Question for experienced observers

Nice report Gene. I have had one “Wow” evening with my 6” and it was enough to convince me that really like this size for my location . A larger aperture may be in the works down the road, but for now I’m going to have as much fun as I can with this one. I also have to admit that tweaking the scope is half the fun and there’s lots one can do with a Newtonian.

I have tried to get out as much as possible this year but this terrible weather has taken its toll on me. I actually saw Mars for the first time last week. It was still low on the Horizon (less than 30 degrees) but the seeing was a bit better than average. I got to see the ice cap and I could just make out some markings on the planet. This was with my 127Mak (still waiting for the Newt to return). Not bad for a 36% CO . Now I have to wait for these weather swings to finish. We were at 77F yesterday and down to 32F today!!! This is not good for seeing

Eric

today we have 3 inches of snow on ground and 30 degrees out. i don't mind. it'll be gone tomorrow. i kind of missed the lunat thing though last night. i'm a very patient man. one has to be with my seeing and weather here. what keeps a guy glued to an eyepiece for hours waiting for better seeing that might not come. crazy isn't it.

eric, after going out the last few weeks with smaller refractors and othercopes, i've found that i still see more in larger aperetures in poor seeing . i tried out the , cut through the seeing with quality refractors. at the small exit pupils provided, i didn't see more, what i did see was nicely resolved, but i didn't really see more. even the seeing affected the smaller apereture scopes too. i have gotten sub arc second seeeing here, the 6in reflector and mak were able to provide views that will forever remain etched in my mind, the 8in dob also. so after all this ed ting stuff of trying to cut through bad seeing, i'm findinding out that the mid aperetures scopes work best for me and my eyes. john is right, seeing, apereture, then design . even terrence dickinson said, if you find that at your site a larger scope is normally seeing limited, then buy a slightly smaller one of great quality. at 6 inches, we're still seeing limited, but when that good or great seeing does come, woo hoo. and your first thought is not going to be, i wonder how my central obstruction is affecting my scope, your not going to run in to get another scope to compare your view to the other, no, you'll be firmly planted at your eyepiece saying how could this be. the detail you'll see on jupiter even in a 6in. scope, you can't comprehend it. that's what i wait for everytime i'm out, it'll come again. and you never know when that will be. sunday night was all hazed out here, but warm. the only thing visible other than the moon was acturus, mars and jupiter. i couldn't even see the surface features of the moon well. jupiter provided better detail than i had seen on subsequent nights, but not great, just better. but mars, it was a lot better, i was able to make out the cap easily and some surface markings. in retrospect, i probably wouldn't have even gone out judging by what the sky looked like. but it was 70 degrees out, the air was still and the tree frogs were singing, so out i went. it was a great time being out.

Really that's what it's all about just being out there when your surroundings are all perfect and taking what you can get from the night sky, sometimes great and other times not so ! When it's like that who really cares ?

It's snowing here!

As I said, thanks for a great exchange of ideas.

And yes, this the Beginner forum. But I see that people learn fast!

Now if the clouds could go away...

--Christian

It's snowing here!

As I said, thanks for a great exchange of ideas.

And yes, this the Beginner forum. But I see that people learn fast!

Now if the clouds could go away...

--Christian

It did snow in San Diego but it stopped. That happened when I was 19 years old... I am now 66...

It does snow in the local mountains, generally at our little astro-hideaway we get about 4-5 light snows/year but this year, none..

Jon

It's snowing here!

As I said, thanks for a great exchange of ideas.

And yes, this the Beginner forum. But I see that people learn fast!

Now if the clouds could go away...

--Christian

3" of snow after 1.5" of rain and -3 C for a high today, in a day or to it will all be gone. Time of year !

Denver weather forecast last weekend: Saturday 71* and partly cloudy, with 3-6 inches of snow Overnight and Sunday morning, high of 40*........

I keep reading about the effect of CO (central obstruction)on planetary viewing (visual only) and I often read that the impact becomes significant at a CO above 30% of the aperture by diameter. I keep trying to visualize this impact and when I find information showing this, I can’t help but think that to an unexperienced observer, the impact is negligible. Take for example the simulation provided on this site:

http://www.beugungsb...iffraction.html

When I look at the difference between a non-obstructed scope and a 30% obstructed scope I can’t for the life me see a significant difference between the two. On the next site (below) I see the difference a bit more, but again not enough to make me spend thousand on a 6” unobstructed scope.

http://www.damianpea.../simulation.htm


I find this issue important for beginners such as myself since it’s easy to get carried away with trying to find that perfect instrument and spend thousands of dollars. Until the observer has gained enough experience to appreciate the difference in contrast, thousands of dollars may be spent only to be disappointed in the result. For a planetary specialist, I can understand how the differences in the above sites may be significant, but for a casual observer it may go unnoticed.

So my question to the seasoned observers is the following: Are the images shown on these sites a realistic representation of the effect of CO?

I also realize that seeing conditions come into play since these images assume perfect conditions. But that only reinforces the fact the any difference between a 30% CO scope and an unobstructed scope may go unnoticed to a casual observer.

Thank you for your time
Eric

Eric,
The most important factors for producing a sharp image are:
--collimation of the optics.
--cooling of the optics to near ambient temperature
--the conditions (seeing, especially, but also transparency)
--the quality of the optics
--the ability to attain a sharp focus with the eyepiece and focuser combo.

