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Difference between 25" and 30"

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#1 bujin9

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Posted 23 July 2021 - 11:53 AM

Hi,
in the market to sell my home...which means I may be in the market to finally get a big dob.
Is there any visual difference between a 25" and a 30"? The price is a bit significant so I'd like to 'choose wisely'. I live in a bortle 5 area. Looking at Starstructure.

Also...will living in a Bortle 5 area make it a 'less than enthusiastic' experience?

Thanks,

Joe

 



#2 Feidb

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Posted 23 July 2021 - 12:19 PM

I have no idea what Bortle dee Bortle dum stuff means, so I can't relate.

 

On the other hand, I've always wondered that myself, whether there's a significant difference in the views from something like 25 to 30 inches. I know the surface area of the mirror is a big jump say compared to 16 to 20 inch, or 15 to 20 inch, and I can say the difference is there, but not sure whether it's...no, from 16 to 20 is probably worth it. However, 25 to 30 may or may not be given the significant jump in not only cost but size and weight etc. I'd have to really consider that unless you are just rolling in money. If I was, and portability was no object, I'd, of course go for the gusto.



#3 daveco2

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Posted 23 July 2021 - 12:53 PM

The gain in resolution would be on the order of 20%; the gain in light collection 44%; and the gain in mirror weight also 44% for the same thickness.



#4 MitchAlsup

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Posted 23 July 2021 - 01:13 PM

25"-to-30" is a gain of ½M in limiting magnitude and probably 1.5× visible galaxy count and close to double the weight.

 

Actual resolution in these large scope is almost never realized due entirely to the atmosphere.


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#5 sanbai

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Posted 23 July 2021 - 01:21 PM

If I could pay and manage the 30", I would. Take into account that currently it may take more than a year to get it at your place *after* ordering. This is extra time to save. However, there are other considerations. The main one is whether it's a permanent installation or a portable one. Permanent 30" is great, portable not so. In-between the "just push out of the garage". Another is feet on ground or not, which is also linked to the former question. Add age and fitness into the equation.

I guess that at this level most of the matching accessories are already purchased or just a small dent in the bank account. It wasn't the case for my 12.5", so I had to take it into account.
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#6 bbasiaga

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Posted 23 July 2021 - 01:59 PM

25*25=625

30*30=900

 

So you have 60% additional light gathering.  Similar to the jump from an 16 to a 20".

 

The old rule of thumb is to double your light gathering to see the difference.  So not quite there.  But you're not upgrading from a 25" to a 30"...so what size is your current scope? 

 

Aside from the weight comments above, focal length is a big issue.  at F5, the EP height will be 125" (10'5") off the ground for the 25, and 150" (12' 6").  That 2' will make a big difference in what size ladder you need and how hard it is to transport that.  You could of course go down to F4 from some folks, or F3-3.3 from Lockwood and help yourself in that aspect. 

 

-Brian


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#7 Astro-Master

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Posted 23 July 2021 - 02:41 PM

I would rather have a 20" portable Dob. I could transport to a Bortle 2 (21:89 - 21:93 MPSAS) zone, than a 30" stuck in a Bortle 5 (20:00 - 20:50 MPSAS) zone.

 

I've found that doubling the surface area of the mirror gives you a noticeable wow factor.  Moving up from a 12.5" scope to a 17.5" scope almost doubles the surface area, and gives you a wow factor, especially on DSO's.

 

Going from a 25" to a 30" is only a 44% increase in light gathering ability, but with the light pollution of a Bortle 5 zone I don't know how much better a galaxy would look, planetary nebula with higher surface brightness would be less affected.

 

Then there is the added cool down time for the larger mirror to consider, and the effect of seeing on the larger scope.  Perhaps attending some star parties, you could see the difference for yourself.


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#8 Sky_LO

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Posted 23 July 2021 - 02:57 PM

My apologies...former math major here....

 

The actual formula for the area of a circle is PI times the radius squared 

Squaring the diameter might be ballpark but is not correct.   

 

Radius ( 1/2 the diameter ) is 12.5 squared times  3.14 =  490

 

and 15 squared times 3.14 -=  706

 

706 / 490  =  1.44     Daveco2  has it right at a 44% increase in mirror surface

Not 60% but still a large difference.     

 

-Lauren   


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#9 aeajr

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Posted 23 July 2021 - 02:58 PM

Hi,
in the market to sell my home...which means I may be in the market to finally get a big dob.
Is there any visual difference between a 25" and a 30"? The price is a bit significant so I'd like to 'choose wisely'. I live in a bortle 5 area. Looking at Starstructure.

Also...will living in a Bortle 5 area make it a 'less than enthusiastic' experience?

Thanks,

Joe

Have you ever owned a telescope before?   Do you have any experience managing something this large?

 

 

Are you planning to transport this to dark sites or is it just for home use?  Will you have an observatory or will you have to move this around your property to use it?  How would you plan do do that?

 

Where will you store it?  Aperture is king but you have to be able to handle it in order to take advantage of the aperture.

 

My friend has a 32".   He has a minivan dedicated to the scope.  He has an electric winch to help get it in and out of the van.  But what amazing views!

 

 

If I were putting it in an observatory, I would go for the 30" for 44% more aperture. 

 

If I were going to be moving this around my property and taking it to remote sites frequently, I would go for a 20" truss Dob that I could dismantle and still be able to move by myself.  

 

My 12" F5 solid tube Dob is the largest scope I will likely ever own for moving around and packing into the car.   If I were to go larger it would be going into a home observatory. 


Edited by aeajr, 23 July 2021 - 03:05 PM.

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#10 George N

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Posted 23 July 2021 - 04:36 PM

......

 

My friend has a 32".   He has a minivan dedicated to the scope.  He has an electric winch to help get it in and out of the van.  But what amazing views!

 

If I were putting it in an observatory, I would go for the 30" for 44% more aperture. 

 

If I were going to be moving this around my property and taking it to remote sites frequently, I would go for a 20" truss Dob that I could dismantle and still be able to move by myself.  

 

.....

I once watched a guy with a 32" at Stellafane -- he sat in his lawn chair drinking something and said to his two teenage daughters "OK girls, get the scope out of the trailer and set it up."  -- and they did! wink.gif

 

I saw another guy there with one of the four Obsession 36 F/5 -- roll it down the trailer ramp, fold up wheels, insert poles, use a winch to crank it down to attach UTA -- it went back in the trailer pulled by an electric winch powered by the tow vehicle battery. Years later - observing with me and my 20" - he said "I really wish I had just stayed with a 20 like yours! I think I'll sell the 36 and go back." -- but he never has. The 36 is currently in a 'Dob-servatory'. His best line: "Owning a 36 is a life-style commitment - like getting married or becoming a monk!"

 

I have a friend with a 36" F/4 that he stuffs under the cap of his Chevy Silverado pick-up - poles sticking out the back! While two very committed people can get it out and assembled - it really takes 4 ( no ramps - all lifting ).

 

I expect my on-order 20" F/3.5 to be "grab & go" at home. I grab the wheelbarrow handles and roll the fully assembled scope out thru the double-wide French doors on my house to the observing area - tweak collimation - insert eyepiece - observe - end of night reverse the procedure. I have a trailer with a ramp rear door - but only very rarely travel alone for observing - and two people can easily lift this scope into a pickup bed. I would bet that even a 22" is a 'single person scope' -- 25 is pushing it, a 30 needs two people.

 

For some reason I've only rarely observed thru a 25" -- I'm much more familiar with the 'jump' between a 20 and a 32 to 36 - where the difference is quite noticeable. I would just guess that the visual difference between a 25 and 30 would require careful looks thru side-by-side telescopes pointed at a dim, low contrast target - galaxy cluster or maybe M-101? I doubt that the owner of the 25 would slink back to his telescope with a glum look on his face and start haunting A'mart for used 30's.


Edited by George N, 23 July 2021 - 04:46 PM.

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#11 MitchAlsup

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Posted 23 July 2021 - 04:45 PM

25*25=625

30*30=900

 

So you have 60% additional light gathering.  Similar to the jump from an 16 to a 20".

 

The old rule of thumb is to double your light gathering to see the difference.  So not quite there.  But you're not upgrading from a 25" to a 30"...so what size is your current scope? 

Adding to what I wrote above, because this post made me realize some missing data that is pertinent::

 

A bit more than 25 years ago I was faced with the choice of 20" or 25" scope. I drew up plans for both, fretted for way more than enough time, and finally decided that I would buy whatever mirror came up on Ebay first. Then for 22 of thee years I have been grateful that a 20" came up before a 25" came up.

 

The 20" Was enough bigger than my then C11 yet remained small enough that I could manover it by myself (into and out of the SUV)--something the 25" would not have been able to do.


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#12 bbasiaga

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Posted 23 July 2021 - 04:47 PM

My apologies...former math major here....

The actual formula for the area of a circle is PI times the radius squared
Squaring the diameter might be ballpark but is not correct.

Radius ( 1/2 the diameter ) is 12.5 squared times 3.14 = 490

and 15 squared times 3.14 -= 706

706 / 490 = 1.44 Daveco2 has it right at a 44% increase in mirror surface
Not 60% but still a large difference.

-Lauren


Area = pi*d^2/4 ...a re-write of the formula to use diameter instead of radius. If you compare area of one mirror to the other, all factors in the equation cancel except the ratio of the square of the diameters. So it is a useful comparison.

My mistake was that the smaller mirror has ~60% of the area of the larger. So a ~40% increase is correct. I just regurgitated the wrong half of the arithmetic!

Brian
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#13 bujin9

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Posted 23 July 2021 - 05:14 PM

Hi all, thanks for the reply’s. A little more info you asked about…I currently use an AT 130mm refractor and a C11. I primarily do imaging although I occasionally take out the C11 for some visual. My ‘big plan’ would be to keep it setup in the garage and be able to wheel it out. The 30” is $3k more which is why I was wondering. But if the only way to see the difference would be a side by side comparison, then that’s my answer….but if someone had said I’d definitely see the difference, then that’s a different story. Physically, I should be fine.



#14 Bill Jensen

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Posted 23 July 2021 - 05:31 PM

Not sure it is something you wanted immediately, but Camera Concepts lists a used Obsession 30 inch for sale. they note the mirror needs a re-coat. 

 

https://www.cameraco...-stellarca.html

 

they are located on long island. Oddly, the link indicates 25, but the description online is for a 30 inch. 

 

I have the opportunity to get some views through a 24 inch and a 28 inch scope when I travel west to a star party under dark skies, and I think that is where a scope of that size will be best utilized. After looking at objects through those scopes, my 16 inch f/4 which is stored out there seems... dim smile.gif



#15 MeridianStarGazer

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Posted 23 July 2021 - 05:53 PM

Hi,
in the market to sell my home...which means I may be in the market to finally get a big dob.
Is there any visual difference between a 25" and a 30"? The price is a bit significant so I'd like to 'choose wisely'. I live in a bortle 5 area. Looking at Starstructure.

Also...will living in a Bortle 5 area make it a 'less than enthusiastic' experience?

Thanks,

Joe

That is like 10" vs 12". I've not seen them side by side, but I heard there is a nice difference. Either should so noticeably better views of M51 as aperture increases.

 

A lot may depend on the mirror and structure.

 

 

If you read old posts about what scope sizes are portable, you need to keep in mind that scopes used to be taller and heavier back in the day. You need to research modern scope designs for a better idea.

