TheSkySearchers

For AP larger aperture helps with exposure time and S/N of course. You can find sites where the microclimates favor good seeing. Site surveys to assess seeing are SOP for the big guys.

Interesting topic. I too have contemplated getting a 10" f4 Newt.

I just realized that all the observatories in my area (near sea level) max out at 14" apertures. I guess it's pretty obvious why large telescopes are placed way up in the mountains - to get rid of as much of the pesky atmosphere as possible. Unless I move to that mountain in Arizona(???) I guess I'll stick with my 8" Newt. In another thread I posted on my first use of my club's Mak-Cassy. The relevant line that in hind sight I find amusingly relevant to this thread is the fact that after waiting 5 minutes I had NO moment of clarity due to atmospherics and gave up (I had more AP images to take).

Thanks nFA for your insight.

notFritzArgelander wrote: ↑Thu Oct 01, 2020 4:01 pm For AP larger aperture helps with exposure time and S/N of course.

That is true. For a second I was thinking for example, a 12" F/4 image would lead to the same result as a 8" F/4 cropped to that same section of the sky by a factor 8/12. The resolution of those images is the same indeed, but the total illumination of that sky section is not. Assuming even illumination and everything else being equal, the 8" will only have received (8/12)^2=4/9 of the photons on the section of the sensor depicting that section of the sky than the 12" did. The same per-object argument holds for visual so that's why a big aperture (for the same F ratio) is still better.

So there's still a good reason to go big - you won't gain resolution but you will gain illumination (per object, not over all) so the SNR of the image is better and you can go deeper. Many galaxies are small so you would want to crop images taken by a smaller aperture scope, and cropping to a given object will work much better for larger apertures. And of course on those rare nights of good seeing if you could pick them and be at the right place, you will indeed gain resolution. Let me know if I mistook.

So I may have to pull the trigger for a 12". Newts are so cheap compared to other scopes, and work really well with a good CC (see Kathy's results for instance). I have a 127 mm F/7.5 Apo but my favorite scope is the 6" F/4.8 Mak-Newt by far. Having to expose 2.5 times longer is rather significant. Aside from that I can't tell difference in the views, the Mak-Newt in fact seems more crisp but of course the illumination is also better. I prefer the Mak-Newt for visual and AP so maybe it's time to get a 12" Newt and dispose of a bunch of scopes including my Z12.

SkyHiker wrote: ↑Thu Oct 01, 2020 6:37 pm

notFritzArgelander wrote: ↑Thu Oct 01, 2020 4:01 pm For AP larger aperture helps with exposure time and S/N of course.

That is true. For a second I was thinking for example, a 12" F/4 image would lead to the same result as a 8" F/4 cropped to that same section of the sky by a factor 8/12. The resolution of those images is the same indeed, but the total illumination of that sky section is not. Assuming even illumination and everything else being equal, the 8" will only have received (8/12)^2=4/9 of the photons on the section of the sensor depicting that section of the sky than the 12" did. The same per-object argument holds for visual so that's why a big aperture (for the same F ratio) is still better.

So there's still a good reason to go big - you won't gain resolution but you will gain illumination (per object, not over all) so the SNR of the image is better and you can go deeper. Many galaxies are small so you would want to crop images taken by a smaller aperture scope, and cropping to a given object will work much better for larger apertures. And of course on those rare nights of good seeing if you could pick them and be at the right place, you will indeed gain resolution. Let me know if I mistook.

So I may have to pull the trigger for a 12". Newts are so cheap compared to other scopes, and work really well with a good CC (see Kathy's results for instance). I have a 127 mm F/7.5 Apo but my favorite scope is the 6" F/4.8 Mak-Newt by far. Having to expose 2.5 times longer is rather significant. Aside from that I can't tell difference in the views, the Mak-Newt in fact seems more crisp but of course the illumination is also better. I prefer the Mak-Newt for visual and AP so maybe it's time to get a 12" Newt and dispose of a bunch of scopes including my Z12.

I used to have the same Mak-Newt that you have. It's a very fine performer indeed! I haven't used my Z12 since moving. It was damaged in the move and is repaired, but I can't seem to manage it as well as I did. I've got a brand new hernia.

It is interesting how a bit of truth and a bit of logic can be moulded into folklore….you just need the right number of people with big enough “halos” to repeat it and then it becomes a “undisputed fact”.....at least a respectable publication like “Sky and Telescope” gave us some objective information. Their article have given me a better understanding of what exactly “seeing” is, without overcomplicating the subject.
Enjoy : https://skyandtelescope.org/astronomy-e ... he-seeing/

Piet Le Roux wrote: ↑Fri Oct 02, 2020 8:18 am It is interesting how a bit of truth and a bit of logic can be moulded into folklore….you just need the right number of people with big enough “halos” to repeat it and then it becomes a “undisputed fact”.....at least a respectable publication like “Sky and Telescope” gave us some objective information. Their article have given me a better understanding of what exactly “seeing” is, without overcomplicating the subject.
Enjoy : https://skyandtelescope.org/astronomy-e ... he-seeing/

This link is thorough and correct enough and is in complete agreement with everything I have been saying in this thread. In particular I like the distinction between slow and fast seeing that the author discusses. There are two pieces of "folklore" that need to be cast out because of bad logic.

One is "small telescopes are not more affected by seeing than large telescopes". The other is "small telescopes are better at resolving features than large telescopes in poor seeing. As stated both bits of folklore are oversimplified to the point of being false.

When the atmosphere is slow seeing bad extra aperture provides zero improvement. The atmosphere is the limiting factor. Thanks for an excellent link that is fully acceptable.

Very often those "folkloristic factoids" are just oversimplified rules of thumb to beginners, that probably couldn't (yet) understand a further elaboration. They are often meant as quick pieces of advice for some first timers trying to choose their first telescope.

turboscrew wrote: ↑Fri Oct 02, 2020 6:16 pm Very often those "folkloristic factoids" are just oversimplified rules of thumb to beginners, that probably couldn't (yet) understand a further elaboration. They are often meant as quick pieces of advice for some first timers trying to choose their first telescope.

Exactly. Often the facts that 1) results depend on the object you are observing and 2) the purpose of observation matter too! Resolution is only one of a telescope's functions. For low power DSO viewing seeing and resolution are often irrelevant.

*** Moved topic to new subforum.****

Hi Piet, nFA and all. A very interesting thread here. I wanted to add a couple of items on the affects of atmospheric spherules (cells) on the aperture size and magnification.
The first explanation is an excerpt from The Night Sky Through Your Telescope (2006) by Robin Scagell: Next, is an article from Sky & Telescope (March, 2015) by Thomas Dobbins. I was going to cut and paste parts of this, but the entire writing, although it repeats some of what has already been said on here, still includes a lot of interesting information: I hope this is also useful to all of you.

I have apreciated this fine discussion.
I generally choose the scope/aperture I will deploy based on my evaluation of "sky conditions".
For me it is a matter of efficiency.

Makuser wrote: ↑Sun Oct 04, 2020 5:27 pm Hi Piet, nFA and all. A very interesting thread here. I wanted to add a couple of items on the affects of atmospheric spherules (cells) on the aperture size and magnification.
The first explanation is an excerpt from The Night Sky Through Your Telescope (2006) by Robin Scagell:

Next, is an article from Sky & Telescope (March, 2015) by Thomas Dobbins. I was going to cut and paste parts of this, but the entire writing, although it repeats some of what has already been said on here, still includes a lot of interesting information:



I hope this is also useful to all of you.

Very good reading to me! Thanks!

From Telescope Optics.

