Post by howie1 on Aug 29, 2017 6:20:34 GMT
Thanks to Ken for suggesting fitting a fan to the primary mirror end of my 200mm x 1000mm Newtonian.
Been out twice now with it. Both times no dewing up of the mirrors, and a surprising benefit is my DSLR is showing little, if any, hot pixels!! The air blowing up the tube is cold air from down the primary mirror cell area, as I fitted my fan to blow up the OTA through the primary cell area/edge up towards the secondary mirror. I can only summise that as it passes the open 2" nosepiece of the DSLR it may create a venturi effect pulling hot air out of the camera. And given my focuser is a thumbscrew attachment (not an expanding band/ring attachment), cold air from outside the camera may also be sucked through the focuser and camera to help cool it? Well, so much for my thoughts on the matter, all I know is I'll be darned if I can see any hot pixels when using the fan!
BTW, there's several threads around the web on suck air down through the tube vs blow air up through the tube. Here's my summary ...
Suck air out of the tube:
Pro's:
* supposed to provide more laminar (no eddies or turbulence) airflow down through the tube through the light path. There's a counter argument that this is BS as (a) the air passes by the secondary mirror as well as the focuser area and so it is actually disturbed and has eddies and such; and (b) as it approaches the primary mirror it curves out towards the edges of the OTA as it is pulled towards the gap around the mirror through which the air is being drawn. And in doing so the airflow is quite fast as it is being drawn through a small gap, so cools the mirror edge much more than the central area. But then there's posts which show airflow shots from a wind tunnel ... they show a venturi effect as the vaccuum created at the mirror central area by the air streaming around the edges rapidly, pulls the main central airmass down close to the mirror. IE still get even cooling across the primary mirror face. One post I saw says if you place a baffle about 0.2 to 0.5 times the primary mirrors diameter up above the mirror within the OTA itself, the baffle greatly helps the air mass coming down the tube to pass the baffle and stream sideways across the mirror face to the gap at the edges.
Cons:
* sucks insects down towards the mirror surface.
Blow air up through the tube:
Pro's:
* Takes cold air and forces it immediately up against the back of the primary mirror cooling it down much faster than the sucking air down the tube where there's little contact between the mirror and the airflow as it narrows down to go past the opening around the primary mirror cell. Thus the big thermal mass of the mirror is chilled much faster and more so than the effect from sucking air down the tube. The mirror is also cooled more evenly across the whole surface edge to edge than sucking the air through (where again the airflow thins to go past the edge of the mirror only really chilling the edges and less so the centre. Of course, there are arguments against all this by those who believe sucking the air down is better! LOL. See the above argument about a vaccuum being created above the mirror surface thus the main air mass still makes contact right across the mirror surface, and in fact chills it faster as the its the mirror surface which is cooled (not the back of the mirror as for blowing up through the tube!)
Cons:
* Said to not provide that all important laminar airflow which the Suck air pundits love talking about. Its been countered by those in the blow brigade pointing to the baffle studies done by the suck brigades own arguments... when the fan blows against the back of the primary cell, all the air is forced to move sideways towards the OTA edges where it goes through the gap to the primary mirror and speeds up ... IE a similar venturi/vaccuum principle happens on the mirror surface side as airflow streams up the sides of the tube, taking heat with it.
* Blows dust and pollen from down near the ground which is where people are walking around, up into the OTA. The counter argument on the web was "Well ... ok ... but the air is blowing it all up through the tube and out the top."
Bottom line for me, was most people reckon blowing at the back of the primary mirror up through the tube worked better to get rid of dew and no one has seen any bad effect on sharpness/optics in doing so. Plus my big 12" dob's mirror blow onto the back of the mirror and up the OTA and no one has proven otherwise to Orion to do any different. So I put my fan blowing against back of the primary mirror and up through the OTA.
Been out twice now with it. Both times no dewing up of the mirrors, and a surprising benefit is my DSLR is showing little, if any, hot pixels!! The air blowing up the tube is cold air from down the primary mirror cell area, as I fitted my fan to blow up the OTA through the primary cell area/edge up towards the secondary mirror. I can only summise that as it passes the open 2" nosepiece of the DSLR it may create a venturi effect pulling hot air out of the camera. And given my focuser is a thumbscrew attachment (not an expanding band/ring attachment), cold air from outside the camera may also be sucked through the focuser and camera to help cool it? Well, so much for my thoughts on the matter, all I know is I'll be darned if I can see any hot pixels when using the fan!
BTW, there's several threads around the web on suck air down through the tube vs blow air up through the tube. Here's my summary ...
Suck air out of the tube:
Pro's:
* supposed to provide more laminar (no eddies or turbulence) airflow down through the tube through the light path. There's a counter argument that this is BS as (a) the air passes by the secondary mirror as well as the focuser area and so it is actually disturbed and has eddies and such; and (b) as it approaches the primary mirror it curves out towards the edges of the OTA as it is pulled towards the gap around the mirror through which the air is being drawn. And in doing so the airflow is quite fast as it is being drawn through a small gap, so cools the mirror edge much more than the central area. But then there's posts which show airflow shots from a wind tunnel ... they show a venturi effect as the vaccuum created at the mirror central area by the air streaming around the edges rapidly, pulls the main central airmass down close to the mirror. IE still get even cooling across the primary mirror face. One post I saw says if you place a baffle about 0.2 to 0.5 times the primary mirrors diameter up above the mirror within the OTA itself, the baffle greatly helps the air mass coming down the tube to pass the baffle and stream sideways across the mirror face to the gap at the edges.
Cons:
* sucks insects down towards the mirror surface.
Blow air up through the tube:
Pro's:
* Takes cold air and forces it immediately up against the back of the primary mirror cooling it down much faster than the sucking air down the tube where there's little contact between the mirror and the airflow as it narrows down to go past the opening around the primary mirror cell. Thus the big thermal mass of the mirror is chilled much faster and more so than the effect from sucking air down the tube. The mirror is also cooled more evenly across the whole surface edge to edge than sucking the air through (where again the airflow thins to go past the edge of the mirror only really chilling the edges and less so the centre. Of course, there are arguments against all this by those who believe sucking the air down is better! LOL. See the above argument about a vaccuum being created above the mirror surface thus the main air mass still makes contact right across the mirror surface, and in fact chills it faster as the its the mirror surface which is cooled (not the back of the mirror as for blowing up through the tube!)
Cons:
* Said to not provide that all important laminar airflow which the Suck air pundits love talking about. Its been countered by those in the blow brigade pointing to the baffle studies done by the suck brigades own arguments... when the fan blows against the back of the primary cell, all the air is forced to move sideways towards the OTA edges where it goes through the gap to the primary mirror and speeds up ... IE a similar venturi/vaccuum principle happens on the mirror surface side as airflow streams up the sides of the tube, taking heat with it.
* Blows dust and pollen from down near the ground which is where people are walking around, up into the OTA. The counter argument on the web was "Well ... ok ... but the air is blowing it all up through the tube and out the top."
Bottom line for me, was most people reckon blowing at the back of the primary mirror up through the tube worked better to get rid of dew and no one has seen any bad effect on sharpness/optics in doing so. Plus my big 12" dob's mirror blow onto the back of the mirror and up the OTA and no one has proven otherwise to Orion to do any different. So I put my fan blowing against back of the primary mirror and up through the OTA.