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Difference between 25" and 30"

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#26 Codbear

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Posted 24 July 2021 - 02:09 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   

You are correct of course in that squaring the diameter does not give the true are of the mirror, but that is not what is trying to be ascertained.

 

When comparing the increase in light-gathering power, it is easiest (and 100% correct) to compare the ratio of the squares of the diameters of the two mirrors. 

 

Why go through the whole calculation when 30^2/25^2 = 900/625 = 1.44 or a 44% increase.

 

Sam


Edited by Codbear, 24 July 2021 - 02:10 PM.

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#27 sixela

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Posted 24 July 2021 - 02:24 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.   

 

When computing the ratio of two quantities through an algebraic fraction, you can multiply the numerator and the denominator by the same (non zero) real number without changing the result...and that real number can 'happen' to be 4/π, which indeed is fairly close to but not identical to 1.


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#28 aeajr

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Posted 24 July 2021 - 02:32 PM

When computing the ratio of two quantities through an algebraic fraction, you can multiply the numerator and the denominator by the same (non zero) real number without changing the result...and that real number can 'happen' to be 4/π, which indeed is fairly close to but not identical to 1.

I quite agree.   We are not solving for the actual area of the aperture, only the ratio of the areas of two apertures.   Comparing squared diameters is certainly close enough. If we wanted to be even more accurate we would have to subtract the actual areas of the center obstructions which could be significant in this case, but we generally ignore that, especially when comparing two scopes of like architecture.   


Edited by aeajr, 24 July 2021 - 02:34 PM.


#29 Augustus

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Posted 24 July 2021 - 03:30 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 use my 24 pretty much any clear night when the Moon isn't up (which hasn't been often this summer, but normally an average of 4 times a month). If I can get at least 90 minutes of actual observing at my dark site which is 90 minutes away, I consider it worthwhile.

 

I'm contemplating a 32" f/3.5 with a cellular mirror, which would still fit in my minivan and be roughly the same weight, albeit a bit taller. 

 

My grab n' go is a 14.7".


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#30 bjkaras

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Posted 24 July 2021 - 05:48 PM

You gain 0.03 arcsec in resolution, about 44% more light gathering and a magnitude gain of 0.4.



#31 MeridianStarGazer

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Posted 24 July 2021 - 05:56 PM

When I saw M51 at a dark sky site with good transparency in a 20", I was delighted. Finally something close to a Hubble image. It was not that small, but still a bit small. I wanted to see it in bigger aperture. There is a 25" that I need to see it in if this smoke will clear. I bet the mirror is covered in smoke dust though right now.

 

That scope is good for looking at smaller more distant galaxies that don't normally fill up an eyepiece while staying bright. With larger aperture, you can make the view bigger. With the low resolution of the dark adapted eye, that matters a lot. Take the power up, and the bigger aperture stays bright.

 

I'm guessing this will have a sandwich mirror, so that cooling is a non-issue. If so, and you have a good ladder and collimation skills and can transport it and afford it, why not go big? Yes, the difference will be there at a dark site. No difference on planets unless you have exceptional seeing.


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#32 a__l

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Posted 24 July 2021 - 06:59 PM

I think that for galaxies and hydrogen nebulae it is much easier to add to the 25" scope NV. Which I did with my 24" ...



#33 Sky_LO

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Posted 24 July 2021 - 08:51 PM

The axiom "aperture wins" is correct just about all the time !   

 

In the end the math doesn't really matter.

 

What matters is the weight of each piece of the scope and whether you can handle it and how much that aperture increase affects the usability of the instrument for you. 

 

 

-Lauren  


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#34 Keith Rivich

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Posted 24 July 2021 - 08:59 PM

I have spent many years observing side by side with friends that have 36" f/5  and 30" f/4 scopes. I have a 25" f/5. In marginal skies there is not a huge difference in the scopes. I have never ever seen any DSO in their scopes that I could not see in my 25. The difference in apertures does stand out when the skies are very good to spectacular. Here the marginal DSO's disappear in my scope while they are still visible in theirs. The big scopes have a better "wow" factor when observing bright objects.