WAY down the list of important points would be the size of the central obstruction. I have personally looked through excellent and poor examples of every kind of telescope, and the excellent examples gave profoundly good images, regardless of the size of the obstruction.
It's why I always advocate choosing a secondary size in a reflector that is appropriate for deep sky use, even if a slightly smaller secondary might be usable for purely planetary observing. Because the observer will not see the difference, and deep sky viewing will be better with the slightly larger secondary.

I've used scopes with obstructions as large as 40% and seen excellent star images and great lunar images in the scope. Is it possible that another scope with a small (say, 18%) secondary and uber-expensive orthoscopic eyepieces (like ZAO-IIs) might reveal a tiny feature somewhat obscured in the scope with the larger secondary? Sure. Do I care? No. What matters to me is the presentation of a wide image with excellent sharpness, so I can watch that planet drift across a really wide field, staying in sharp focus all the way through. And so I can see the context around the deep-sky object.

People worry a lot about the CO of the scope because they read how bad it is to have one slightly larger when the difference in seeing from moment to moment has much more impact on the image quality. Sigh.
Well, we are a detail-oriented group of hardware geeks, I guess.

So, in answer to your question, if all other factors are equal, and they rarely are, then those images are reasonable simulations. But put the smaller secondary scope in a poorer seeing environment and the larger secondary scope in skies with better seeing, and the more-obstructed optics will perform better.

I see highly variable seeing where I observe. If you do too, concentrate on collimation and cooling of the optics and wait for better seeing. Observe more and you will see more. Regardless of the size of the secondary mirror.

Hi Larry

Mine is an F5 that was designed more for astrophotography than visual, hence the distance from the secondary to the focal plane is longer to provide more back focus. This in turn means that secondary diameter is greater. Using a flashlight test I measured it at 48mm. Using a low profile focuser and extending the tube a few inches I could get it down to 25%, but I doubt I would notice the difference.

Eric

this isn't to be contrary, but the orion scopes have a 47mm secondary also and only a 1.25 focuser. i don't know and understand all the numbers like john issacs does, but it seems as its a standard diagonal size in the synta 6in f5's. i talked with orion last friday about putting 2 in focuser's on their 6 in f8's and f5's. skywatcher has a definite foot up on orion there. does this mean anything, nope, just that even the 1.25 models have same secondary, gene

I don't think focuser size has anything to do with CO % ? It is the area of the secondary in comparison to the area of the primary in % which will give you the CO and in turn, theoretically at least, that gives you a / the restriction, effects on your views. This is just what I have read / can remember as I have never seen or experienced what I can say was CO problems / effects. There are others that are a lot more experienced / familiar with this concern I will say so maybe my numbers aren't all correct ? Most of my experience are with SW brand Newts but I have looked at / reviewed the specs on others during my frequent searches if they were available, sometimes not, we never seem to stop looking and comparing.

The larger the size of field to be illuminated, the larger the secondary needs to be. The optimum secondary size for 1.25" will be smaller than the optimum secondary size for 2".

Thank you for your reply Don. This confirms what I have suspected and hence why toying with the size of the secondary is one mod that I will not be doing. I love to be able to go from low power wide field to sharp high power views with this scope.

Eric

I've used scopes with obstructions as large as 40% and seen excellent star images and great lunar images in the scope.

Have you ever used a 4 inch scope with a 40% Central Obstruction? In general I agree with what you have said but at the same time, the effect of the CO is real and greater than that of the eyepiece, at least in my experience.

It's an easy comparison to make. Take that 4 inch refractor, cut a circle 1.6 inches in diameter and suspend over the objective to provide a 40% CO. Do some comparisons on the planets.

Jon

I've used scopes with obstructions as large as 40% and seen excellent star images and great lunar images in the scope.

Have you ever used a 4 inch scope with a 40% Central Obstruction? In general I agree with what you have said but at the same time, the effect of the CO is real and greater than that of the eyepiece, at least in my experience.

It's an easy comparison to make. Take that 4 inch refractor, cut a circle 1.6 inches in diameter and suspend over the objective to provide a 40% CO. Do some comparisons on the planets.

Jon

2 instruments:
Short focal ratio 6" Maksutov and a Kletsov 8" Maksutov with a sub-aperture corrector. That last one had thick vanes, too, so the coverage may have exceeded 40%.

2 instruments:
Short focal ratio 6" Maksutov and a Kletsov 8" Maksutov with a sub-aperture corrector. That last one had thick vanes, too, so the coverage may have exceeded 40%.

I specifically chose the 4 inch size because at that aperture, you can work at the limits of the optics and quality apo refractors are reasonably affordable. The math says a scope with a 40% CO is giving up a serious amount of planetary scale contrast. I think it shows if one is comparing equal aperture scopes.

Jon

2 instruments:
Short focal ratio 6" Maksutov and a Kletsov 8" Maksutov with a sub-aperture corrector. That last one had thick vanes, too, so the coverage may have exceeded 40%.

I specifically chose the 4 inch size because at that aperture, you can work at the limits of the optics and quality apo refractors are reasonably affordable. The math says a scope with a 40% CO is giving up a serious amount of planetary scale contrast. I think it shows if one is comparing equal aperture scopes.

Jon