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Equipment Discussions >> Reflectors

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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: James Pierce]
      #6480898 - 04/22/14 10:20 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.

Quote:

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).




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André Heijkoop
member
*****

Reged: 05/02/10

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: Starman1]
      #6481371 - 04/22/14 02:36 PM

Quote:

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.


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: André Heijkoop]
      #6481462 - 04/22/14 03:17 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.


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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]
      #6481699 - 04/22/14 05:27 PM

Quote:

Quote:

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


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stratocaster
sage
*****

Reged: 10/27/11

Re: Jumping from a 16" to a 20" dob, worth it? new [Re: mantrain]
      #6484863 - 04/24/14 02:58 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.


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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Jumping from a 16" to a 20" dob, worth it? new [Re: JimMo]
      #6486558 - 04/24/14 09:02 PM

Quote:

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.


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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: stratocaster]
      #6486625 - 04/24/14 09:32 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.

Quote:

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.




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aatt
sage


Reged: 07/26/12

Loc: CT
Re: Jumping from a 16" to a 20" dob, worth it? [Re: Peter Natscher]
      #6486791 - 04/24/14 10:57 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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