 

I'm with Lockwood on this: If the scope is permanently placed, like in an observatory, definitely go big. If you plan on hauling the scope in a trailer, like I do, go big. If you have to stuff the scope in a minivan...go small. 

 

FWIW I have been contemplating moving up to a 30 or 32" scope. The increase in aperture is worth it to me. 


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#35 Allan Wade

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Posted 24 July 2021 - 09:03 PM

There’s a guy with a 28” dob near me. It would be one of the finest 28” scopes in the world. Using it under his Bortle 5 sky is an extremely under whelming experience. My 16” at my dark site would outperform it by some margin.

 

So my main tip is get a dob you can comfortably take to a dark site. While a 30” class dob can be moved, the novelty would wear off real fast and it would stay stuck at home. Getting a dob that big for Bortle 5 observing is a waste of time.

 

I was going to get a 24” dob plus trailer which I think is a viable option for regular dark sky trips. I only went for a 30” class dob after buying a dedicated dark site where I could leave it permanently. 
 

I remember a star party where we had a couple of 32” dobs setup next to a 24” and a 25”. By the time you get to 24” in relatively dark skies, everything looks great. But I was amazed how significantly better galaxies and planetary nebula looked in the bigger dobs. 


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#36 MeridianStarGazer

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Posted 24 July 2021 - 09:52 PM

An 80 inch won't fit M51. There are other galaxies to look at instead, and much more of them. But 80" is no longer for beginners. The messier objects are too big for the view, I heard. Though I don't know what eyepiece that person was using.

 

A 5mm exit pupil at 300x is 5 feet, or 60". 17mm Ethos. 1/3 degree fov. With a 7mm you could fit stuff in the 80".



#37 John Fitzgerald

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Posted 24 July 2021 - 10:20 PM

80 inch?  We were discussing a 25 inch vs 30 inch.  An 80 inch is still a professional class observatory instrument costing millions.


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#38 sopticals

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Posted 25 July 2021 - 03:39 AM

Can't give comparisons of 25" v 30", but I have 25" and 33" and there is definite difference, especially on the "dim stuff", and also of more interest to me the "brightening" of the usual

DSO like the well known globs etc. Of course a 33" provides a 74% light increase over the 25" which is substantial (compared to the 44% a 30" would provide). Unfortunately I may have

to "move" both these larger dobs on due to recent loss of dark sky (LED street lights have moved into my locality). A 30" and up to maybe 36" could still be practical if you have shed space

and scope is on wheels. I have a wheeled ground board so am able to push the 33" out to observing point and back (back into the shed is a little more difficult due to gravel to shed concrete

floor 2" step). The 33" probably doesn't weigh more than my classic 1980 GS1100 m/cycle but small wheels on dob are not so "forgiving" when "rolling". Will add photo of dobs,(note the

wheels on 33" ground board.)

 

Stephen.(45deg.S.)

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#39 CHASLX200

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Posted 25 July 2021 - 05:44 AM

Looking at deep sky in city skies is like driving a 900hp car at 30mph. I am sure a 25" or 30" F/5 or 6 would be great for the planets. But i am gonna need a much bigger house. A 18" is as much as i want to handle and even have trouble getting it out the door that it just sits most of the time well until prime time for planets in about 2 months.

 

I want a scope i can roll out all setup easy and a longer FL like F/6 not F/4.5. A 16" f/5.5 would be my choice for my door.


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#40 George N

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Posted 25 July 2021 - 10:09 AM

Looking at deep sky in city skies is like driving a 900hp car at 30mph. I am sure a 25" or 30" F/5 or 6 would be great for the planets. But i am gonna need a much bigger house. A 18" is as much as i want to handle and even have trouble getting it out the door that it just sits most of the time well until prime time for planets in about 2 months.

 

I want a scope i can roll out all setup easy and a longer FL like F/6 not F/4.5. A 16" f/5.5 would be my choice for my door.

