Telescope resolution

[jetstream]

Well I have been pondering things here trying to understand some things.It is said that greater aperture gives more resolution(at a given wavelength of light) beacuse as aperture increases the Airy disk gets smaller.I get that part.

Is resolution also dependent on light gathering ability?If I took my 10" reflector and filtered the light down say 50% would it still maintain the same resolution?I would expect it to hurt DSO,but bright objects like planets?

[rl]

Resolution depends only on aperture and wavelength (assuming an unobstructed light path and decent seeing, optical quality to meet the Rayleigh criterion.). You could add a neutral density filter to cut down the intensity without compromising the resolution. Very useful on the moon..

RL

[russ.will]

As I understand it; you do loose angular resolution by stopping the scope down, but you gain in the stability of image. The smaller column of atmosphere you are now looking through, will be occupied by less turbulence. The other advantage of stopping down a dob with a sub aperture mask, is that if you make the aperture no larger than the distance from the edge of your secondary to the edge of the OTA, you will remove the contrast smearing diffraction spikes of the secondary spider vanes by firing the aperture between them.

Thus, you theoretically get a dimmer, less detailed image, but it's more stable and higher contrast, which will help differentiate things like subtle colour differences in cloud banding. As you have also stopped the scope down without changing the focal length, you have increased the focal ratio making giving you EPs an easier ride too.

I made a mask with multiple apertures and a rotating mask for my SW 300p FlexTube and the difference was quite marked on most nights. But and it's a big BUT; on the rare nights where the atmosphere was truly stable, there was little to match the staggering detail of the unfettered full aperture. I think that night happened once in the year that I had it though.

Russell

[jetstream]

Thanks for the replies,I'm playin around here with some stuff and its working.Thanks to a friends advice I made a weird shaped aperture mask giving about 4.7" or so clear aperture,and eventhough my Airy disk is not a disk-it really worked on Jupiter.My VX10 gives fantastic views but gets "light swamped" at times.I have had great views before darkness with my single polarized filter and a bit later without@ full aperture during clear sky but mediocre seeing.When seeing is excellent the unmasked dob is awesome.I thought this strange shaped mask would never work,but it does.Now to try something else.

[russ.will]

If the strange shape is effectively a quarter circle cutout between the vanes and secondary, I found a smaller circle worked better - Cardboard is your friend when prototyping! Just my 2p.

Russell

[jetstream]

If the strange shape is effectively a quarter circle cutout between the vanes and secondary, I found a smaller circle worked better - Cardboard is your friend when prototyping! Just my 2p.

Russell

Exactly what it is Russell.I can use a 90mm circle in there which I may try....or buy a 120Ed after I sell my 90mm!lol!I bet the almost 120mm round mask you made worked well in the 300mm,have you compared that to a 120mm class refractor?

[John]

With my 12" dob, the max aperture I can use without the secondary or a secondary support vane intruding, is around 11cm. Having tried such a mask I felt the views were very much like you get with an apochromat 4" refractor. 

Very nice but I do already have 4" and 4.7" ED refractors 

[jetstream]

With my 12" dob, the max aperture I can use without the secondary or a secondary support vane intruding, is around 11cm. Having tried such a mask I felt the views were very much like you get with an apochromat 4" refractor. 

Very nice but I do already have 4" and 4.7" ED refractors 

John I'm  wondering how good the 120ED is for planetary?I see that you have the big Istar mounted,how do they compare?I have been getting some awesome views with the 10" but at times it seems as if there is too much light gathered for the seeing conditions.My 90mm is great for white light solar and would presume the 120ED would match or best it?

[John]

John I'm  wondering how good the 120ED is for planetary?I see that you have the big Istar mounted,how do they compare?I have been getting some awesome views with the 10" but at times it seems as if there is too much light gathered for the seeing conditions.My 90mm is great for white light solar and would presume the 120ED would match or best it?

My ED120 bests my Vixen ED102 on most things apart from maximum field of view where the faster focal ratio of the Vixen wins. I've not been able to use the big Istar enough to compare the views with the ED120 to be honest.

[jetstream]

Thanks John,I may consider a 120mm class refractor,but need better cool down times,more light gathering power and more resolution than the 90mm Raptor provides.I also want to keep the white light views I'm getting with the 90mm Raptor,they are very good.It could be I just need some more seat time with the VX10 to maximize its potential.My really clear sky doesn't coincide with great seeing a lot though....

[AlexB67]

Gerry could be with your pristine transparency and ultra dark skies you often mention may be as much a hindrance in a way.

I suppose I am only trying to make myself feel better by looking at some positives  in light polluted skies, for planetary viewing it may not actually be so bad with the reduced brightness I am seeing.  It is easy if I don't want to dark adapt properly, one look over my fence and there is some street light.   

A colour filter maybe even a polarising filter to reduce brightness perhaps in the 10 inch would help you ?

[John]

Gerry could be with your pristine transparency and ultra dark skies you often mention may be as much a hindrance in a way.

I suppose I am only trying to make myself feel better by looking at some positives  in light polluted skies, for planetary viewing it may not actually be so bad with the reduced brightness I am seeing.  It is easy if I don't want to dark adapt properly, one look over my fence and there is some street light.   

A colour filter maybe even a polarising filter to reduce brightness perhaps in the 10 inch would help you ?

Interestingly, I had the best views of Mars I've ever had last night during a period when I was effectively clouded out with a thin cloud layer. I could see Mars shining through it so I turned my 12" dob onto it and saw amazing clarity in the sharpness and contrast. Then the cloud layer thickened up and Mars faded away. Just for around 40 mins though everything must have just fallen into place for Mars viewing.

