Telescope size, magnification, and seeing conditions?

[Dave1]

So guys I need some help?

I'm opening what could be a can of worms here, but I'm trying to get to the bottom of this, so that I can make my next astronomy equipment investment decision wisely.

I'm thinking of getting an 8" Newtonian telescope with 1/10 PTV mirrors. But I'm worried about 8" telescope not being supported by UK seeing conditions and having a bright image with little detail? Which is why I am tempted to keep my Skylight 4" F15. 

I read an article years ago stating the optimum aperture for UK seeing conditions is between 4"-8", obviously 8" is at the upper end of this bracket, which translates to me that it will be pushing it in average seeing conditions.

Am I missing the point? Is it more a case of the chosen magnification being supported by the seeing conditions regardless of aperture?

For instance if the seeing conditions will only support 150x, will it matter if I'm observing with a 80mm at 150x vs 8" at 150x, will the amount of detail appear the same in both? Or will the bigger aperture be bright, washed out, less sharp, and less details? Because of the bigger aperture not being supported by the seeing conditions? ( smaller air cell size than 8" )

David

[John]

I find that my 12 inch dobsonian usually noticeably outperforms my refractors which range from 100mm to 130mm in aperture. On deep sky objects the difference is very striking. On the moon and planets the refractors do very well for their aperture but usually the 12 inch will show finer details.

Your Skylight F/15 is s super refractor (as are mine) but personally I have always found having some additional aperture beneficial on most targets.

 

 

[joe aguiar]

I would prefer a 8” with 150x power vs 80mm at 150x

i don’t live in uk but I guess I can give u my experience 

the 80 will be maxed out almost where the 8” won’t be

also u will see more dimmer detail overal to

or if it’s manilly for the sun moon and planets and double stars a 4 to 5 inch top end apo refractor will also be great cristal views

joejaguar

Edited March 28, 2020 by joe aguiar

[miguel87]

Views will never get worse with more aperture unless the optical quality of the scope is worse. Like for like, more aperture is always a better view.

I have heard similar things about 'optimal aperture' it is nonsense. If it was true, all professional observatory telescopes would be 8 inches or smaller so that they have optimal views!

Edited April 16, 2020 by miguel87

[niallk]

My 15" gives better views than my 10", which is better than my 5"

(... on planets and DSOs.  But, if views at 200x aren't good, it just isn't a good night)

Edited April 19, 2020 by niallk

[Dave1]

See this thread just proves just how subjective this is. There are plenty of examples here and on Cloudy Nights. Also comments from people historically that belong to Astronomy Clubs, where many telescopes have been compared at the same time at the same target. That will say on a night of poor seeing, that a small telescope put up the better image, with the details on offer more readily seen in the smaller aperture. Some science seems to point to this being because of the smaller amount of atmosphere you are looking through in poor seeing conditions. Basically the air cell size would only support a 4" aperture over a 8" aperture.

Since I've started posting here again this year. I have tried to get out under the star once a week. Seeing generally has not been that good, with boiling seen very obviouly in a 3" telescope. The details on offer on the planets not that great. And I've certainly seen far better in a 3". I've done comparisons with my bigger telescope in the past in poor seeing conditions. I certainly don't think it is worth bringing out my 4" in poor seeing, and I have a very fine 4".

I see a lot of comments based around the science of resolution, aperture. Not many comment including the science of seeing conditions. 

David

[ollypenrice]

2 hours ago, Dave1 said:

See this thread just proves just how subjective this is. There are plenty of examples here and on Cloudy Nights. Also comments from people historically that belong to Astronomy Clubs, where many telescopes have been compared at the same time at the same target. That will say on a night of poor seeing, that a small telescope put up the better image, with the details on offer more readily seen in the smaller aperture. Some science seems to point to this being because of the smaller amount of atmosphere you are looking through in poor seeing conditions. Basically the air cell size would only support a 4" aperture over a 8" aperture.

Since I've started posting here again this year. I have tried to get out under the star once a week. Seeing generally has not been that good, with boiling seen very obviouly in a 3" telescope. The details on offer on the planets not that great. And I've certainly seen far better in a 3". I've done comparisons with my bigger telescope in the past in poor seeing conditions. I certainly don't think it is worth bringing out my 4" in poor seeing, and I have a very fine 4".

I see a lot of comments based around the science of resolution, aperture. Not many comment including the science of seeing conditions. 

David

The 'smaller aperture beats the seeing' argument, if it is correct at all, is correct for the resolution of fine detail and will apply to planetary and lunar observing. I have no fixed idea of the validity of the argument and have never done back to back comparisons to test it.

However, it would be absurd to suggest that a small telescope at 150x can compete with a large one at 150x on faint targets - and I rather doubt that anyone has advanced such an argument, have they?

This is one of those threads in which I somehow feel I'm missing something - which I often do!

Olly

[Dave1]

@ollypenrice My observations are planetary, and stars mainly. When I do critical test it is mainly on Jupiter. I generally very rarely venture out of it. I know Olly that you spend a lot of time on fuzzy targets, galaxies and nebular. Which I really have no experience of. So I can't really comment in that regard. My astro equipment really isn't the best choice for fuzzies. Chris Lord of http://brayebrookobservatory.org/ . Has written some very good articles on seeing, air cell size, with the science to back it up. I have to admit one of my weaknesses is advanced math. Somebody may look at the article, I think good, see it differently, if they understand advance math then they might find fault with the articles.

I agree with you on faint targets regarding fuzzy targets in regards to aperture. I would have to guess on faint targets how seeing affects the image at the eyepiece.

David

 

[ollypenrice]

1 hour ago, Dave1 said:

@ollypenrice My observations are planetary, and stars mainly. When I do critical test it is mainly on Jupiter. I generally very rarely venture out of it. I know Olly that you spend a lot of time on fuzzy targets, galaxies and nebular. Which I really have no experience of. So I can't really comment in that regard. My astro equipment really isn't the best choice for fuzzies. Chris Lord of http://brayebrookobservatory.org/ . Has written some very good articles on seeing, air cell size, with the science to back it up. I have to admit one of my weaknesses is advanced math. Somebody may look at the article, I think good, see it differently, if they understand advance math then they might find fault with the articles.

I agree with you on faint targets regarding fuzzy targets in regards to aperture. I would have to guess on faint targets how seeing affects the image at the eyepiece.

David

 

In that case I have nothing to contribute since the aperture/seeing issue lies outside my competence.

With regard to Mr Lord, I'll reply via PM.

Olly

[jjbag]

I can't give any UK advice but prior to living where I do currently I lived in a bortles 8/9 location, 6 years ago i started off with a 130 Meade Mini (5") and it got me hooked enough that a year latter I upgraded to a 10XTI and I will say Light Pollution didn't hamper the upgraded performance that I got. I was able to see object I was unable to see in the 5" and Jupiter and Saturn were OMG views though the 10", I now live in a bortels 4.5 location with the 10" and of course aperture fever is kicking me hard so I'm now thinking of going to a 16 or 18". But for this thread I'd go with the upgrade to the 8".