Peter Shah Posted September 30, 2018 Share Posted September 30, 2018 When I first used a Newtonian for imaging I remember my stars looking like tadpoles. I battled for ages adjusting and tweaking collimation with no change. I'm sure there are many of you who can relate to the frustration, I guess its all part of the imaging experience. All this time i had my fans running. One night I was going through the motions trying to solve the issue and unplugged the fans. It was like a light switch, the stars almost instantly became round. I cant tell you how i felt, my scope wasnt a dud after all. Delighted with what I had discovered I set to start imaging Andromeda which was in the East and rising nicely. Odd stars were back again, Completely deflated I packed up and went to bed. The next clear night I thought I would try a few things. Testing the scope at different altitudes, the stars were good while the scope faced up but bad when I moved closer to the horizon. You could be forgiven thinking it might be flex or just the seeing as you move through thicker atmosphere, but these stars change shape almost looking like a flame from a candle. With flex you would get odd stars but more or less stable in one position and bad seeing wouldn't be bias to one side, the wobble would appear across the whole star. So knowing what I found with the fans I tried running the fans but backward. Well the difference was instant....pin stars! The solution was to run my fans on suck. My conclusion was when the fans are on blow air passes the back of the mirror over its edges creating disturbed air flow. When the fans are on suck the tube currents are straitened and flow down the tube nice and evenly. The obvious problem is with the fans on suck is your mirror is likely to dew up sooner and then there is the potential for dust. This is based around a closed tube Newtonian, I would imagine an open truss tube would be unaffected by much of this. Many years on I'm now using a much larger scope with a wider aperture tube but I did grab a couple of frames to illustrate the effects of tube currents and the result with the fans on suck. Fans on Suck Fans switched off Bottletopburly, +RonC, Tweedledee and 1 other 4 Link to comment Share on other sites More sharing options...
Tweedledee Posted September 30, 2018 Share Posted September 30, 2018 Very interesting. Since starting to build my 20" dob, I've been reading quite a lot about this and have learned that there are many, often surprising, factors that can modify how effective the fans are. Structural differences around the mirror as well as the size of gap around the mirror create their own particular effects, detrimental or good. Some successfully run fans by blowing and some by sucking. Some introduce small fans blowing gently across the mirror surface to remove the boundary layer better. Some have these small fans blowing down on the centre of the mirror suspended on wires in the shadow of the secondary. Also the mounting of the fans can introduce unwanted vibrations. My eventual conclusion is for the need to be prepared to modify things and experiment with what works best for a particular system. To this end, in my case, I plan to have a fan at the back of the mirror that I can easily change from blow to suck and small fans that will blow across the mirror surface. The fans will be separately adjustable for speed. The fans will also be mounted so as to avoid vibration. Of course some of this flexibility can be a little more difficult to build in, if not designing the scope from scratch. Also it occurs to me that may be for visual use as against imaging, the eye can be a little more forgiving of some smaller detrimental effects. It seems to me that this area can be a bit of a minefield, but a little pragmatism and willingness to experiment should sort it out. My above comments are mainly based on what I have read on Cloudy Nights and other forums rather than my own trial and error. I imagine I may be back on here at a later stage requesting further advice and help on this matter. ? Glad you got yours sorted Peter. ? +RonC, Smithysteve, BAZ and 1 other 4 Link to comment Share on other sites More sharing options...
BAZ Posted September 30, 2018 Share Posted September 30, 2018 Although Pete Shah is reaching out to imagers here, it, as Pete Sullivan mentioned has a remarkable effect on visual scopes too. From my personal experience, without my fan running (on blow) I get what I thought was really bad seeing. Turning the fan on almost instantly improves this and I can only assume this is caused by turbulent and mostly static tube currents. Apparently even you body heat next to a Dob can cause issues with the view. I have made a plate up that sits behind the mirror and forces the draught up and around the edge of the mirror, although this might contribute to it's own effects, altering the fan speed seems to be enough to move air without causing too much turbulence seems to work fine for me. I have it on full whack to cool the mirror and then turn it down to around 1/4 speed. Peter Shah, +RonC and Tweedledee 3 Link to comment Share on other sites More sharing options...
+Bino-viewer Posted September 30, 2018 Share Posted September 30, 2018 (edited) Pete : i was always under the impression that all OO scopes sucked air down the tube, over the primary ? Mine does, i know. I assumed the closed tubes did likewise. This has been debated at length as i'm sure we all know. But i've always believed if a fan works the other way round, ie blows air on to the rear of a mirror it will only cool the rear of the mirror ; the surface of the mirror stays warmer with a small plume of warmer air (the boundary layer) staying put and degrading views / imaging. This is why alot of premium US Dob scope makers, ie Obsession, Teeter, all suck air downwards, AND, MORE IMPORTANTLY, add 'boundary air' fans which blow across laterally to remove this troublesome air. Mick is right about body heat too, although this primarily affects open, truss type scopes. A full length tube mitigates this pretty much, as does a well designed shroud on a larger scope. Edited September 30, 2018 by Bino-viewer Tweedledee and Peter Shah 2 Link to comment Share on other sites More sharing options...
