Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Equipment Discussions >> Reflectors

Pages: 1 | 2 | 3 | (show all)
Achernar
Postmaster
*****

Reged: 02/25/06

Loc: Mobile, Alabama, USA
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6480147 - 04/21/14 10:31 PM

The best way to answer this question is to attend a regional or national star party where folks bring these very large Dobs. Bring yours along too, then compare what you see through yours and the larger scopes. Then you can decide if a bigger telescope is worth the greater bulk, height, weight and expense. One reason why I settled on a 15-inch was that if I went much bigger, it would be impossible to get it through the door. Another was the fact the optics would be nearly twice the price, and there's no way I can get it into my car.

Taras


Post Extras: Print Post   Remind Me!   Notify Moderator  
rocco13
Got Milk?


Reged: 07/29/06

Loc: Phoenix, Arizona
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: davidmcgo]
      #6480151 - 04/21/14 10:34 PM

On a slightly smaller scale, I went from a 12 to a 15 and honestly was not wow'ed like I expected. Perhaps my expectations were too high, although I don't really think so. Sure, the 15 showed a little more, and went a bit deeper, but after two years of handling the bigger & heavier scope, I noticed I was taking it out less and less. So, I sold it and went back to a 12.

Unfortunately I was forced to sell off that 12 and currently have a 10, but I discovered through trial and error that the 12 is my personal 'sweet spot'.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mantrain
scholastic sledgehammer
*****

Reged: 07/22/11

Loc: San Diego
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: davidmcgo]
      #6480185 - 04/21/14 10:52 PM

Oh, and how to I spend the $$ on the glass and not have the wife find out??

Post Extras: Print Post   Remind Me!   Notify Moderator  
Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6480196 - 04/21/14 11:00 PM

Once you've decided that your area you observe from has decent seeing and darkness to support a +15" Dob, I think the next issue is transporting it. Build it so that it can be rolled into your vehicle so that you don't have to struggle lifting anything. Choose a thin primary mirror for less scope weight and a faster cooling time. That's what I'v egot in my 24" 3.3 StarStructure Dob. A fast and thin primary on a great cell with small boxes and rigid frame. I've observed with Dobs of 14.5", 18", 20" and two 24's and have enjoyed all of them. I had great views with my past 20" Starmasters -- completed both Herschel Object lists (I & II) with them. My current 24" f/3.3 StarStructure LE with a Kennedy primary, is fantastic on everything from DSO's to Lunar and planets. Just last week, I bino'd Mars at 450X and it was awesome -- fine detail and lot's of planetary color. It shows much more galaxy halo and arms than my past smaller Dobs. Side by side with another observer's 15" f/4.5 Obsession, the Hickson galaxy clusters were much brighter. The brighter images of larger aperture provides a better view of DSO's when using narrow-band filters. I say go for a 20" or even a 24" if you can transport it. You'll build up the physical strength needed to comfortably manage a larger scope just by using it. You only live once. Managing my 24" SST helps motivate me to keep myself in good physical condition.

Quote:

I made my own 16 " dob, and I really love it. But all my tools, and extra wood just sit there now with nothing to be done. There is this 20" primary and secondary for sale, and I am really considering it. But am not sure how much better the viewing will be with a 20." One thing is for sure, the 16" is at the edge of being loaded by one person w/out wheel-barrow handles. So yeah, the 20" would need that.
Just wondering, would a 20" be that much better? 200sq" of glass vs 314 sq ", plus $4000 in parts and lots of labor (of love).

But right now I am not totally convinced it's going to be that much different. I mean, I think they are both sort of in the same visual realm. But I have not really looked too much through a 20." I have looked through an 18" and I couldn't see too much difference.




Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6480209 - 04/21/14 11:11 PM Attachment (7 downloads)

Quote:

Oh, and how to I spend the $$ on the glass and not have the wife find out??




that's not for us to tell you.

I'd spend the money on a better mirror than a bigger mirror.

if you go bigger, better chances not as good a figure, rolled edge.

from .707 zone to the edge is the most important part of the mirror cause it delivers more than 1/2 the light to the airy disc.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mantrain
scholastic sledgehammer
*****

Reged: 07/22/11

Loc: San Diego
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: davidmcgo]
      #6480217 - 04/21/14 11:18 PM

Quote:

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.

Edited by mantrain (04/21/14 11:21 PM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
davidmcgo
Pooh-Bah
*****

Reged: 10/09/04

Loc: San Diego, CA
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6480270 - 04/21/14 11:50 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
JMW
Pooh-Bah


Reged: 02/11/07

Loc: Nevada
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: davidmcgo]
      #6480373 - 04/22/14 12:57 AM

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
mantrain
scholastic sledgehammer
*****

Reged: 07/22/11

Loc: San Diego
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: davidmcgo]
      #6480387 - 04/22/14 01:07 AM

Quote:

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
André Heijkoop
member
*****

Reged: 05/02/10

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6480419 - 04/22/14 01:39 AM

Quote:


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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: André Heijkoop]
      #6480432 - 04/22/14 01:54 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: André Heijkoop]
      #6480435 - 04/22/14 02:02 AM

Quote:

Quote:


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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Bill Weir
Carpal Tunnel
*****

Reged: 06/01/04

Loc: Metchosin (Victoria), Canada
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: Starman1]
      #6480454 - 04/22/14 02:39 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
Deep13
Carpal Tunnel
*****

Reged: 01/25/05

Loc: NE Ohio
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: Bill Weir]
      #6480477 - 04/22/14 03:06 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
JustaBoy
Post Laureate
*****

Reged: 06/19/12

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: Deep13]
      #6480516 - 04/22/14 03:53 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!


Post Extras: Print Post   Remind Me!   Notify Moderator  
jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: Deep13]
      #6480626 - 04/22/14 06:56 AM

Quote:

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
JustaBoy
Post Laureate
*****

Reged: 06/19/12

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: jpcannavo]
      #6480646 - 04/22/14 07:14 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
jpcannavo
scholastic sledgehammer
*****

Reged: 02/21/05

Loc: Ex NYCer, Now in Denver CO!
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: JustaBoy]
      #6480670 - 04/22/14 07:33 AM

Quote:

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!


Post Extras: Print Post   Remind Me!   Notify Moderator  
James Pierce
member


Reged: 08/11/09

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: jpcannavo]
      #6480686 - 04/22/14 07:53 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).


Post Extras: Print Post   Remind Me!   Notify Moderator  
Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: JustaBoy]
      #6480880 - 04/22/14 10:07 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.

Quote:

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.




Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
18 registered and 26 anonymous users are browsing this forum.

Moderator:  ausastronomer, Phillip Creed, JayinUT, okieav8r 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1464

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics