BAZ Posted March 27, 2016 Share Posted March 27, 2016 I have had a bit of an issue on the 300P with a small line on one side of an object, which moves to the other side as it went through focus. I have never been happy with it, but the collimation looked good. This goes back a while now, even before the last Kelling. So today I decided to sort it out. I took the mirror cell out and had a look there, I found one of the mirror clips just a nudge too tight. I loosened this off and made sure they were all just right. If they are correct, just sliding a bit of baking parchment under the clips gives around 6 thou clearance. Loose, but not rattling and the mirror should be able to rotate Further down the line I found a focuser adjustment locking screw loose, so in certain positions the focuser moved. I have lined that back up and collimated it again, so I am now waiting on a clear night to star test it. Always something eh! red dwalf 1 Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 27, 2016 Share Posted March 27, 2016 (edited) There is always something Martyn, you're right. Cooldown is my main issue with the Dob at present. I've read a general 'rule of thumb' which says about 5 mins per inch of aperture should be enough cooldown time for a scope to give an acceptable view. Last week i tried this out. I let my scope cool for over an hour, then did a star test on Arcturus. Huge thermal issues. The star really did look like the defocused image below right, only worse. The Airy rings looked ok though. This image shows the effects of atmospheric turbulance and / or cooldown / thermal issues. I think 5 mins / inch is insufficient or it was in this case. Edited March 27, 2016 by Bino-viewer Link to comment Share on other sites More sharing options...
BAZ Posted March 27, 2016 Author Share Posted March 27, 2016 Somewhere round an hour should be producing decent views. I think the seeing has been more of a problem of late. If you are doing a star test try Polaris at around 150 -200X, that's what was recommended as a bright star causes it's own problems. Link to comment Share on other sites More sharing options...
Daz Type-R Posted March 27, 2016 Share Posted March 27, 2016 Fingers crossed that's sorted it Martyn, I have something else to do to mine but its finding the time (and the tools). Link to comment Share on other sites More sharing options...
+Ibbo Posted March 27, 2016 Share Posted March 27, 2016 seeing has been absolutly awful and will mess up any airy rings Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 27, 2016 Share Posted March 27, 2016 43 minutes ago, BAZ said: Somewhere round an hour should be producing decent views. I think the seeing has been more of a problem of late. If you are doing a star test try Polaris at around 150 -200X, that's what was recommended as a bright star causes it's own problems. Ok, i will try the same test on Polaris next time out. I appreciate Arcturus was maybe a bit bright, but it well placed at the time. If i can image Polaris i will. Link to comment Share on other sites More sharing options...
BAZ Posted March 28, 2016 Author Share Posted March 28, 2016 Seeing is 100% pants at the moment (09.00 Monday) It's snowing. Who said we could have snow? Link to comment Share on other sites More sharing options...
philjay Posted March 28, 2016 Share Posted March 28, 2016 Seeing has been abismal this winter with the jet stream pushed over us most of the time, planetary and lunar plus solar have been difficult to use decent mags Link to comment Share on other sites More sharing options...
+RonC Posted March 28, 2016 Share Posted March 28, 2016 Would a fan not aid with your cooldown Rob? Cheers Ron Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 28, 2016 Share Posted March 28, 2016 My OO Dob does have a fan Ron. Its only a very small one (40mm?) and draws air down the tube from the top to the bottom. A fan mod is on my list of 'things to do' Link to comment Share on other sites More sharing options...
BAZ Posted March 28, 2016 Author Share Posted March 28, 2016 (edited) I wonder if that could be part of the problem, all the others, including mine blow the other way, so it vents out of the top. Bizarrely having the fan on stabilises the image. I don't know if sucking the air into the tube would have an adverse effect, but you would be pulling moisture laden air towards the mirror surface as opposed to pushing past it from the back. Maybe upgrading your fan to an 80mm would help. I made a plate which fits over the entire back end, so the air is forced up the tube rather than bouncing back off the back of the mirror. http://s1230.photobucket.com/user/BBQBAZ/media/Dob%20Mods/DSCF1886_zpsf233676b.jpg.html Edited March 28, 2016 by BAZ Link to comment Share on other sites More sharing options...
catman161 Posted March 28, 2016 Share Posted March 28, 2016 Yeah I'm with Martyn on this one. When I had my dob with a bespoke made fan for it (which currently resides on Daz type-R's 200P) he back plate is made exactly to fit snugly so that air is forced up through the tube in order to irradiate the layer of air that sits across the top of the primary. I would turn it on full power when I first setup and leave e scope to cool as normal with the fan running full whack then when it was time to observe/image I would turn it down to its lowest setting and leave it running whilst observing/imaging. It certainly helped in my opinion. Maybe Daz will chip in with what he does currently. Link to comment Share on other sites More sharing options...
