My 20" is a sort of Obsession style scope but uses many older design features that do slow down the setup. Nevertheless I can be up and running inside 20 minutes if I don't muck about too much. My record is about 13 minutes including collimation. I have nowhere to store it setup so have to wheel it out and put it together each time.
I have seen Erick setup his 12" SDM in about 3 minutes, plus about another 5 to finish off plugging in cables etc for AN, Servocat and dew heaters.
Dude, that's one small house you've got there
Great instrument though. Quality skies would be critical to use that thing to its full potential. Must be pretty good where you are.
I am sorry to say this, But, every time I see the heading of this thread
" Dob Trumps Refractor"
I cannot help thinking it is about Donald Trump
Sad isn't it
Cheers
I am sorry to say this, But, every time I see the heading of this thread
" Dob Trumps Refractor"
I cannot help thinking it is about Donald Trump
Sad isn't it
Cheers
Agree. It should be called "Dob Complements Refractor"
Not sure if that Trump guy would know anything about scopes. Or astronomy. Or physics. Or science.
Incidentally there is no smiley icon showing a dob on here.
While I've looked at a few deep sky objects through large Dobs at star parties I've never seen high-power planetary views through one. I'm curious how often it's possible to use the 1000x powers being talked about here.
Not trying to be controversial - if anything I'm wondering how much time to spend pestering big scope owners at IISAC
There is usually a night sky tour at IISAC, open to all who are interested. Last year we enjoyed some great views through an 18" (I think), with Argo Navis and ServoCat to make things easier with the crowds.
With Mars close by I'm sure it will get a lot of scope time this year.
There is usually a night sky tour at IISAC, open to all who are interested. Last year we enjoyed some great views through an 18" (I think), with Argo Navis and ServoCat to make things easier with the crowds.
With Mars close by I'm sure it will get a lot of scope time this year.
Cheers
Steffen.
I'd like to find a big scope that isn't part of a sky tour so I can have a really good look at Mars
While I've looked at a few deep sky objects through large Dobs at star parties I've never seen high-power planetary views through one. I'm curious how often it's possible to use the 1000x powers being talked about here.
Not trying to be controversial - if anything I'm wondering how much time to spend pestering big scope owners at IISAC
I'm looking forward to a 1000x night myself. Favouring the 10mm Ethos and Paracorr II I usually go to 300-350x when the scope has cooled sufficiently. On rare really good nights I can push it to around 600x with the 6mm but that's about it so far.
If the seeing is good there is really not much advantage in taking the power much beyond 1 mm pupil - as at this power the first ring of the airy disc becomes visible , and like putting your face close to a newsprint photo at which point you see the dots the magnification becomes empty . ( this all assumes you have well functioning eyes ). I demonstrated this on a good night with my 12" 8" and 3" refractors al;l showing the first ring of the Airy pattern at magnifications of 300, 200 and 80X respectively .
Above this power floaters in the eye and the crystalline structure of the eye also begin to obscure and compete with subtle detail on planetary surface.
At 1mm there is still a good brightness gain of the disc compared to features coming from the eye. While you can hear tales of retractors being pushed to 100X per inch on bright high contrast objects like the Moon and Saturn there will be no gain over lower magnifications.
My current 350mm reflector is revealing to me astonishing detail - all that the aperture is capable of in moments of steady seeing on Mars at 350X with a 5mm Radian ( an eyepiece of focal length equal to the f number will give a 1mm pupil ! ) . So while I often here anecdotally of people using up to X1000 `without image breakdown ' - I know it is nonsense in the sense that low contrast detail on the planets will become compromised by the small exit pupil which will cause a lot of `grain' in the image in all but say a 40" telescope at these magnifications. You may use X1000 but you certainly wont see more and much more likely to see a lot less.
That would explain why I keep coming back to the 10mm as my favoured eyepiece because 350x is the sweet spot on nights of good seeing?. The other night Mars and Saturn were spectacular at 350x best I've ever seen them. When I went higher the view wasn't as good.
I'd heard of keeping the exit pupil at 0.5mm or greater to avoid seeing your floaters, but not the 1mm 'limit' Mark is talking about.
However, now that I think about it, my 6.7mm eyepiece gives around 1mm or more in all my scopes, which might explain why it gives particularly satisfying and comfortable views of the planets.
I'd heard of keeping the exit pupil at 0.5mm or greater to avoid seeing your floaters, but not the 1mm 'limit' Mark is talking about.
