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Jumping from a 16" to a 20" dob, worth it?

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#26 mantrain

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Posted 21 April 2014 - 10:18 PM

I can see a pretty pronounced difference between my 15" and a friend's 18" when we set up together, especially on galaxies. Planets are pretty equivalent where cool down, collimation, and tracking matter more, but under dark skies, the 18" seems to reach almost twice as deep.

I have a 15" instead of an 18" because my little house, detached garage, and steps between mean I hand carry everything to the car and can't use the wheelbarrow handles. The 15" mirror box is my max to carry. But if that wasn't a constraint, you can bet I would have an 18" instead.

So I think there is a big jump from 16" to 20" and would certainly entertain the idea if I could handle the physical aspects of it. I would still want something in the 4" to 8" range for home use, and would not have a 16" and a 20" at the same time. But for the dark sky trips, a 20" would be awesome.

Dave


Is it possible you have a very discerning sense of objects?
I do not see much of a difference bx my 16" and my buddies 18." But every additional inch is more glass than the inch before it, because that circle the inch of glass has to wrap
around is a larger circle.

so 15" = 177 sq" of glass
16" = 200 SQ"
18" = 254
20" = 314

Hey I got a great Idea, my wife spends much too much time in her large vanity mirror. I bet I could cut a 30" mirror out of that and I just need a secondary. And the wifey could get a new vanity to boot. we both win.

#27 davidmcgo

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Posted 21 April 2014 - 10:50 PM

Could be, I spent a lot of years with nothing bigger than a 130f8 Apo and tracking down some Palomar clusters, seeing the G1 glob in M31, tracking Pluto, etc with it and another decade with a 10" as my biggest scope. I also tend to really study the brighter objects, I spent over 2 hours just on M31, 32, and 110 at all kinds of powers seeing lots of fine structure in the 15" at Az Sky Village last fall.

Dave

#28 JMW

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Posted 21 April 2014 - 11:57 PM

I hauled around my club's Obsession 20 inch f/5 for 5 years. Eventually passed the job onto another member of the club and bought a used Webster D14. I can't see quite as deep but the thinner Zambuto mirror knocks the socks off the Obsession thicker mirror for planetary viewing. If I find a rural dark sky property that I can afford, I may but a larger scope that doesn't need to be transported each time I want to use it. When at home I am perfectly content to use my C11 EdgeHD or TEC 140 under my light polluted skies. The Webster is a lot more manageable when pushing it up the ramp into my Highlander. The 20 inch Obsession felt like a bit of a strain going up the ramp. I also am glad I don't have to carry around a full size ladder anymore. I use a 3 step stool with handle around for shorter people using my TEC 140 or Webster.

I can look through the club's Obsession several times a year at members dark site star parties. I am happy to let someone else do the hard work. I get my really big scope fix each year at the Golden State Star party. I usually spend a couple of hours on one of the 4 nights looking through the 25-28 inch Dobs.

I would suggest setting up your 16 next to someone else's 20 inch. It would be worth a trip to a regional star party under dark skies to try out the views. Cheaper way to figure out what right for you.

#29 mantrain

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Posted 22 April 2014 - 12:07 AM

Could be, I spent a lot of years with nothing bigger than a 130f8 Apo and tracking down some Palomar clusters, seeing the G1 glob in M31, tracking Pluto, etc with it and another decade with a 10" as my biggest scope. I also tend to really study the brighter objects, I spent over 2 hours just on M31, 32, and 110 at all kinds of powers seeing lots of fine structure in the 15" at Az Sky Village last fall.

Dave


I think that is the epitome of amateur astronomy and I tend to move from object to object too quickly. that's a whole other thread.

#30 André Heijkoop

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Posted 22 April 2014 - 12:39 AM

Now tell me about the 24..Others here are saying it takes long to cool. hard to move, etc, etc.
I am thinking the wheelbarrow handles will help the move aspect. But what about the cooling?

The 24" takes a longer time to cool than the 14" for sure. But I never find it a problem. I set up the 24" at sunset, do a collimation, drink some coffee. And when the skies are dark enough my 58mm thick mirror is sufficient cooled down.
I don't have fans for cooling, IMHO an uneven cooling with fans is a lot worse than cooling without fans.

Wheelbarrow handles are a must if you want to spare your back.

My mirror is 24" f3.7, I just need one step on a ladder.

#31 Starman1

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Posted 22 April 2014 - 12:54 AM

OK, a 20" is 56% larger in area than a 16". That gains you less than 1/2 magnitude, which can be within the fluctuation from night to night, in terms of transparency and depth of magnitude seen.
Is a 20" larger? Sure. Is it significant? I would argue it is not.
But it is a lot larger and heavier.
Be sure that's what you want to move before you start down that road.

