18 replies to this topic

[JHGIII]

I have had the great fortune to view the Southern sky through a 24 inch Obsession in the Atacama desert region of Chile and it was amazing. So I have been bitten by the aperture bug.

 

I know I can get too much house for my needs (did that then downsized and was able to build an observatory).

I could get too much car for my roads. Sure there are cars that go 200 mph and up, but assuming I drive legally, I'd never be able to utilize that potential. Just owning it does nothing for me

I could get a twin screw 60 ft yacht but I live on an inland lake, so there is no way I can use that to it's potential.

 

My question is how may one determine the practical limit for a telescope based on the current level of light pollution and the typical astronomical forecast for one's place of residence. I enjoy astronomy from my rural (green to dark green) skies in Alabama, weather permitting most weekends and occasional week nights. At the new moon, the milky way is easily visible with dark lane from treeline to treeline. The Andromeda galaxy and double cluster in Perseus are naked eye. I use a dew heater every night. Best FWHM that BYEOS shows me most nights is 2.5 to 3. I do not plan on moving to blue or black skies.

 

I foresee working toward retirement in the next ten years and am toying with the ideas of a "lifetime" scope or scopes which I would plan to save for and purchase while still working full time. I like most all parts of astronomy, but am most drawn to DSO's for both visual observation and currently rudimentary DSLR astrophotography. Some nights I am purely visual. Many nights I look through one scope while the second acquires images. I truly enjoy being out in the solitude of the night and am not interested in robotic imaging while I sleep. With that as background, let us for the sake of argument, suspend limits on financial resources but agree that ASSUMING there is a theoretical point at which there is no longer a perceptual difference in image quality (visual or that captured by imaging equipment) then it does not make sense to buy bigger. I also stipulate that I am not going to climb ladders in the dark, so impose a one step limit which will determine the max EP height of a reflector for a 6 ft male. I'll also stipulate that we leave EAA out of the discussion since one could always add that capability later should the mood strike, conditions deteriorate etc.

 

For example, I have an 8 inch mass produced dob and a C11 on a G11 Gemini 2 as my visual scopes. I would love a premium reflector like JP Astrocraft, Webster, Teeter...., possibly with Servocat, but definitely with Argo Navis type computer to help find obscure objects. Under my current skies, would an average observer with say 10-12 years of experience be able to tell the difference between a 16 inch F4-5 and a 20 inch F3 reflector? An 18 and a 20? 24? .... Moving the beast is not really an issue as I have an observatory. Where do the curves cross? I know there will always be exceptional nights of transparency and seeing, but if they are only 1 in 100, it is probably easier to travel on another astro-vacation to seek out black skies with someone else's equipment.

 

From an imaging standpoint, would my skies support a 17 inch iDK or is 12 or even 10 inches really a more reasonable expectation (mount would be adjusted according to needs of scope). Here you may substitute refractors for the imaging scope if you wish (I currently am learning with a SV 90T Raptor). The point is when do my conditions limit my aperture fever for imaging? I don't want to toss every third or fourth sub. I am interested in pretty pictures that I can enjoy sharing with friends and studying myself for colors and details that escape my poor human eyes.

 

It is my hope that there is indeed an externally imposed practical limit to my ability to see and to image that allows me to enjoy both with superb optics but also to rest easy in retirement, not worrying if another inch or two would let me see better, because at that point, I may not be able to move up. Obviously there will be some compromise in the final decision just as there always is. I am just trying to better understand the parameters in order that I have the best chance of maximizing my fun in the future. I look forward to your thoughts.

 

John

[MikeRatcliff]

I do quite a bit of visual observing in rural green/dark green skies in S. California. Roughly 15 years of experience. A 20-22" inch telescope definitely can see more than a 14-16" in rural skies.

 

However, in my drab red/white skies at home, if I were stuck with observing at home, I would not get a 20+ scope. Galaxies are too washed out in such skies. In terms of galaxies the payback with larger aperture is too small. Electronics would be worth exploring.

 

Given your situation and if no money issues, I would strongly consider a 20+ f/3 scope.

 

Mike

[SimonLowther]

You should only be limited by 1) $$$'s and 2) what size you are willing to struggle with; if its too cumbersome you won't use it.

 

From your ideas above I would say yes to all of them, this is not the dress rehearsal.

 

Regards

Si

[Allan Wade]

As for your visual scope question, a 24" f/3.3 is a one step dob, and is a fabulous setup for DSO from reasonably dark skies. That size performs significantly better than the 16"/18" level. I've spent time sharing views between a 24" and my 12" and the difference can't be described.

 

Sounds like you have fairly dark skies, so every step up in aperture is going to be beneficial. You've already imposed an aperture ceiling on yourself by limiting the dob to one step up. That's not a bad thing, as big dobs climb exponentially in price as the size continues to sneak up. So I think something around 24" will meet your requirements as a lifetime big scope.

[JimMo]

As for your visual scope question, a 24" f/3.3 is a one step dob, and is a fabulous setup for DSO from reasonably dark skies. That size performs significantly better than the 16"/18" level. I've spent time sharing views between a 24" and my 12" and the difference can't be described.

 

Sounds like you have fairly dark skies, so every step up in aperture is going to be beneficial. You've already imposed an aperture ceiling on yourself by limiting the dob to one step up. That's not a bad thing, as big dobs climb exponentially in price as the size continues to sneak up. So I think something around 24" will meet your requirements as a lifetime big scope.

Alan beat me to it.  Agree 100% with his post.  My "dream" dob is a 22" f/3 something with a Lockwood mirror set.

[GlennLeDrew]

There is no such thing as `too much scope` (unless perhaps the matter of atmospheric seeing is the determinant.) Every increase in aperture always affords the greater and expected depth of penetration or detail, no matter the sky brightness.

 

For stellar limiting magnitude, 6-inch to 12-inch change affords a gain of 1.5 magnitudes. So does upping from 12 to 24 inches, or from 100 to 200 inches. And whether the sky is urban bright or country dark, the same ratio applies.

 

For extended objects, under a given sky quality the same degree of contrast discrimination applies irrespective of aperture (and as long as the target is resolved so as to be perceived as having area.) And so the larger aperture affords to see objects or details of given contrast that are smaller by the inverse of the ratio of aperture gain. A 12-incher will reveal stuff half the size of that detected through the 6-incher. A 200-incher will show things half the size of that revealed by the 100-incher.

[JHGIII]

Thank you to all for your thoughtful and helpful contributions. I'm pretty sure I understand the " pure math" of the aperture issue. The challenge for me is that I don't know of any nearby opportunity to view through such a scope. I think Glenn's parenthetical qualifier in his opening sentence is the crux of the biscuit. I am at about 700 ft above sea level about 70 ft above a 21,000 acre lake in the very humid deep south. Clear Sky Chart lists my seeing and transparency as average most clear nights, Maybe good but very rarely if ever excellent. I'm not sure of the correct terminology but I think resolution is what I am concerned about. I don't want to see a bigger image that is less clear. That is not worth extra cash to me. 

 

Also most replies have centered on a visual scope, but I do want to image (especially in retirement when I'll have more time to process). Let's hear from the imagers Especially any in the SE USA. 

 

Thank you again

 

John

[jrbarnett]

I have had the great fortune to view the Southern sky through a 24 inch Obsession in the Atacama desert region of Chile and it was amazing. So I have been bitten by the aperture bug.

 

I know I can get too much house for my needs (did that then downsized and was able to build an observatory).

I could get too much car for my roads. Sure there are cars that go 200 mph and up, but assuming I drive legally, I'd never be able to utilize that potential. Just owning it does nothing for me

I could get a twin screw 60 ft yacht but I live on an inland lake, so there is no way I can use that to it's potential.