FIGURE 77: Illustration of a point source (stellar) image degradation caused by atmospheric turbulence (linear pattern size, identical f-ratio). Left column shows best possible average seeing error in 2 arc seconds seeing (r0~70mm @ 550nm) for four aperture sizes. The errors are generated from Eq. 53-54, with 2" aperture having only the roughness component (Eq. 54), and larger apertures having tilt component added at a rate of 20% for every next level of the aperture size, as a rough approximation of its increasing contribution to the total visual error (the way it is handled by the human eye is pretty much uncharted territory). Columns to the right show the possible range of error fluctuation, between half and double the average error. Best possible average RMS error is approximately 0.05, 0.1, 0.2 and 0.4 wave, from top to bottom (the effect would be identical if the aperture was kept constant, and r0 reduced). The 2" aperture is little affected most of the time;  4" is already mainly below "diffraction-limited", while 8" has very little chance of ever reaching it, even for briefly. The 16" is, evidently, affected the most; D/ro ratio for its x2 error level is over 10, resulting in clearly developed speckle structure (magnification shown is over 1000x per inch of aperture, or roughly 10 to 50 times over the practical limits for 2"-16" aperture range, respectively. Also, since the angular blur size is inverse to the aperture size, the x2 blur in the 16" and 2" aperture are roughly of similar size angularly).

Late to the party, but maybe it does not matter.

The pic, as posted above by Gabby, is a very good explanation of what can be seen through the eyepiece.

Now, the problem with the models of seeing, as in the S&T is, that the turbulence is better understood with the light waves and with the Fourier optics than with the light rays. In plain language, the turbulence is something what happens in the frequency domain.

In the low frequency limit of turbulence eddies in a thin atmospheric layer,
the Airy disc will be dancing across a low number of arcseconds. So, the planet or double star will basically stay sharp, but randomly moving within some angular range.

Increasing the frequency, or alternatively the thickness of the atmospheric turbulent layer,
the visibilty of the first diffraction ring will drop, and the Airy diffraction pattern slightly flattens.

At an even higher frequency of the turbulent eddies and a thicker turbulent atmospheric layer,
the Airy diffraction pattern will be flattened loosing much of its visiblity. It turns slowly into a blur.
...
And at the high frequency limit you won't know what happens, as you will stay at home.


Masking the telescope aperture you can't increase the visual resolution when viewing through a turbulent atmosphere.
However, you can make your observing session more comfortable, and you can improve the visibilty of the diffraction ring(s).

As the turbulent atmospheric blur is polychromatic, and as it increases with the wavelengths of light decreasing,
you can help yourself using the narrow or medium passband interference filters. A typical example is the red H-Alpha 35nm passband filter for the Moon, when viewed during the day light. And similarly, the Solar Continuum Filter (545nm 10nm passband) for observing the Moon during night.

Best,
JG

It’s never too late for good advice.

Thanks for all the different viewpoints, advice and information, unlike some members I don't perceive that as disrespectful. At this stage I am not in the big aperture game anymore, having sold my 15" Obsession, so at this stage my all efforts would be concentrated to my 8" SCT's for now.....

Piet Le Roux wrote: ↑Tue Apr 13, 2021 6:27 am Thanks for all the different viewpoints, advice and information, unlike some members I don't perceive that as disrespectful. At this stage I am not in the big aperture game anymore, having sold my 15" Obsession, so at this stage my all efforts would be concentrated to my 8" SCT's for now.....

Good choice! Best compromise between aperture, weight and portability.

Makuser wrote: ↑Sun Oct 04, 2020 5:27 pm Hi Piet, nFA and all. A very interesting thread here. I wanted to add a couple of items on the affects of atmospheric spherules (cells) on the aperture size and magnification.
The first explanation is an excerpt from The Night Sky Through Your Telescope (2006) by Robin Scagell:

Next, is an article from Sky & Telescope (March, 2015) by Thomas Dobbins. I was going to cut and paste parts of this, but the entire writing, although it repeats some of what has already been said on here, still includes a lot of interesting information:



I hope this is also useful to all of you.

Hi Marshall,
Only just saw this thread. Thanks for posting those articles. Interesting reads. I have forwarded the 2015 article to some of my local visual observing colleagues.
Joe