#32 Starman1

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Posted 22 April 2014 - 01:02 AM


Now tell me about the 24..Others here are saying it takes long to cool. hard to move, etc, etc.
I am thinking the wheelbarrow handles will help the move aspect. But what about the cooling?

The 24" takes a longer time to cool than the 14" for sure. But I never find it a problem. I set up the 24" at sunset, do a collimation, drink some coffee. And when the skies are dark enough my 58mm thick mirror is sufficient cooled down.
I don't have fans for cooling, IMHO an uneven cooling with fans is a lot worse than cooling without fans.

Wheelbarrow handles are a must if you want to spare your back.

My mirror is 24" f3.7, I just need one step on a ladder.

You need to get a temperature gauge for your mirror (I recommend a laser type like they use in the heating/air conditioning business). Without fans, your mirror won't be cooled down to ambient by dawn at 58mm thick. I just presumed a 10 degree temperature drop and the cooling calculator indicated your mirror never got anywhere near the ambient temperature over an 8 hour period. In fact, it grows worse assuming it starts out at ambient temperature. If you assume the mirror is warmer than the ambient, it never even gets close. You need fans. On a 24", 3 to 5 of them.

#33 Bill Weir

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Posted 22 April 2014 - 01:39 AM

20" f/?, you don't say.

I have a f/3.3 20" and observe regularly with a friend who has a f/5 15". At the eyepiece on the same objects the difference between the two is quite noticeable. Sure some it has to do with the difference in aperture but also the difference in mirror quality. Then there is that focal ratio thing. His is so tall. My wider shorter scope is so much easier to use. If that is a longer focal ratio mirror then I think give it a pass.

I think put that wood aside in a safe way. (it won't go bad) Get to know the 16" scope that you just built. Push it to its limits and see if you think something is lacking. Then after a bit of time you think you really do need/want a larger scope you can build it then. Another big mirror will come up and by then if you've been dilligent with saving you might be able to get a better mirror. When I bought my 20 I was using a nice 12.5" and although fully enjoying it was aching for something a bit bigger. I thought about it long and hard and ended up with my 20". It's no more work to transport and setup but it does take up quite a bit more space in my vehicle and in storage at home. This is something to seriously take into account if thinking of of going to the next level in size of scope.

Or... is it that f/4 20" Zambuto for sale on the Mart? If so then all bets are off. Consider it.

Bill

#34 Deep13

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Posted 22 April 2014 - 02:06 AM

well, you could do the math. For an arithmetic increase in perceived brightness, one needs a geometric increase in actual brightness. Calculate the difference in surface area of the two sizes (subtract area of the secondary using the minor axis for diameter). If you have 4x increase in area, the object will seem twice as bright. Doubling surface area will make an object seem 1.5x as bright.

#35 JustaBoy

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Posted 22 April 2014 - 02:53 AM

It doesn't matter if we're talking about circles or squares, the ratio is still the same as long as we are talking about like shapes - No need for Pi.

So 20/16 = 1.25 . . Now square the result for area difference = 1.5625

22/16 = 1.375 Squared = 1.89

For sq inch area it's just 16x16 X 1/4 Pi which is .7854 so = 201sq inches surface area.

Keeping it simple just makes it faster:-)

Another thing is which eyepiece has the widest True field, Quick...

Compare them by multiplying the focal length times the apparent field. - So a 41mm Pan vs a 31mm Nagler gives us 41x68 = 2788 for the pan, and 31x82 = 2542 for the Nagler.

2788 and 2542, What" - You might ask.. Who cares what it is, it's just a ratio the will work every time.

Now, 2788/2542 = 1.097 increase in True Field DIAMETER, which squared give an increase of 1.20x in AREA or 20% more. - Taking the reciprocal of this we see that the 31 Nagler has 83.1% in AREA of what a 41 Pan gives us.

I'm sorry - I know that I'm no fun:-)

But Thanks!

#36 jpcannavo

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Posted 22 April 2014 - 05:56 AM

well, you could do the math. For an arithmetic increase in perceived brightness, one needs a geometric increase in actual brightness. Calculate the difference in surface area of the two sizes (subtract area of the secondary using the minor axis for diameter). If you have 4x increase in area, the object will seem twice as bright. Doubling surface area will make an object seem 1.5x as bright.


And even more complicated than this, since the power functions governing perceptual brightness elicited by a visual target vary with the various physical/photometric properties of the target and background. See Steven's law etc.

#37 JustaBoy

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Posted 22 April 2014 - 06:14 AM

Going to a star party really is the best way:-) - But, be sure to get the guys to let you help lift all their stuff if you really want to see how it is going to be.

#38 jpcannavo

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Posted 22 April 2014 - 06:33 AM

Going to a star party really is the best way:-) - But, be sure to get the guys to let you help lift all their stuff if you really want to see how it is going to be.