Hi Morton,
I have no problems at all using exit pupils down to about .5mm, particularly in my 14" and 18" scopes and gaining additional detail to what is visible with a 1mm exit pupil. The smaller exit pupil provides a larger image scale. If the seeing supports it, the telescope aperture supports it and the observer's eye supports it, then additional detail can be obtained. Larger telescopes are better supported to use small exit pupils, as the larger telescope collects exponentially more light for a given exit pupil. Using a .5mm exit pupil in an 18" telescope is a whole lot different than using a .5mm exit pupil in a 4" telescope, simply because the larger telescope collects over 18X more light and throws up a much brighter image. The ability to use small exit pupils can also depend notably on the individuals eye physiology. To make a blanket statement that exit pupils under 1mm provide no additional details is somewhat misleading. It depends on a whole lot of variables.
Quote:
Originally Posted by MortonH
While I've looked at a few deep sky objects through large Dobs at star parties I've never seen high-power planetary views through one. I'm curious how often it's possible to use the 1000x powers being talked about here.
Not trying to be controversial - if anything I'm wondering how much time to spend pestering big scope owners at IISAC
Not very often at all. Maybe 4 or 5 times in the 8 years I have had the 18" telescope. I used to get excellent seeing from my backyard at Killarney Vale looking north across Tuggerah Lakes. The seeing over large bodies of water can be exceptional as the body of water does not emit thermals like land masses. This occurs because the temperature of the body of water is very stable from day to night. About 4 or 5 times a year I could push the 18" scope to 750x on the Moon, Mars, Saturn and double star splits, but the more common magnification cap under normal very good seeing nights was 525X, which I could attain about 1 in 4 observing sessions. The more common magnification cap was 300X which I could reach on 2 of the 4 nights and one night in 4 I was limited to about 200X, or just under. From my present location in Kiama, due to the close proximity of the escarpment and its prevailing thermals I haven't had any of my 3 telescopes over 400X in the 3 years I have lived here. It's worth noting that an 18" telescope at 300X shows a whole lot more detail at 300X on any given target than a smaller telescope due to its greater light gathering power and greater resolution. It's probably worth noting that in my 18"/F4.5 scope 420X represents a 1.1mm exit pupil.
There is usually a night sky tour at IISAC, open to all who are interested. Last year we enjoyed some great views through an 18" (I think), with Argo Navis and ServoCat to make things easier with the crowds.
With Mars close by I'm sure it will get a lot of scope time this year.
Cheers
Steffen.
Andrew Murrell and Gary Kopff will have two of the 3RF scopes at IISAC this year. A 25"/F5 Obsession and an 18"/F4.5 Obsession. I cannot attend this year as I have to work on ANZAC day, being our single busiest trading day of the year.
We had an exceptional view of Mars last week in Andrews 25"/F5 Obsession at Coonabarabran. We had a couple of nights of excellent seeing at Ozsky and took advantage of the conditions for some excellent views of Mars and Saturn. We had a photograph image of Mars in the 25" scope at 450X using a 7mm Nagler eyepiece.
I have no problems at all using exit pupils down to about .5mm, particularly in my 14" and 18" scopes and gaining additional detail to what is visible with a 1mm exit pupil. The smaller exit pupil provides a larger image scale. If the seeing supports it, the telescope aperture supports it and the observer's eye supports it, then additional detail can be obtained.
Hi John
Its a general rule of thumb I use assuming good seeing and good optics. Because it is enough to reveal the diffraction ring in these circumstances there is not so much value in increasing the magnification - the image will be dimmer. If the seeing is not so good it may be that you can see more with 0.5mm pupil simply because you are making it easier to see as the scope is not working at the diffraction limit, and the point of empty magnification will be much less apparent. . 1mm pupil is just the point that if the seeing and optics are good , the image can already reveal everything that there is to be seen resolution wise at least for high contrast features. If the seeing is good enough to resolve some diffraction pattern structure at 1mm then there should be no gain in resolution at a higher power / smaller pupil.
I would imagine you would use 450X on the planets much more frequently than 900X ( 0.5mm pupil ) with the 18" - " seeing" effects the planetary detail just the same at both powers but the 1mm pupil will be just a lot more satisfying aesthetically as the `signature' of your eyes structures in transmission and floating internal debris will appear to be a lot less objectionable.
Because they are in an observatory all set up all the time, they just flick couple of switches and away they go.
Most amateur "dobs" would be lucky to be used once a week due to the hassle of setting them up.
Oooooh my....never a ' truer ' word spoken .... .... com'on not having a ' go ' at anyone ... but there's a lot of truth in that..... cause I was like that decades ago.....just plain lazy I was
Flash.....
Last edited by FlashDrive; 11-04-2014 at 09:01 AM.