 

I read that a 4" scope at a dark site is as good as 10" in suburban. And 10" at a darker than rural is as good as 25" in suburban. I'd go with the 10". If both of these are to be observatory scopes, get the 30". If you are transporting it, make sure you can.


Edited by MeridianStarGazer, 23 July 2021 - 06:12 PM.


#16 Tom Stock

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Posted 23 July 2021 - 06:17 PM

That is like 10" vs 12". I've not seen them side by side, but I heard there is a nice difference. Either should so noticeably better views of M51 as aperture increases.

 

A lot may depend on the mirror and structure.

 

 

If you read old posts about what scope sizes are portable, you need to keep in mind that scopes used to be taller and heavier back in the day. You need to research modern scope designs for a better idea.

 

I read that a 4" scope at a dark site is as good as 10" in suburban. And 10" at a darker than rural is as good as 25" in suburban. I'd go with the 10". If both of these are to be observatory scopes, get the 30". If you are transporting it, make sure you can.

There is a difference yes and I definitely prefer the 16 but even between my 10" and 16" the difference is not huge in bortle 5.  I dont think I'd want to set up a 30" for bortle 5 skies.  I am in Bortle 5 and I take out the 10" more than the 16".  


Edited by Tom Stock, 24 July 2021 - 03:37 PM.

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#17 CHASLX200

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Posted 23 July 2021 - 06:31 PM

My avg nite is MAG 3 and the best if MAG 5. Even in MAG 5 skies deep sky is washed out in my 18". Globs are cored out easy, but lack a dark sky background so look lack luster. No wonder i only view planets and i got the seeing for that in spades.  25 and 30" is something i will never have. 18" is the max for my house with a narrow door.



#18 stubeeef

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Posted 23 July 2021 - 06:42 PM

Not sure it is something you wanted immediately, but Camera Concepts lists a used Obsession 30 inch for sale. they note the mirror needs a re-coat. 

 

https://www.cameraco...-stellarca.html

 

they are located on long island. Oddly, the link indicates 25, but the description online is for a 30 inch. 

 

I have the opportunity to get some views through a 24 inch and a 28 inch scope when I travel west to a star party under dark skies, and I think that is where a scope of that size will be best utilized. After looking at objects through those scopes, my 16 inch f/4 which is stored out there seems... dim smile.gif

I don't want a 30 but that's VERY tempting...I'd send the mirror to Ostahowski and have him re-coat it and tell him to make any corrections it might need....I'd still be ahead by $15k



#19 CHASLX200

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Posted 23 July 2021 - 06:47 PM

I could never send a 30" mirror out without thinking it would be lost or broken during shipping.  Just the cost of shipping and coating would cost a ton.



#20 daquad

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Posted 23 July 2021 - 06:54 PM

Difference between a 25" and 30" is the same as the difference between a 5" and a 6".  5log(5/6) = -0.396.  So for stars the 30" will go ~0.4 magnitudes deeper.  Can you tell the difference?  As for resolution, the atmosphere will probably not permit either scope to perform to its theoretical maximum on most nights.  

 

Consider the difference in ergonomics.  If it does not matter to you, then the 30" will show more detail in extended objects when the atmosphere will allow it.  What are your requirements?  Cost?  Main objects to observe?  The 30" will show more stars, but will it be much better for nebulae, or planets, doubt it.  (Keep in mind the difference between a 5" and a 6" scope of the same design.)

 

Good luck in your choice.  Not a bad decision either way.  Many of us would envy your position.

 

Dom Q.


Edited by daquad, 23 July 2021 - 06:56 PM.

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#21 Mike Lockwood

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Posted 24 July 2021 - 11:02 AM

There is a significant jump in the work required to move a telescope when you go from ~24"-25" up to 30".  Many can put a 24" in a vehicle, but few put a 30" in a vehicle, they opt for a trailer or toy hauler instead.  However, all issues can be addressed with smart planning and some mechanical devices to help move it.  Investing in these make your observing more enjoyable and make you likely to bring out the telescope more times each year.  (An easier to set up scope is a more often used scope.)