Absolutely!

#39 James Pierce

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Posted 22 April 2014 - 06:53 AM

I'm biased with a 16" myself, but I honesty believe that 16" is a kinda magical size for scopes. Big enough to see 10,000+ objects, and great structure in perhaps 1000 of those. Small enough to fit into the back of a pretty normal car.

Having enjoyed a week of observing recently with everything from 12 to 30 inch dobs on the one field I can say it's not until you get to a good ~25 inch that you see a dramatic change in the view (and I say change rather than improvement because many 25 and larger scopes never really thermally equilibrate unless they have new very thin mirrors). The degree of difficult moving, storing and living with a 25" is a whole other thing. The frustration of always viewing through a scope that's almost there is pretty frustrating too. (My 16" has 30mm thick mirror so it cools quite fast)

Which one can you, and will you get to a very dark sky most often ? A 20" under dark, but not truly dark skies will be beaten out by a 16" under truly dark skies every time. (PS I still want a 25"+ too, but only when I have a perm location for it somewhere very dark).

#40 Peter Natscher

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Posted 22 April 2014 - 09:07 AM

To compare the performance and light gathering differences between a 16 and 20, you need to do the comparison with the same object and take note of the differences. In the time it takes to walk around looking for these two aperture sizes at a busy star party, one will lose perspective in what these two closely sized scopes can show.

Going to a star party really is the best way:-) - But, be sure to get the guys to let you help lift all their stuff if you really want to see how it is going to be.



#41 Peter Natscher

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Posted 22 April 2014 - 09:20 AM

My 24" f/3.3 Kennedy mirror StarStructure LE has a thin 40mm edge thickness (~25mm at center!) and cools fairly fast housed in its all-aluminum welded boxes. The telescope build and material do have a positive or negative effect on mirror cooling time. The StarSTructure aluminum boxes pull heat out of the mirror with aluminum's higher thermal coefficient than wood, esp. glue-layered wood like plywood. OTOH, my past 24" *plywood* Dob retained heat longer into the night. During summertime observing with a 40° temp. drop by midnight, the thicker 45mm mirror and plywood boxes never reached equilibrium. I could feel the heat retained in the plywood well after sunset. I haven't found the need to add fans to my 24" StarStructure. So, bottom line: choose your Dob's build materials wisely.

I'm biased with a 16" myself, but I honesty believe that 16" is a kinda magical size for scopes. Big enough to see 10,000+ objects, and great structure in perhaps 1000 of those. Small enough to fit into the back of a pretty normal car.

Having enjoyed a week of observing recently with everything from 12 to 30 inch dobs on the one field I can say it's not until you get to a good ~25 inch that you see a dramatic change in the view (and I say change rather than improvement because many 25 and larger scopes never really thermally equilibrate unless they have new very thin mirrors). The degree of difficult moving, storing and living with a 25" is a whole other thing. The frustration of always viewing through a scope that's almost there is pretty frustrating too. (My 16" has 30mm thick mirror so it cools quite fast)

Which one can you, and will you get to a very dark sky most often ? A 20" under dark, but not truly dark skies will be beaten out by a 16" under truly dark skies every time. (PS I still want a 25"+ too, but only when I have a perm location for it somewhere very dark).



#42 André Heijkoop

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Posted 22 April 2014 - 01:36 PM

You need to get a temperature gauge for your mirror (I recommend a laser type like they use in the heating/air conditioning business). Without fans, your mirror won't be cooled down to ambient by dawn at 58mm thick. I just presumed a 10 degree temperature drop and the cooling calculator indicated your mirror never got anywhere near the ambient temperature over an 8 hour period. In fact, it grows worse assuming it starts out at ambient temperature. If you assume the mirror is warmer than the ambient, it never even gets close. You need fans. On a 24", 3 to 5 of them.


I always have an IR temperature gauge with me and regularly measure the temp at the backside of the mirror. Seldom I see a temp difference of more then 2 degrees Celsius measured at different parts of the mirror.
Just a month ago at a Dutch starparty I was able to have a magnification of 890x on M57 (XW5 with Powermate 2x). You can't go that deep if your mirror is not at equilibrium and the seeing is so so.

#43 Mike B

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Posted 22 April 2014 - 02:17 PM

Personally, i view the scope as a "tool", or maybe like a pair of boots; how does it fit my hand, my feet, my use... my comfort? It's of limited benefit to me if i can't (or don't) use it often and effectively.

So go for the Dob that FITS you best- your personal ergonomics, viewing habits, "tastes", & style, ability to handle, transport, & store. Considering that you already have a 16", perhaps use it some more before looking beyond, to better ascertain how THIS one fits you.