Live in a big city? Want a 30-inch? John Dobson taught us the way!!  Get an old school bus almost ready for the dump - remove 2/3 of the seats to open up the back - install a ramp in the tail  to get the scope in/out -- head with your friends to the high country national parks in 'observing season', use your 6" in the city during the winter to look at Saturn and maybe M-42.  If you get caught in a high-country snow storm -- pull over and sleep inside the telescope tube!  wink.gif

 

For a planetary observer - I agree that it would be hard to believe that anything bigger than a 20" in the F/5 to F/6 range is needed - and even that is pushing it.

 

We've had some very experienced observers tell us in this thread - yes, under good sky, a 30" will show more than a 25" - especially if your goal is to "push" beyond what you have seen before. Is the better view of 'show objects' like the Messier list improved in a 30? Probably a little - but I always wonder when seeing a 30+ incher pointed at something like M-27 -- unless of course at a star party and the owner has a long line of APO/SCT owners waiting to be impressed! wink.gif

 

Can using "NV" catch up? It will certainly show a lot more in the 25 if that's 'your thing' -- but it will show even more in a 30. I've looked thru Al Nagler's electronic gizmo on a 36"  --  very impressive -- so impressive that Al was practically dancing around in joy! It also was 'impressive' on a 90mm APO 20 feet away - but nothing like the 36.

 

So -- a potential 30" owner is back to the old questions -- can I get it under good sky (darkness and seeing)? can I transport it if need be? storage? can I climb up to the high eyepiece all night, 4 nights in a row and still take it apart to go home? - am I healthy enough to use such a beast? maybe the biggest -- do I have the $$$???? Very few will answer 'yes' to all those questions -- and they are the lucky few who should have a 30-inch...... that the rest of us can maybe look thru at the next star party.

 

Personal note - of limited value to readers here - my NI (natural intelligence system) went thru this exercise a while back and concluded - I could handle, live with, and afford a 22" to 25" sub-F/4 Dob of the 'modern design' made by one of the top say 5 or 6 pro makers -- but -- how much longer will my health hold out? My final decision was to stick with the old familiar 20-inch Dob size - re-imagined as a F/3.5 and in the 100 lb weight range. So -- now I have extra $$ to spend on more imaging gear!! Tak e-150 on a G-11 mount???  What works for me certainly will not work for you!

 

Anyone with a 30-inch -- message me your phone # and when I can come over!  wink.gif


Edited by George N, 25 July 2021 - 10:39 AM.

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#41 Starman1

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Posted 25 July 2021 - 10:26 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

This is a ratio of 156.25 / 225, so the 30" has an area increase of 44%

That is almost exactly 0.4 magnitudes deeper.

Will that yield a significantly deeper reach?  Not really.

It's the difference between an 8" scope and a 9.6" scope and not too many people would think that was a significant change.

Larger aperture is larger aperture, but in this case, a significant difference would be to gain a full magnitude, and that is to a 40" scope.

25" gains a full magnitude over a 16", but the next equivalent jump is to 40".

Will you transport the scope to darker skies?  If not, and the scope will be permanently mounted in an observatory, then go as big as you can afford.

Just be aware the 25" at a darker site will outperform a bigger scope at home, so if the 25" can be transported, the answer is obvious.

[a friend's 32" at a dark site radically outperforms the 60" on Mt. Wilson, for an example]


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#42 grzesznypl

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Posted 25 July 2021 - 08:23 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.

 

As mentioned before, very large telescopes are notoriously know to preform poorly in light polluted skies. Bortle 5 is far from being ideal. For that kind of environment 14"-16" telescope would be much more suitable. 

Secondly, the price difference you mentioned ($3000) is for structure only and does not take under consideration primary and secondary optics. The real difference between 25 and 30" telescope will be north of $8-10k. 



#43 sixela

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Posted 26 July 2021 - 08:24 AM

As mentioned before, very large telescopes are notoriously know to preform poorly in light polluted skies. 

There are a lot of myths about that. They don't "perform [more] poorly", they just don't beat the physics and the difference between a smaller scope and a larger one diminishes (while the cost doesn't, so the cost/benefit ratio worsens). But they don't "perform poorly" in the way that they'd show less than a smaller scope in a Bortle 5 sky.