Peter Shah Posted September 30, 2018 Author Share Posted September 30, 2018 I do agree Rob and Martyn you have hit an extremely good point with the gap differences and variable design from scope to scope. Rob you are correct OO scopes all have their fans on suck. But this wasnt so for the early models. It was my OO 8inch f4.5 that had these problems back in 2004. Its a huge debate and there is no real simple answer....really you should try it and see which best suits your kit. What I will say about my AG12 is I have three quite powerful fans that draw lots of air down the tube. The amount of influence my fans have seems to reduce the effect of the boundary layer. I have seen other imagers use fans on the side of the ota to remove this BL. On occasion when the air is stable enough I do image with the fans completely off. The other problem with drawing air down the tube is dew and dust. I am able to run my heaters while the fans are running with very little issue. Its when the scope is pointing directly up and the heat has a tendency to rise out of the influence of the fans where i have problems. Link to comment Share on other sites More sharing options...
Bottletopburly Posted September 30, 2018 Share Posted September 30, 2018 i have fans on both my newts once i get running properly i will check the results out both blowing not sucking two articles from gary seronik here https://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-1/ https://garyseronik.com/beat-the-heat-conquering-newtonian-reflector-thermals-part-2/ Smithysteve, Peter Shah, BAZ and 1 other 4 Link to comment Share on other sites More sharing options...
philjay Posted September 30, 2018 Share Posted September 30, 2018 Interesting Peter. Some years ago I had a rush of blood to the head and bought a classic 90s 8" OO newt with the intention of updating it for imaging. The cells on these earlyish OOs were pretty basic and were totally closed. I drilled vent holes then fitted fans which I could reverse. I spent a few nights experimenting with direction and speed and came to simikar conclusions. I had front and rear thermocouples across the cell and could monitor when things stabilised more effectively then turn the speed down. Peter Shah and BAZ 2 Link to comment Share on other sites More sharing options...
Bottletopburly Posted October 1, 2018 Share Posted October 1, 2018 (edited) @Peter Shah What effect do you think having the tube flocked has ,positive or a negative effect on tube thermals there must be some insulation property to having it done , both mine are done . Edited October 1, 2018 by Bottletopburly Peter Shah 1 Link to comment Share on other sites More sharing options...
Peter Shah Posted October 1, 2018 Author Share Posted October 1, 2018 3 hours ago, Bottletopburly said: @Peter Shah What effect do you think having the tube flocked has ,positive or a negative effect on tube thermals there must be some insulation property to having it done , both mine are done . Not sure, I haven't done a lot on this . A while back I did have the outside of my tube insulated. Only to prevent the tube changing temperature rapidly for when the sun hit the observatory early in the morning. I found that that is the time when my scope tended to dew up. I now have the rear of the tube insulated to prevent heat escaping from the primary heater band. Only really intended to keep the optics pristine when not in use. I will have to do some experiments this winter. As dew free imaging is quite difficult from here..... I'm sure we all have similar issues with this too. Bottletopburly and Smithysteve 2 Link to comment Share on other sites More sharing options...
Daz Type-R Posted October 1, 2018 Share Posted October 1, 2018 Interesting article this, thanks for posting. I initially did not have a fan on my Newt and suffered with dew on the primary, so I jumped on the band wagon and attached a fan, blowing air at the back of the primary and ever since, I have never had a dewed up primary. Im only visual, but keen to explore the differences suck vs blow will do, may just swap out my fan for a reversible one and give it a go. Peter Shah 1 Link to comment Share on other sites More sharing options...
Peter Shah Posted October 1, 2018 Author Share Posted October 1, 2018 3 hours ago, Daz Type-R said: Interesting article this, thanks for posting. I initially did not have a fan on my Newt and suffered with dew on the primary, so I jumped on the band wagon and attached a fan, blowing air at the back of the primary and ever since, I have never had a dewed up primary. Im only visual, but keen to explore the differences suck vs blow will do, may just swap out my fan for a reversible one and give it a go. That's interesting that you have used a fan to keep dew at bay Link to comment Share on other sites More sharing options...
tuckstar Posted October 2, 2018 Share Posted October 2, 2018 22 hours ago, Daz Type-R said: may just swap out my fan for a reversible one and give it a Can't you just switch the wires round? Link to comment Share on other sites More sharing options...
Tweedledee Posted October 2, 2018 Share Posted October 2, 2018 5 hours ago, tuckstar said: Can't you just switch the wires round? Mostly they have arrows on the edge pointing in the direction of airflow. I think the motors normally only rotate in one direction, so they have to be physically turned around to reverse the flow. BAZ and tuckstar 2 Link to comment Share on other sites More sharing options...
+Graham Posted October 3, 2018 Share Posted October 3, 2018 (edited) I think you might find its the pitch and shape of the fan blades that govern the optimum direction of airflow. Very interesting topic this. Been taking notes. Edited October 3, 2018 by Graham Tweedledee 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted October 3, 2018 Share Posted October 3, 2018 1 hour ago, Graham said: I think you might find its the pitch and shape of the fan blades that govern the optimum direction of airflow. Very interesting topic this. Been taking notes. Yes, I have noticed that some of the latest hi-tech fans seem to have little tweaks in the shape of the blades and the surrounding plastic to create better airflow. These tweaks are not symmetrical so the blades should only turn one way. This allows maximum airflow with minimum turbulence and therefore less vibration. ? Link to comment Share on other sites More sharing options...
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