Depth of Focus & Stopping Down Fast Newtonians

[floyd_2 (Dean)]

I thought that I would share a link that I found that clearly discusses depth of focus, seeing, stopping down fast Newtonians and so on. It's a great read and probably a must read for anyone currently considering a fast newtonian.

I'm going to experiment with stopping my 15" down to 5" for planetary and lunar viewing if seeing doesn't allow decent viewing at full aperture. I have already made an aperture mask from protostar flock board and will try it out next time it looks like full aperture planetary viewing just isn't working out.

Certainly, I don't think I have ever owned a fast telescope that could top the planetary views that my original 10" f/6 offered. Anyway, for any who are interested, it's a really clear, good read:

http://www.brayebrookobservatory.org...thoffocus.html

Dean

[Terry B]

Interesting. Thanks

[Satchmo]

Dean - the simplest way to improve the depth of focus is just to use a barlow lens. An F5 becomes an F10 light cone with a 2X amplifier. Aperture stops deliver a superficially sharper image on nights of poorer seeing. I have never however seen more genuine detail with stopped down aperture than with full aperture. It requires more patience but larger aperture always seems to deliver more. With Jupiter particularly, aperture brings the ability to see colour differentiation in features and even in poor seeing , this colour differentiation allows a whole layer of detail that disappears with smaller aperture ( even if the view is superficially sharper )

Most often I find the culprit when people think their larger scopes are sensitive to the seeing is that the optics are not adequately cooled. If you have an Obsession you are likely to have a 2" thick mirror - which are difficult to work with thermally. If you only have a single fan on the back - fit another two- also fit a boundary layer fan to blow across the face of the mirror and lastly fit a low cost digital thermometer with a probe attached to the mirror.

If your delta between ambient and mirror is not better than 1.5 degrees C you will not have much chance of exploiting what the aperture is capable of delivering. Hope this helps. Often people who fit temp monitoring are shocked to see how much their mirrors can lag behind ambient during the night .

[floyd_2 (Dean)]

Thanks for your insights Mark, and for taking the time to reply in such detail. You're right about the thickness of my mirror. Without actually measuring, I have always assumed that I would be chasing temperature equilibrium all night. For the most part, I'm a deep sky man. However, from time to time I like to keep an eye on the planets, so I'm keen to improve the scope where possible for planetary viewing.

The main reason why I thought that I might try an aperture mask was to stop the scope down to an aperture that might have a greater chance of being smaller than the air cell size under average (or less) seeing conditions. As you noted, there will certainly be a contrast and detail hit - with a hopeful benefit of longer periods of perceived better seeing. I used to own 4" refractor, and the difference in image brightness and contrast between that and a 15" is huge. It's all a trade off I guess. Certainly your comments about thermal equilibrium should be my first target for a tuning exercise though Mark - thanks very much.

I found another good article specifically about mirror cooling, fan installation schemes, boundary layer cooling techniques etc. Well worth a read for anyone headed down this path. I found it interesting that there is a point at which the effectiveness of (more) fans increases at a significantly decreasing rate. I think it noted that 30-45cfm was about the sweet spot for a 15" mirror - which isn't a lot. A decent 80mm PC pancake fan can push that out. I guess fans need to be properly baffled so that the effectiveness of their air flow is maximised. Here's the link for anyone who is interested. It's not a bad read:

http://www.fpi-protostar.com/bgreer/fanselect.htm

Dean

[floyd_2 (Dean)]

I thought I'd post the results of the 5" aperture mask testing that I did the other night for anyone who might be interested. I stopped my 15" down to 5" with an off axis aperture mask on Sunday night and compared the views of Saturn with and without the mask using a 7mm Nagler.

Being a previous owner of a 4" refractor, I can remember the planetary views through it. The view through my 15" stopped down to 5" was very similar. There was a definite loss of detail, and an even more definite loss of contrast. On the plus side, my suspicion of increased periods of better seeing were correct (which is what I was really after). Periods of markedly better seeing increased substantially, so it really was a tradeoff.

I agree with Mark after this testing - it's better to be patient and view the planets without an off axis aperture mask, as the detail and contrast are to die for by comparison.

Dean