 

Assuming everything else is equal, increasing aperture shows more detail and more features in deep sky objects.  Things are easier to see, and dark features show noticeably more contrast.  A 30" f/3.0 is a great balance between large aperture and easily-accessed eyepiece height/position, and a SIPS or Paracorr 2 will make the field of view look as good as a slower instrument.


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#22 MeridianStarGazer

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Posted 24 July 2021 - 11:14 AM

Good point. Bigger does not have to mean used less. It just means you need some power tools and trailer. Crane, winch, or such.



#23 George N

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Posted 24 July 2021 - 11:56 AM

I could never send a 30" mirror out without thinking it would be lost or broken during shipping.  Just the cost of shipping and coating would cost a ton.

The 30" in question is available for pick-up on Long Island NY. I'd pick the thing up - do a half-day drive to Ithaca, NY - and drop the mirror off (OK, maybe not "drop") at EMF - they would turn it around in 5 days or so (and would probably even help lift it out of the mirror box) - while I visited the Finger Lakes wineries or went to a race at Watkin's Glen or boating on the lakes or observed the bald eagles at the National Wildlife Refuge or..... Maybe Cornell has their old 19th Cent refractor open or have resumed their Astro Dept's weekly public lectures? Lots of fast running water falls to see right now - thanks to all our rain. Bottom line - there's a good chance here to mix a week's vacation with a 30-inch Dob pick-up and a mirror re-coat at 98% reflectance, no shipping or packing - for around what $600 - including help with getting it out of the telescope. What a deal!!!  ;)


Edited by George N, 24 July 2021 - 12:02 PM.

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#24 George N

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Posted 24 July 2021 - 12:20 PM

Good point. Bigger does not have to mean used less. It just means you need some power tools and trailer. Crane, winch, or such.

Or have a place to store it 10 feet from the observing area - or even in a "Dob-servatory". My friend who owns a 36" is a dairy farmer (now mostly retired) with plenty of room for telescopes - and he also has an 18 and a 22. The guy I know who use to haul an Obsession 36 F/5 around in a big trailer now has it installed in a observatory in Northern NH. I don't think it goes on the road any longer.

 

Personally -- I would not see the goal of owning a 30-incher as being "used as often as possible" -- but rather "pushing deeper into the Universe than ever before" when it is used. I would bet that most 30" owners have a smaller Dob of some sort - or a different scope type - SCT, APO, for your average 2 hour observing session what the sky clears unexpectedly.

 

But -- a 30 is simply not for any but that one in a thousand person with the right location - or maybe a small astro club. 


Edited by George N, 24 July 2021 - 12:22 PM.

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#25 CHASLX200

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Posted 24 July 2021 - 01:54 PM

Or have a place to store it 10 feet from the observing area - or even in a "Dob-servatory". My friend who owns a 36" is a dairy farmer (now mostly retired) with plenty of room for telescopes - and he also has an 18 and a 22. The guy I know who use to haul an Obsession 36 F/5 around in a big trailer now has it installed in a observatory in Northern NH. I don't think it goes on the road any longer.

 

Personally -- I would not see the goal of owning a 30-incher as being "used as often as possible" -- but rather "pushing deeper into the Universe than ever before" when it is used. I would bet that most 30" owners have a smaller Dob of some sort - or a different scope type - SCT, APO, for your average 2 hour observing session what the sky clears unexpectedly.

 

But -- a 30 is simply not for any but that one in a thousand person with the right location - or maybe a small astro club. 

I never had a wish for a scope over 20" let alone i am not that rich to buy a 30" even if i wanted one. Guess if i had my own building in my back yard and lots of money then maybe i would have a 30". I think for planets a 20" F/5 or slower Dob would be all i would ever need.  Not sure what a 30" mirror alone would even cost. Maybe 10k?


Edited by CHASLX200, 24 July 2021 - 01:55 PM.



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