I gotta pop a cap in this thing once and for all. In-light of this recent thread https://www.cloudyni...ral-refractors/ I decided to do this "visual" experiment over the course of the past couple of nights with a few of my observing buddies, DarrenT, David Person and mikona who are also members here on CN. I wanted to gather their opinions to see if they agreed or not and overwhelmingly they were blown away at the results. This is very common knowledge among many seasoned double star observers, yet there's still an astonishing amount of experienced observers who are still under the impression that larger aperture is literally "always" better and that concept just blows me away. I have noticed that whenever I discuss this in the double star forum, many observers understand it and really get it, yet whenever I post this in the refractors forum, It's almost always met with skepticism.
To me there are many myths in astronomy like, how important it is to have dark skies, yet 80% of where I enjoy my observing is in light polluted skies. For example, Alvin Huey and Don Pensack love to observe faint fuzzies and I totally get that. If I was into faint fuzzies more, I'd use a big Dob and find the darkest skies possible to see them. We all know that term "clear skies". Read any of my posts and you will rarely ever see me write "clear skies". My signatures almost always read "steady skies" because most of what I care about is steady air. I absolutely despise bad seeing with a passion. I'll take light pollution just fine, but I absolutely dread bad seeing because everything I love to observe relies heavily on steady skies. Just recently, I was viewing double stars in a really dark sky. At higher magnifications, I actually found it slightly annoying because it makes it harder to frame carbon stars and double stars in the center of the FOV because I can hardly see the field-stop of the eyepiece which I use to frame stars. I totally get it if others want their dark skies, but they're really not needed for everything observers look at and I think there's still a countless number of observers who still don't really quite understand this concept. I often keep ambient lights around me in the yard which are kept on constantly while I'm observing the solar system, Moon and certain stars and that's how I prefer it for several reasons which I've shared in other threads. There are some advantages to these methods and some disadvantages. I know that's hard for countless observers to believe, but it's true.
But, let's get back to those useless smaller apertures so many others are convinced of. Have a look here. How could anyone use a petty 3" telescope? It's not big enough you say, but that's just really bad information when someone gives you that impression. If anything, it merely reflects their own lack of personal experience on what a small lens is actually capable of showing them.
So, let me put this in bold letters so it's actually more clear. There are many instances where increased aperture is actually a hindrance to images. I have a 60mm, 90mm, 102mm, 128mm, 152mm, 250mm. There are countless targets the smaller 60mm refractor will literally and clearly outperform all of the larger apertures. I've sometimes asked myself why Burnham's Celestial Handbook has been such a goldmine to me over these past 25 years and I've come to realize it's because Burnham gave so much attention to individual stars. Robert Burnham Jr. was a 20th century jewel. It's a companion with lists that read like this....
1.) LIST OF DOUBLE AND MULTIPLE STARS
2.) LIST OF VARIABLE STARS
3.) LIST OF STAR CLUSTERS, NEBULA, AND GALAXIES
When I read deeper into that, some interesting thoughts come to mind. The first thing is that I can observe categories 1 and 2 from the convenience of my light polluted skies perfectly fine. The only time light pollution is of a hindrance is if the targets are too faint, but don't forget, there are countless targets that are not too faint or unresolvable in smaller apertures which can be studied and observed. Still, many insist that astronomy isn't enjoyable unless it's under dark skies. For the vast majority of beginners who are primarily looking at showpiece objects like M13 and M31, of course darker skies help, but you have to remember that observational astronomy is not always about deep sky targets like those listed in category 3.
The late great Leslie Peltier who wrote "Starlight Nights" observed over 100,000 variable stars said this about his 6" and 12" achromatic refractors. "Each telescope has its own particular sphere of usefulness. Each one can perform its own specific duties much better than could the other one, so there is really no cause for any rivalry between the two and I, for my part, have always done my best to insure domestic tranquility by alotting them equal observing time". Those were the words of a very wise and extremely experienced observer, so if anyone tells you aperture is a must, ask them if they've ever observed 100,000 variable stars before.