 

My question is how may one determine the practical limit for a telescope based on the current level of light pollution and the typical astronomical forecast for one's place of residence. I enjoy astronomy from my rural (green to dark green) skies in Alabama, weather permitting most weekends and occasional week nights. At the new moon, the milky way is easily visible with dark lane from treeline to treeline. The Andromeda galaxy and double cluster in Perseus are naked eye. I use a dew heater every night. Best FWHM that BYEOS shows me most nights is 2.5 to 3. I do not plan on moving to blue or black skies.

 

I foresee working toward retirement in the next ten years and am toying with the ideas of a "lifetime" scope or scopes which I would plan to save for and purchase while still working full time. I like most all parts of astronomy, but am most drawn to DSO's for both visual observation and currently rudimentary DSLR astrophotography. Some nights I am purely visual. Many nights I look through one scope while the second acquires images. I truly enjoy being out in the solitude of the night and am not interested in robotic imaging while I sleep. With that as background, let us for the sake of argument, suspend limits on financial resources but agree that ASSUMING there is a theoretical point at which there is no longer a perceptual difference in image quality (visual or that captured by imaging equipment) then it does not make sense to buy bigger. I also stipulate that I am not going to climb ladders in the dark, so impose a one step limit which will determine the max EP height of a reflector for a 6 ft male. I'll also stipulate that we leave EAA out of the discussion since one could always add that capability later should the mood strike, conditions deteriorate etc.

 

For example, I have an 8 inch mass produced dob and a C11 on a G11 Gemini 2 as my visual scopes. I would love a premium reflector like JP Astrocraft, Webster, Teeter...., possibly with Servocat, but definitely with Argo Navis type computer to help find obscure objects. Under my current skies, would an average observer with say 10-12 years of experience be able to tell the difference between a 16 inch F4-5 and a 20 inch F3 reflector? An 18 and a 20? 24? .... Moving the beast is not really an issue as I have an observatory. Where do the curves cross? I know there will always be exceptional nights of transparency and seeing, but if they are only 1 in 100, it is probably easier to travel on another astro-vacation to seek out black skies with someone else's equipment.

 

From an imaging standpoint, would my skies support a 17 inch iDK or is 12 or even 10 inches really a more reasonable expectation (mount would be adjusted according to needs of scope). Here you may substitute refractors for the imaging scope if you wish (I currently am learning with a SV 90T Raptor). The point is when do my conditions limit my aperture fever for imaging? I don't want to toss every third or fourth sub. I am interested in pretty pictures that I can enjoy sharing with friends and studying myself for colors and details that escape my poor human eyes.

 

It is my hope that there is indeed an externally imposed practical limit to my ability to see and to image that allows me to enjoy both with superb optics but also to rest easy in retirement, not worrying if another inch or two would let me see better, because at that point, I may not be able to move up. Obviously there will be some compromise in the final decision just as there always is. I am just trying to better understand the parameters in order that I have the best chance of maximizing my fun in the future. I look forward to your thoughts.

 

John

Bigger under light polluted skies offers the same gains over smaller as under very dark skies.  There's no such thing as too much light grasp under any skies if hunting DSOs, but...

 

It is extremely easy to get too much scope for comfortable and convenient use, storage and transport.  The best scope is always the one that gets used most often.

 

For DSOs especially get the biggest Dob that you think you won't tire of setting up, schlepping around and storing.

 

For me that's in the 12" to 16" range.  If I were retired someplace where I could store a larger scope ready to deploy by rolling it out of a storage structure, I'd go bigger, but given that as often as not I transport my scopes by car and generally disassemble them between uses, 12" to 16" is my limit.  I have one of each - a 12.5" and a 16" and am in debating mode as to which suits my needs and tolerances better.

 

- Jim

[LennyM]

Just a thought, I have found that it is true the saying, "the best scope is the one you will use the most."

If money is not an object I would suggest a scope you know you will use. That includes considering the time it takes to set it up and take it down. Also, it's weight and size, and the complications involved in its use. Another consideration is what objects you choose to observe. If you desire to view faint DSOs than you will want larger aperature. You should consider the the type of sky conditions you will normally experience as that will impact size and type of scope you will use. I think you should consider you age or more so, that you will age and may not want to take out larger scopes and or more complex scope as time goes on.

