Question On Size

[Johnny Carter]

I would have thought that I could find the answer to what appears to be a simple question, but it seems I can’t. I have read many articles that state that you can see an object with a medium sized telescope, or a large, sometimes a small. What I am curious about on this is there a set range, and does that range perhaps vary depending on the type of scope? I have seen some pretty big (to me) dobs, but when they say something can be viewed with a large scope, I don’t know if they mean a 16” dob or the Hubble telescope. We do plan to get a refractor at some point so we do a lot of comparisons, we would ideally like to be able to set up 2 scopes with varying objectives, we currently have a 150 Mak, which we have barely got into. Anyway, any advice is appreciated.

[Ylem]

It's kinda subjective, are you reading Star Watch?

Light pollution is the biggest issue, so an 8" at a dark site may show similar that a 10 or even a 12 in the suburbs.

[helicon]

Well Johnny, medium amateur scopes usually are in the 6" to 8" range for reflectors, large is around 12" and bigger. But as Jeff says, an 8" or 10" reflector can outperform a 12" or larger scope if dark skies are enjoyed.

Darker skies are a great equalizer. The noted observer James Muirden used a 4" refractor from very dark skies and was able to document all of the Messier objects easily, even the faint galaxies.

[Johnny Carter]

It's kinda subjective, are you reading Star Watch?

Light pollution is the biggest issue, so an 8" at a dark site may show similar that a 10 or even a 12 in the suburbs.

I’m not sure what Star Watch is but you have given me something to look up. Actually I was looking on Sea and Sky at things that can be seen later this year. I’m definitely not in a dark site at the house but I do a a small place in the Hill Country that is, and I know that the “seeing” is way different. Good thought on that, thanx.

[Johnny Carter]

Well Johnny, medium amateur scopes usually are in the 6" to 8" range for reflectors, large is around 12" and bigger. But as Jeff says, an 8" or 10" reflector can outperform a 12" or larger scope if dark skies are enjoyed.

Darker skies are a great equalizer. The noted observer James Muirden used a 4" refractor from very dark skies and was able to document all of the Messier objects easily, even the faint galaxies.

Gotcha, that was what I was looking for, just a ballpark figure. I know I am not going to see things like the colorized images I see on here, but if I read about seeing something I would like to have a fair idea if it is obtainable. One ice thing about SynScan Pro is that it gives a magnitude scale on objects it shows, very helpful. Thanx.

[Ylem]

Actually I think the book I had in mind was Night Watch, by Phil Harrington I think.

It's a good book, kinda like Left Turn at Orion.

[Bigzmey]

I like when atlases and guide books split targets into small, medium and large telescope groups. But I read it differently. Small scope targets will be easiest to observe with any scope, so I would start my session with those and then try some for medium scope targets and if successful try some for large scopes.

As already mentioned above sky quality, observer experience and optics quality can swing visibility of targets quit a lot. At my dark location I was catching with 4" APO refractor faint galaxies which I was not able to see from light polluted home location with 8" SCT. However, when I bring both 4" and 8" to the dark location, 8" totally dominates.

If you are thinking of adding another scope to your collection for visual observing you should get the LARGEST scope which YOU can COMFORTABLY use. For me that largest scope is 9.25" SCT for Alan is 17.5" DOB, for Terry 6" Newtonian. This depends on your observing style, personal preferences, need to travel, hike to the observing site, etc.

I would put for consideration 6" fast achro refractor like SkyWatcher 150ST

https://www.firstlightoptics.com/startr ... 0-eq5.html

and 8" SCT

https://www.highpointscientific.com/cel ... used-40331

[OhNo]

Bigzmey hits on a very good point, "fast". In AP the faster you gather photons the less likely mistakes can occur. The human eye only gathers light for so long before it sends that info to your brain to recognize what you're seeing.

While large scopes do gather more light but their focal length makes the process "Slow". A f/5 scope will be faster than a f/10. So if we consider the visual astronomer and their eyeball/brain connection a faster scope would benefit, A larger fast scope is great.....

I viewed (but it was difficult) all but 5 Messier objects with a Orion ST-80 refractor. It helped I was doing it from a Bortle 2 (verging on Bortle 1) site. Travel wasn't too bad, right outside my front door!

I should mention that as we age, our eye rods and cones change too. Young undamaged eyes will see better than ole tired ones.

[Johnny Carter]

Thanx all, good info. And about the tired eyes, boy do I know about that. I guess that my scope is bordering on medium where I live, should elevate once I get to the ranch. Like I say, curious, and this all helps.

[TCampbell]

Using telescopes of larger or smaller aperture doesn't necessarily change whether or not you can see a thing ... but rather how much detail you are likely to see.

