces some fine tight stars that larger apertures and COs tend to bloat and soften. Great seeing is another matter.

Ok that's my view from 8" anyway.

Pete


The 15.7 magnitude value is what Astroplanner thinks the limit is on a 16" when you define your scope.

SkySafari Plus goes down to 15.0 so drops list components fainter. I'll probably be upgrading to Pro (which goes much fainter) when observing season gets rolling.

And it doesn't always seem to work "by the numbers". I've caught IC4617 near M13 with my 12.5" Royce - from the backyard no less! SkySafari lists it at magnitude 15.17, where Astroplanner doesn't provide a magnitude at all. I have yet to repeat that observation.

It should be interesting to track success rate vs. magnitude, but after awhile I could see myself putting a brighter limit on searches.

#37 Ed D

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Posted 11 February 2013 - 10:02 AM

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.

Pete


Pete, I attached a picture of my friend's truss Dob he made using a 16" f/4.5 LightBridge mirror he picked up used for $150, not pristine or like new, but it does give great views. As you can see it's not all that tall, requiring using the first step of a folding step ladder only around zenith. He made it using the Kriege and Berry design. BTW, the mirror box weighs 50#.

Ed D

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#38 IVM

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Posted 11 February 2013 - 02:25 PM

I like how this 16" was apparently delivered to the site in the two-door car...

#39 Ed D

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Posted 11 February 2013 - 10:37 PM

Didn't even occur to me to point it out, but that is the car it rode in. It breaks down very compactly and leaves enough room for all the other stuff. We each bring our own cars loaded with our scopes and other gear. The big Dob is actually easier to transport than my other friend's 8" GEM mounted Newt seen in the pic.

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#40 ThreeD

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Posted 11 February 2013 - 11:55 PM

I like how this 16" was apparently delivered to the site in the two-door car...

Since I built a new base for my LB16 easily fits in my Prius. I'm still using the original OTA too and that bottom tube is tall. Lots of room left in the car too...

#41 Mta472

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Posted 13 February 2013 - 07:35 AM

If you do not live in a dark zone, IMHO you would be throwing money away on a 16 inch vs a 12 inch because of light pollution. I live in CT and went from an 8 to 12 inch and it did NOT make much difference.

#42 Starman81

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Posted 13 February 2013 - 05:49 PM

If you do not live in a dark zone, IMHO you would be throwing money away on a 16 inch vs a 12 inch because of light pollution. I live in CT and went from an 8 to 12 inch and it did NOT make much difference.


Really? I would like to hear more about that since I do plan on going to 12" of aperture at some point...

Also, what's with people with big dobs always having smaller cars? It is a trend that I've noticed. It's like the challenge of hauling around a big dob wasn't enough so you want to try to do it with smaller and smaller cars to add to the accomplishment.

#43 GlennLeDrew

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Posted 13 February 2013 - 05:50 PM

It doesn't matter the degree of light pollution, the same benefits from an aperture increase accrue as they do under dark skies. If an object is seen in a smaller scope, more detail will be seen with a bigger scope. The brighter sky merely restricts to objects having higher surface brightness. Of course, beyond some amount of sky glow, practically nothing but the brightest central cores of galaxies is to be seen, with a concomitant dearth of interesting structure to be observed anywhere.

#44 azure1961p

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Posted 13 February 2013 - 10:27 PM

+100 on what Glenn just said. Contrast thresholds don't take a holiday with poor sky's. a poor sky will never look pretty but everything else is the same in terms of resolving power which works on a number of levels. The laws of light are the same - city or country.

Pete

#45 deepskydarrell

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Posted 14 February 2013 - 04:07 AM

Am I wrong, I've always equated a city view thru my 16.5 as about the same as a 4 inch in the dark? Perhaps that's too simplistic.

As for the larger the aperture the smaller the vehicle, the tendency may be the larger the aperture the farther one drives to the dark therefore the better fuel economy needed?

DSD.

#46 deepskydarrell

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Posted 14 February 2013 - 04:07 AM

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#47 drbyyz

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Posted 14 February 2013 - 10:14 AM

Am I wrong, I've always equated a city view thru my 16.5 as about the same as a 4 inch in the dark? Perhaps that's too simplistic.


I think a touch simplistic. Plus it really depends on what you mean by "city." My city sky is a Bortle 4/5, while someone else's may be a 9. Same with "dark" sky, mine is a 3, but others may have a 1.

#48 Tony Flanders

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Posted 14 February 2013 - 10:24 AM

Am I wrong, I've always equated a city view thru my 16.5 as about the same as a 4 inch in the dark?


That doesn't make sense to me. It seems to me that there are many things that will show far better through your 16.5-inch scope in the city than through a 4-inch scope in pristine skies. These are objects that are small but bright -- most obviously planets, but also many planetary nebulae and tight double stars.

Conversely, there are many things that will show far better in the 4-inch scope under dark skies than in any scope, no matter how big, in city skies. These include essentially all galaxies and diffuse nebulae.

The only place where aperture and sky brightness are even close to interchangeable is in the ability to see faint stars. But here, it seems to me that the 16.5-inch should show considerably fainter stars in the city than the 4-inch does under dark skies.

#49 Achernar

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Posted 14 February 2013 - 10:55 AM

Might be due to the cost of gasoline, and the fact bigger vehicles cost more to own in general. Still, it's really cool to be able to build a large Dob that will fit in many cars. I can get my 15-inch into my car, and still take along a passenger.

For every time I do take the 15-inch to a darker site, I use it at least several times at home despite the heavy light pollution. With digital setting circles and nebula filters, a lot of enjoyable observing can be done from a city at home. I have observed quite a good many galaxies from my driveway, even quite dim galaxies. Whether or not one is in a city or a dark site, a larger telescope always shows more than a smaller one at the same site.

Taras

#50 JohnH

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Posted 14 February 2013 - 12:42 PM

The arguments for the step from a 12" to a 16" class follows good principles.

My current project is a 12 1/2" Dobsonian, which is a fair step up from my former go to scope, an 8" Maksutov. I want get some practice at a build while keeping weight under 60 lbs.

I chose this size as it meant about doubling the effective light gathered, which is roughly how much improvement is easily visible.

I was thinking that was where I would end telescope making,if it werent for a run in with a number of shorter, well corrected bunch of scope in the 16 to 22 inch sizes at some star parties.

So now I added 5 12 inch, 4 16 inch and one 18.625 blanks as fodder for larger, faster scopes in the future

Two of the 16s are full thickness, and I plan to send them to Glassfab to turn them into four 1.6 inch blanks more suited to portable instruments.






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