Okay, so let's get back to this aperture thing, shall we? You have a few options. You can take one large refractor and use a couple of different aperture masks as if you had several refractors of different aperture, or if you don't feel like lugging out a big scope and masking it down, then set up a 60mm achromatic or ED lens or a portable 80mm lens. I assure you, you will have a blast if you know what you're doing. The problem is many observers don't understand what targets look best in certain apertures because they're probably just into deep sky showpiece objects and are not familiar with this. The other problem is knowing the limits of your telescope and this is also where the problems stem from. Another issue is countless observers have the incorrect perception that every double or multiple star you want to see has to be the tightest double. Ahhhhhh..... I gotta have a telescope that splits the tightest doubles obtainable and that means its a better telescope for doubles. If you think that, then you're missing the boat. Let's just do some basic stuff. We'll just use the common Dawes limit. 4.56 divided by your aperture in inches.
2" aperture 2.2"
5" aperture .9"
Okay, set your parameters for your 2" telescope. Select doubles that are about 3 arc seconds or more. Just select doubles that are above or close to the Dawes limit of any aperture. Also, for a 2" or 3" glass, select brighter doubles like 0 to 8th magnitude that are typically your showpiece doubles. I used my FS128 which comes with a dust cap that has about a 2" aperture opening that simply unscrews from the center of the cap. The target chosen for this demonstration was Alpha Hercules "Rasalgethi". Here's some info on this double. We looked at about two dozen showpiece objects but I'm just using this one for explaining this. There are many more.
For this comparison we used 104x and 173x with a 10mm and 6mm Ortho and my 1.25" Tak prism diagonal. Seeing looked pretty good but not great because we were in the valley. At 128mm full aperture, Rasalgethi appeared like the images you see here in Pickering 4 and 5. The two stars also looked a bit over exposed, so the colors did not appear as distinct as we would have desired or have seen in the past. In other words, they were slightly washed out from the seeing. The separation and magnitude of these two stars is also within the grasp of each aperture. I then placed the small mask over and the differences were incredible to say the least. Several things changed and I let the guys decide what they liked after we all compared the two images.
1.) With the 2" aperture applied, the airy disks looked larger and clearly more distinct due to the decrease in resolving power. At 5" aperture, the airy disks appeared smaller, brighter, noisier like Pickering 4 and less colorful.
2.) At 2" aperture, all the scintillating was clearly reduced. The brighter component looked like Pickering 8 and 9 while the secondary component looked like Pickering 10 in fact we could only see a very slight diffraction ring around the primary while the secondary was nonexistent altogether. In other words, just two textbook pinballs that looked absolutely breathtaking.
3.) All of us overwhelmingly agreed the colors were clearly more obvious and distinct in the smaller 2" aperture and less over exposed looking like they were at 5".
4.) mikona was especially in awe at this because he was not aware that this is what happens when you reduce aperture for many double stars. mikona also has a SkyWatcher 120ED and when I came over to explain what we were doing, he realized that his telescope also has about the same size aperture opening which can easily be removed from the front dust cap. He later also swung over to Mizar and Alcor with the 2" aperture and dropped his jaw in disbelief at how beautiful and perfectly textbook the images looked.
This then taunts the next question. Many observers will swear stars are more colorful in larger apertures but that's not the only way works. There's a big difference between stars and extended objects like planets and planetary nebula for example. Certainly in those cases, the larger apertures will usually provide you with increased color contrast, but stars are another story. These points of light can often easily appear washed out or over exposed looking in appearance while reducing aperture increases and exposes the airy disk. This in turn makes the stars appear almost immune to some seeing conditions compared to larger apertures. The result is that you can see solid distinct disks with colors displayed more easily. This taunts another question still. Many observers will say that seeing affects apertures the same way. My suggestion is to cut yourself a cardboard aperture mask 80mm to 60mm from a cereal box and see for yourself and you be the judge. I hope others aren't stuck in bad weather, otherwise I'll probably never hear the end of their stat sheets.
Some of the guys laugh when I set up the tiny Takster which is about the size of a fetus, but I can tell you this much, I'll have a pretty good idea what I'm pointing it at and it's highly unlikely that whatever it is will look better in anything larger. Pick your mission and as with all things, everything is relative. Put aside your strong biases and check out some breathtaking showpiece doubles. For me, I love to see clean and crisp textbook images when I can. I'm happy to hear your impressions.
Edited by Daniel Mounsey, 14 September 2018 - 11:12 PM.