My first telescope (90mm) could see that Jupiter had "belts" or "bands" and that Saturn hand "rings". But when I got a larger telescope, I could see that the "belts" had texture (clouds) which wan't evident in the smaller aperture scope. Saturns rings had a gap (Cassini division) that wasn't visible in the smaller scope.

Assuming decent optical quality, a larger scope will reveal more detail than a smaller one. But also consider that larger sometimes (but not always) means a longer focal length -- which translates into viewing a smaller piece of sky. There are many extended objects (e.g. open clusters) that don't look very impressive in long focal-length scopes because you are really only looking at part of the (extended) object -- even when using a low-power eyepiece.

My largest scope (which I have since replaced) was a 14" scope with a focal length of around 3500mm. The double-cluster didn't look very impressive in that scope -- because you could not see both clusters entirely within the field of view at the same time. It was a bit too much. The Andromeda galaxy has an angular width about 6x wider than the moon ... and really looks better in a much shorter focal length scope (say ... a 500mm refractor.)

[chris_g]

It's kinda subjective, are you reading Star Watch?

Light pollution is the biggest issue, so an 8" at a dark site may show similar that a 10 or even a 12 in the suburbs.

I’m not sure what Star Watch is but you have given me something to look up. Actually I was looking on Sea and Sky at things that can be seen later this year. I’m definitely not in a dark site at the house but I do a a small place in the Hill Country that is, and I know that the “seeing” is way different. Good thought on that, thanx.

Hi Andy,

As mentioned skies are really a contributing factor. You can check your Bortle scale for your sites at https://www.lightpollutionmap.info You'll need the GPS coordinates in decimal format. I use the Compass Steel app on my Android to get that for my current location. More information for you to ponder as you research scopes!

Clear Skies,
Chris

[Johnny Carter]

Thanx for that link, looks like I am in a class 4 at the house southwest of Weatherford, my place north of Uvalde is a class 2.

[chris_g]

Thanx for that link, looks like I am in a class 4 at the house southwest of Weatherford, my place north of Uvalde is a class 2.

Jealous!!!! My horizon challenged yard is 5.
I'll be getting a Celestron C8-A end of the week. I researched this series for quite a while. They start at 6 inch. Good OTA for both visual and AP...

https://www.celestron.com/products/c8-o ... e-dovetail

[Richard]

Well In RSA in Gauteng . johannesburg is 7 but we have a thing called Load Sheading where power is switched of in certain arear when its mine turn it certainly improves by a huge amount , I could be a 5 or 6 then ? We used to be part owner of a game farm in Laphalale where its 2 fantastic I could honestly see more with a 5 inch SCT than a 10 inch Dobs at home , but its sold now , but we have unlimited access during off hunting period

[OzEclipse]

Hi Johnny,
Lot's of good advice so far, I'll just add a little to what's already been said. Given a dark sky like your Ulvade property, a good 6" scope and young sensitive eyes can see stars down to somewhere between magnitudes 13 - 13.8. But that is because stars are point sources.

When looking at planets, the diameter is the primary factor that determines resolution.
One criteria is RESOLUTION(arc sec) = 4.5 ÷ Objective diameter (inches)

The quality of the optics and the eyepiece will determine whether you will really be able to achieve or even slightly better theoretical resolution.

Nebulae, galaxies, globular and open clusters all have magnitudes. But they are what are called integrated magnitude. The magnitude is the light from the entire surface of the object integrated as though it was a point source. Through the telescope, you observe surface brightness. A small 11th magnitude galaxy might appear brighter in the scope, than a large 10th magnitude galaxy if the magnification is the same for both objects. The very large Rosette Nebula is slightly easier to spot in my 63mm binos than in my 18" reflector.

So it gets difficult to give you a definitive rule of thumb for visibility of deep sky objects. However with time and experience, I have got to the point where I look at the listed magnitude of a deep sky and it's diameter, and I have a bit of an idea whether a particular scope will be able to see it. Yes all the Messier objects were visible in a 4" scope, some will be very faint and indistinct.

An 8" scope is probably the best compromise between good light gathering ability and ease of portability. Deep sky objects really start to pop in 10-12" aperture scopes and in anything bigger deep sky objects are a pure joy to behold. However with each 2 inch jump in aperture 12, 14, 16, 18 involves a large increase in weight, rapid decrease in portability and only small jumps in light gathering. Some designs increase in weight more than others.

I love using my 18", in my backyard at Bortle 2. I don't take it to star parties. If I lived in the city, I'd do better with a portable 10"-12" scope that I could take to dark skies.

Joe