Doc Posted March 28, 2016 Share Posted March 28, 2016 Me to all my fans in all the dobs I've owned, have always pushed the air out not suck it in. Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 28, 2016 Share Posted March 28, 2016 (edited) This is what i have at present. The mirror cell in OO scopes is well made, although the primary mirror retaining clips are too large imo. However, the fan seems like an afterthought. Pretty naff to be honest. I definitely want to mod the cooling fan, and am also working on fitting a baffle (just above the primary) so some of the upward air is then deflected across the face of the primary. This is the plan, but it's still at the 'drawing board stage' at present. Edited March 28, 2016 by Bino-viewer Link to comment Share on other sites More sharing options...
Daz Type-R Posted March 28, 2016 Share Posted March 28, 2016 (edited) Yep Felix has it correct, here is a picture of mine.... Drawing air down the tube sounds like a bad idea to me as per Martyn's comments above, you will be drawing moistened laidan air onto your primary. Edited March 28, 2016 by Daz Type-R Link to comment Share on other sites More sharing options...
Tweedledee Posted March 28, 2016 Share Posted March 28, 2016 Also dust and other atmospheric debris is more likely to be drawn down and deposited on the primary. I'd say blow it up and out rather than suck it down. Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 28, 2016 Share Posted March 28, 2016 Sorry guys. I know we've been here before....... Darren : is this the Astronomy shed fan mod thats been mentioned ? I can't find anything about it on Dions forum ? Link to comment Share on other sites More sharing options...
Daz Type-R Posted March 28, 2016 Share Posted March 28, 2016 9 minutes ago, Bino-viewer said: Sorry guys. I know we've been here before....... Darren : is this the Astronomy shed fan mod thats been mentioned ? I can't find anything about it on Dions forum ? Found the link on the shed..... http://www.astronomyshed.co.uk/forum/viewtopic.php?f=65&t=977 I think you need to ask Dion directly, I did. Link to comment Share on other sites More sharing options...
+Bino-viewer Posted March 28, 2016 Share Posted March 28, 2016 Thanks for the link Darren Link to comment Share on other sites More sharing options...
tuckstar Posted March 29, 2016 Share Posted March 29, 2016 Apparently on larger Dobs, 18+, it is good to have a fan blow air directly on to the mirrors surface to create a cushion of air that stops dew forming. Those I've seen have been close to the primary in the shadow of the secondary, and open truss design. I agree though that pushing the air out should stop damp air getting in anyway. I just switch mine on and leave it to it. Link to comment Share on other sites More sharing options...
Brantuk Posted March 29, 2016 Share Posted March 29, 2016 I'm sure I've posted this link before elsewhere on the forum - but while we're all on about fans here's a bit of techie stuff you might find interesting. http://www.fpi-protostar.com/bgreer/fanselect.htm Tweedledee, tuckstar and Smithysteve 3 Link to comment Share on other sites More sharing options...
Tweedledee Posted March 29, 2016 Share Posted March 29, 2016 46 minutes ago, Brantuk said: I'm sure I've posted this link before elsewhere on the forum - but while we're all on about fans here's a bit of techie stuff you might find interesting. http://www.fpi-protostar.com/bgreer/fanselect.htm Thanks Kim. Not seen that, but it will come in very handy as just today I've been planning the 20" mirror cell. Brantuk and tuckstar 2 Link to comment Share on other sites More sharing options...
BAZ Posted March 30, 2016 Author Share Posted March 30, 2016 It's stuck out on the backyard cooling down now, I am hopeful that I might get a bit of clear sky and all being well, a good look at Jupiter. I'll let you know if I have sorted it out, either that or be quietly jabbering to myself in a dark corner. Link to comment Share on other sites More sharing options...
BAZ Posted March 30, 2016 Author Share Posted March 30, 2016 Oh well, that didn't work, it is still not right. Link to comment Share on other sites More sharing options...
Daz Type-R Posted March 30, 2016 Share Posted March 30, 2016 This line you have got, is it a line of light or a black line? Has one of your spider veins slipped, I.e, it's not straight in relation to the primary? Link to comment Share on other sites More sharing options...
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