#44 Starman1

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Posted 22 April 2014 - 04:27 PM

You need to get a temperature gauge for your mirror (I recommend a laser type like they use in the heating/air conditioning business). Without fans, your mirror won't be cooled down to ambient by dawn at 58mm thick. I just presumed a 10 degree temperature drop and the cooling calculator indicated your mirror never got anywhere near the ambient temperature over an 8 hour period. In fact, it grows worse assuming it starts out at ambient temperature. If you assume the mirror is warmer than the ambient, it never even gets close. You need fans. On a 24", 3 to 5 of them.


I always have an IR temperature gauge with me and regularly measure the temp at the backside of the mirror. Seldom I see a temp difference of more then 2 degrees Celsius measured at different parts of the mirror.
Just a month ago at a Dutch starparty I was able to have a magnification of 890x on M57 (XW5 with Powermate 2x). You can't go that deep if your mirror is not at equilibrium and the seeing is so so.

Quite likely, you store your scope at or near the ambient temperature, and/or observe in places where the temperature drops very little from sunset until dawn.
We have the same thing with observers in Florida, where daytime and nighttime temperatures are nearly identical. Florida also has very good seeing a lot of the time, so the observers there, like you, often use high powers without issue.
In a good part of the Western US, where I observe, we can drop 15-20C in the course of a night, with most of that occurring in the first two hours.
Even when the mirror is at the ambient temperature at 4pm, it is 10C warmer than the air by 2 hours later. I run 3 fans on a 32mm thick 12.5" mirror and can barely keep up with the temperature drop.
It's obviously good to have more stable conditions

#45 stratocaster

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Posted 24 April 2014 - 01:58 AM

It seems the general consensus is that visually there won't be enough difference to be worth it. I agree with that. As Don P has pointed out, 24" or 25" would be the next size up to be visually worth it.

Is the hassle factor of the larger size worth it to you? Only you can answer.

And the labor of love aspect? If that is the primary motivation then no other justification is necessary. Build the larger scope for the sake of building it.

Personally for me, I would not upgrade from 16" to 20". I have a 10" f5 dob now, and I wouldn't consider upgrading to 16". A 250% increase in light gathering wouldn't be worth it to me all other things considered. If I were to ever upgrade I'd go to something like a 24" f3.

I think 24" would be your absolute minimum for improved visual performance.

#46 GeneT

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Posted 24 April 2014 - 08:02 PM

If I was 6' tall that would have factored in and I probably would have gone with an 18".


You raise an important point. I am 5'11" I have a friend 6'3". He moves his 18 inch telescope around as easily as I do my 12.5 inch. He stands flat footed at his 18, and I have to use a ladder for much of the viewing. He loads his 18 on ramps into his vehicle as easily as I do my 12.5 inch.

One's height does factor into this equation. So does one's age. As we age, larger and heavier becomes a problem. My 12.5 inch Portaball will probably carry me through the rest of my life past my current 71 years of age. I only have to lift it six inches to a small transporter, wheel it to my CRV, and onto the seat of the vehicle. It is all quite easy to do even at my old age. :grin:

#47 Peter Natscher

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Posted 24 April 2014 - 08:32 PM

10" to 16" provides a big 0.8 magnitude gain. Why wouldn't that be worth it? I made a move from 18" to 24" a few years ago providing me a 0.4 gain and I now see a lot more detail in a galaxy's size, the halo's are more defined and detailed, and arms are more extended with more Ha regions showing. Any 0.5 mag. gain is worth it for the increased nebula and galaxy details gained.

It seems the general consensus is that visually there won't be enough difference to be worth it. I agree with that. As Don P has pointed out, 24" or 25" would be the next size up to be visually worth it.

Is the hassle factor of the larger size worth it to you? Only you can answer.

And the labor of love aspect? If that is the primary motivation then no other justification is necessary. Build the larger scope for the sake of building it.

Personally for me, I would not upgrade from 16" to 20". I have a 10" f5 dob now, and I wouldn't consider upgrading to 16". A 250% increase in light gathering wouldn't be worth it to me all other things considered. If I were to ever upgrade I'd go to something like a 24" f3.

I think 24" would be your absolute minimum for improved visual performance.



#48 aatt

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Posted 24 April 2014 - 09:57 PM

I am wrestling with the same question-15"->20". I imagine what I can see, DSO-wise will be brighter and more obviously detailed even under light pollution, but I don't expect to see much deeper.I don't know since I have no experience comparing the two apertures under my typical skies.Can't say if it is worth it or not under an orange/red sky. I do know that a side by side with my 15" and a 17.5" on M101 was a more than noticeable difference in the HII regions and arms at a darker site. The HII's popped in the 17.5 versus being detectable in the 15". Made me want more aperture on the spot.






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