Stopping down...a question

[nicks90]

I also have a question on stopping down big dobs - but slightly different to the ones mentioned so far.

somewhere in my recent past I came across an article that said you could get good focus by using a stopped down mask with 2 holes in it. Out of focus this would give 2 images and when in perfect focus the image would over lay each other and be perfectly aligned when perfectly focussed. So sort of doing the job of a bhatinov mask. But it also has the advantage of increasing the focal ratio of the scope too as per this discussion.

So with that in mind, has anyone tried this before?

Reason I'm considering it is because I think a 4" stop will reduce te brightness too much on jupiter, but 2 x 4" stops would give the equivalent of a single 6" aperture and have the benefit of turning my 12" f5 into a 6"f10 - and also assist in getting focus perfect too.

Nick

[Stu]

It's an interesting concept, and one which I thought about a while back. I guess the only real way to check it is to try it out.

Ultimately I guess it potentially will operate as if you have two very thick, curved spider vanes. The diffraction spikes would be smeared out but I'm not sure how it would perform in terms of contrast. In a way it would be like a binoviewer in reverse, two images being overlaid into one image.

My instinct is to think its worth a try but am suspecting life (and physics!) have a trick or two up their sleeves :-)

Stu

[Moonshane]

I cannot read this link at work but it seems to be about the dual hole theory?

http://autocostruttori.blogspot.com/2008/01/aperture-masks.html

[Qualia]

It's a killer playng around with Jupiter at the moment and I'm going to have to call it a day when work starts (hopefully) on Monday. I mean, staying up till 4am to view the giant as its slowly creeping up through great swarths of atmosphere isn't exactly the best plan of action and this kind of experiment should be conducted nearer winter time. Nevertheless, I had a peak again this morning with the Tal and Moonshane side by side. The Tal with its full aperture, the 10" stopped down to about 90mm.

Visually speaking I couldn't discern a better image. The Tal threw up a lovely, visually pleasant, sharp and contrasty view of Jupiter as did the stopped down Moonshane. For the two hours I stayed on the roof last night viewing Jupiter I could not see any greater detail between the 4" Tal and the masked 10". Both scopes, in there own way, gave a differing but aesthetically pleasing view.

I played around with magnification, jumping from 10mm (Delos), and a 9mm, 7mm, 6mm and 5mm (BGOs) and found that the ideal power through the 4" f/10 was 142x with a 0.7mm exit pupil. This was the Tal's sweet spot. But curiously, through the stopped-down 10", I was comfortably viewing Jupiter at 178x with a 0.5 exit pupil (think I've got the math right). Why was this so, I have no idea but apart from my own physiological mess-ups with the likes of floaters sometimes interferring with the observation the image of Jupiter was cracking.

With the mask removed, the brightness of Jupiter significantly increased but, again, through the full 10" aperture, I found the planet performing comfortably at 178x but now with a more forgiving exit pupil of 1.4mm. I think it's the actual brightness of Jupiter that throws me and sometimes makes it trickier for my eyes to grasp subtle detail. On the Moon I'll just up the magnification, but doing that with Jupiter messes up the image. Nevertheless, when the seeing settled, and my eyes had accustomed themselves to the brightness I found that resolution, contrast and image quality was at its best through the full 10". Neither the 90mm stopped-down scope, nor the 100mm frac could really compete.

So, again, the conclusion is pretty much the same as yesterday. The mask certainly reduces the glare of Jupiter and steadies those moments of bad seeing and perhaps it is here, at this junction, that a mask might be useful. It helps sharpen up the image on the bright gas giant on those nights or moments of bad seeing. It reduces the possibility of 'noise' and for the time and expense it takes to make such a device, there's really no excuse not to do it. But 4" inches of aperture is never going to show as much planetary detail as 10" and this early morning sitting with Jupiter without the mask there were many moments when the Moonshane threw up a significantly better views and blew away the 90mm stopped down and 4" frac.

The only negative thing about playing around with Jupiter like this is that I'm begiining to think a wide field EP might be handy, something around the 8mm to 6mm range for Jupiter, Saturn and the Moon. I have no problems tracking Jupiter with the BGOs (demonstration of the quality and sheer smooth-class of the mount Shane built!) but a wider field would give me a little more time to take note and even sketch what I think I'm seeing. The problem is I'm skint, haven't started work yet, can't see much sense duplicating EP focal lengths that I already have and these kind of EPs are expensive . I know some of you guys have decent zoom EPs but their focal lengths are so small and the seeing conditions generally so variable here in Spain that I can't imagine ever going beyond 180x - 200x. Not sure what to do here, so any advice would be useful.

Another question, has anyone ever built themselves an Apodizing Mask and seen the utility or not of such a device?

[foundaplanet]

Rob, I may be wrongbut I think I have read somewhere on the forum that Paul G Abel uses a mask when he makes his drawings. If you go to the sketches section and search Jupiter you will find some of his drawings in there. ... They are rather mind blowing for a ten inch newt. Obviously the critical factor is conditions, I will always find the time to look at Jupiter if I am out and it is up but I am learning well now how to make the most of those special nights when the skies do it proud...:-)