The point is that if you buy a large scope at the expense of being unable to move it to a better sky you're doing yourself a disservice. Because a slightly smaller scope in a Bortle 2 sky will very likely run circles around a much larger one in a Bortle 5 sky... 


Edited by sixela, 26 July 2021 - 08:24 AM.

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#44 bujin9

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Posted 26 July 2021 - 12:58 PM

Thanks all for your great input.

Augustus (Zane)....I'm not to far from you (in New haven area)...sounds like you like your 24 and are looking to go larger which gives me hope that this won't be a wasted effort (including cash)....especially cause I know your sky conditions in the Stamford area. The hope is that the scope will stay built in my garage and possibly just roll out to my driveway for viewing.



#45 Augustus

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Posted 26 July 2021 - 01:46 PM

Thanks all for your great input.

Augustus (Zane)....I'm not to far from you (in New haven area)...sounds like you like your 24 and are looking to go larger which gives me hope that this won't be a wasted effort (including cash)....especially cause I know your sky conditions in the Stamford area. The hope is that the scope will stay built in my garage and possibly just roll out to my driveway for viewing.

I don't observe at home, ever. Typically I drive up to Litchfield or Lake Taghkanic - anything worse than mag 21 skies are a waste with big scopes. 



#46 Nick Dangerr

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Posted 26 July 2021 - 01:54 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   

Actually, if you're going to take a ratio, it doesn't matter whether you square the diameter or square the radius (as long as you are consistent - if using diameters the x2 factors out: 2^2 / 2^2 = 1) (also a math major smile.gif ) - but yes, if you do the arithmetic right it is 44% (30^2 / 25^2) = 900 / 625 = 1.44


Edited by Nick Dangerr, 26 July 2021 - 01:58 PM.


#47 Starman1

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Posted 26 July 2021 - 02:09 PM

And 0.4 magnitudes is 2.512 0.4 

which is 1.445x, or roughly 44% increase, which is how much deeper a 30" sees than a 25".

 

I gain 3.7 to 4 magnitudes by transporting my scope to a dark site.

It makes the difference between a 25" and 30" seem like peanuts.

Peanuts except for weight, cost, and difficulty in use.

My friend's 32" f/3 is easier to use than many large scopes, but it's still a massively-large scope.

In contrast, a 25" f/3 would require only a 1-step stool to use.

And be a heck of a lot easier to transport.


Edited by Starman1, 26 July 2021 - 02:10 PM.


#48 Keith Rivich

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Posted 26 July 2021 - 03:54 PM

There are a lot of myths about that. They don't "perform [more] poorly", they just don't beat the physics and the difference between a smaller scope and a larger one diminishes (while the cost doesn't, so the cost/benefit ratio worsens). But they don't "perform poorly" in the way that they'd show less than a smaller scope in a Bortle 5 sky.

The point is that if you buy a large scope at the expense of being unable to move it to a better sky you're doing yourself a disservice. Because a slightly smaller scope in a Bortle 2 sky will very likely run circles around a much larger one in a Bortle 5 sky... 

I concur.

 

Our 36" at the George Observatory is in in pretty bright skies. My 25" in dark skies will go much deeper. What the 25" cannot exceed is the resolution of the 36". On bright DSO's, such as M57, even in the light pollution, the 36" will blow you away. On a 15th magnitude smudge of a galaxy, not so much. 


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#49 Sky_LO

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Posted 26 July 2021 - 06:39 PM

Folks seems to be enjoying displaying their math skills as well, and I feel like the lowly target. 

 

The only reason I responded with some math is the 60% increase mentioned....seemed too high.

(and it was).   Kudos to you if you have better / new / easier math, enough already though ok (wimper, wimper, uncle) ? 

 

-Lauren 


Edited by Sky_LO, 26 July 2021 - 06:42 PM.


#50 CHASLX200

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Posted 26 July 2021 - 07:18 PM

I would love to give some 30" and bigger Dobs a go at planets in my steady seeing.


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