In all it's hard to say. I would suggest you take time to think over all the above (and of course the other advice given) and be realistic about your expectations.

Good luck

[JHGIII]

Thanks. These type decisions usually take me a year or so and in this case will probably require a trip to something like WSP. I fully agree with the "scope you use" concept. That was my main motivation for building the observatory. I guess viewing height for the dob will be the determining factor. As to imaging, more research and a field trip seem to be in order, but I suspect I'll go smaller aperture here in order to go larger on the dob.

[csrlice12]

I'd look at the plusses and minuses of the scopes you are considering.....with an eye towards which set of "deficiencies" you can live with the best.  Sounds like you may need dew control regardless....

[oldtimer]

For DSOs aperture is king pretty much regardless of light pollution. Planetary and high power double star splitting is a whole different ball game. 'Seeing' or air steadiness is the issue. Here in the midwest seeing is almost never ideal. On most nights my 80mm refractor shows the best planetary detail. Only on very rare nights can my 10" f6.5 newt with custom optics show the detail its capable of. Its interesting that on the 'Clear Sky Clock' home page they mention that their seeing predictions are largely intended for people with scopes of 11 to 14 inches of aperture.

[csrlice12]

I will say this, for many, a 10" Dob is a great all round schlep it around, any bigger they just ain't grabngo...the 10" is one of the best "throw it in the car and go to the dark site at the last minute scopes. Even if you get a bigger one, ther'll be plenty of nights the 10" will be the one you grab.

[tomykay12]

10" is a really nice size, no doubt. In the OP's situation, I can see him with something quite a bit bigger as his stay at home scope. Good thing the Lord gave us so many different scopes.

[davidpitre]

Without a doubt you should get to a star party with big dobs to try them out.

[My 2 Stars]

Without a doubt you should get to a star party with big dobs to try them out.

Mostly everyone here has way more experience than I do and the one thing I have learned about ANY equipment question is that going to a club or star party to try what you are interested in, is the best advice you can get.

 

Heres a similar post recently.

 

http://www.cloudynig...n/#entry7151746

Edited by My 2 Stars, 10 April 2016 - 06:23 AM.

[Pauls72]

For imaging, aperture is not king, it's the mount that is king.

You indicated you where learning AP with a Stellavue 90mm f/7 630mm fl triplet. After you feel comfortable with the 90mm, try moving up to doing imaging with your C11 with a f/6.3 reducer or a Hyperstar. You should be able to get by with your G11 for awhile. That should keep you busy for quite some time. After you have master that, then you might consider moving up. (I assume you have the Gemini Goto on your G11.)

Edited by Pauls72, 10 April 2016 - 10:30 AM.

[Tony Flanders]

On most nights my 80mm refractor shows the best planetary detail. Only on very rare nights can my 10" f6.5 newt with custom optics show the detail its capable of.

I'm suprised that your 80-mm refractor outperforms your Dob on the planets so often. It suggests something wrong with the Dob -- perhaps inadequate cooling.

It is indeed a rare night that my 7-inch Dob performs to its theoretical limits. But it almost always shows vastly better planetary images than my 70-mm refractor. After all, it has more than twice the resolution!

Moreover, even when both scopes are limited to lowish magnification (say 120X), I get to use my Dob at that magnification with a luxurious 1.5-mm exit pupil, whereas with the refractor I'm fighting floaters (and a very dim, grainy image) at an 0.6-mm exit pupil.

[oldtimer]

Tony;

Maine 'ain't'  Chicagoland. Just for laughs I pulled up the seeing chart for the USA today. Nearly all of Maine was rated 'average' seeing. Illinois was almost all below average. The midwest is almost always under the dreaded jet stream. By the way my 5" Mak often outperforms my 80mm

Edited by oldtimer, 10 April 2016 - 05:38 PM.