[AlexB67]

Interestingly, I had the best views of Mars I've ever had last night during a period when I was effectively clouded out with a thin cloud layer. I could see Mars shining through it so I turned my 12" dob onto it and saw amazing clarity in the sharpness and contrast. Then the cloud layer thickened up and Mars faded away. Just for around 40 mins though everything must have just fallen into place for Mars viewing.

I had that happen earlier in the week or before, do not exactly recall when, mist and clouds. I actually noticed that as soon as the sky cleared again, the melting pot and unsteady seeing returned as well.

It is amazing how much cloud you can cut through with moon and bright planets. It has happened to me a view times now were my eyes were glued to the eyepiece for a long period not realising how cloudy it had got. All of a sudden the view got very dark and it was fully clouded when I looked up. I had some of the best views with the slightly darker seeing and slight clouds as well. 

Happen to me on Jupiter as well a few times this year.

[jetstream]

John and Alex-I do think my skies that are good for DSO hurt me on the planets at times,washing out some contrast on Jupiter in particular.In light cloud,with steady skies this helps me as well.During late evening viewing while there still some light that can be polarized with the filter(Baader single polarizer) the views can be much better than later on when the darkness enhances Jupiters brightness.I have to say this,at my dark site Jupiter can bug my dark adaptation when DSO hunting,but from my house its bright but with light pollution-however the sky transparency and seeing conditions would be the same.

Mars was great here last night,the brightness of it doesn't have the same effect Jupiter does on my viewing.I guess I'm looking to maximizing my planetary viewing,and keeping the same level of detail that the VX10 can give.Any suggestions appreciated.

PS I was going to try a ND filter to reduce light level to 6" aperture levels...thoughts?

[Mike73]

John and Alex-I do think my skies that are good for DSO hurt me on the planets at times,washing out some contrast on Jupiter in particular.In light cloud,with steady skies this helps me as well.During late evening viewing while there still some light that can be polarized with the filter(Baader single polarizer) the views can be much better than later on when the darkness enhances Jupiters brightness.I have to say this,at my dark site Jupiter can bug my dark adaptation when DSO hunting,but from my house its bright but with light pollution-however the sky transparency and seeing conditions would be the same.

Mars was great here last night,the brightness of it doesn't have the same effect Jupiter does on my viewing.I guess I'm looking to maximizing my planetary viewing,and keeping the same level of detail that the VX10 can give.Any suggestions appreciated.

PS I was going to try a ND filter to reduce light level to 6" aperture levels...thoughts?

I've recently bought a 2" 0.6 ND filter Gerry for the very reason yourself, John and Alex have mentioned.

I've had some great nights observing planets when the transparency has been really poor, some nights have been so cloudy/misty that you just wouldnt think any kind of astronomy was possible but I've had some excellent views because the cloud has acted like a natural ND filter.

I bought the filter to use with the VX10L but that has some teething issues and wont be in action till this time next week but I will try the ND filter in my 16" from a dark site tomorrow night (before I get stuck into the DSO's) and report back.

[russ.will]

Exactly what it is Russell.I can use a 90mm circle in there which I may try....or buy a 120Ed after I sell my 90mm!lol!I bet the almost 120mm round mask you made worked well in the 300mm,have you compared that to a 120mm class refractor?

You mean other than the 127mm frac in my signature?

Okay, it's hardly an Apo, but a F9+, it ain't too shabby on planets either. I've wandered around a bit, tried and sold eight scopes over the mere four years I've been messing in this hobby and come to the conclusions of what gave me the most pleasure in both quality of function and views. This was narrowed down to another 12" Dob (without Goto) and a short 80mm Apo on a Goto EQ mount.

The thing about the dob in particular is that once cooled, I've yet to look through anything that gives me the same wow factor. CA free, you can deal with too much light and diffraction spikes with a mask for planets to give 4-5" frac like views and then, blow that same frac out of the water on any DSO. It is the true one-scope-to-rule-them-all.

I'm clearly not anti frac, as the previous 80ED I had, was one of the few scopes I had that had me sitting under clouds hoping for glimpses in the gaps as it gave lovely wide field, low mag views. That scope was mechanically inept though, so whilst I was convinced of the format, it could not remain. As a totally visual user, it is perhaps a slightly irrational choice, but this is a hobby and so there is room for 'because I want one'.

So there is the slight dichotomy of me liking a small frac for wide field views and a big Dob for planets, but for me the two have certain complimentary crossover in my generally favoured DSOs. Others will almost certainly disagree with my choice, but I don't care.

Russell

[jetstream]

I need a 12" dob and don't have one! So then I could run a good sized(4.7" or so) round hole in the mask.The chance of me selling my VX10 here is slim to none-the VX12L would be the one,just like John has or similar.I really do like the relatively wide FOV of the f4.8/1200mm.Your right about the wow factor,from 21.7mag skies some things are absolutely stunning to view in the dob.I was just so happy when my VX10 made it 3700 miles to my house unharmed!whats the chances of that again...Exchange rate bad now too for me,so a VX12 aint lookin good! I am looking at used 15" Obsessions,but I really do like turning the scope in its rings to position that focuser...

[michaelmorris]

From my understanding, the edges of any lens or mirror introduce interference patterns to the image you see. The larger the aperture, the small the ratio of lens/mirror edge to the area of the lens/mirror, hence the higher the theoretical resolving power of the system.

In the real world, many other factors such as seeing, internal reflections, optical quality and collimation also play a part.

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