yeah youll notice a reddish ring around part of the pic thats where the power plug is supposed to be seated... but the supplied nut and bolts came loose and now i've lost them
Excellent, thanks Anthony! I was thinking of building something similar for my 10".
A question for you dew experts, would it be better mounting a fan under the mirror mount as in the above pics, or mounting the fan onto some form of board/"plug" (which would require bigger mirror-locking bolts) to force the fan to draw air from inside the tube/around the mirror, as in stringscope's setup on the first page.
Originally posted by iceman Excellent points Ian.. I'll definitely have to make a dew shield/tube extension for my dob.
What can be done about dew on the primary mirror?
Mike,
I have a dew shield that I made that extends my tube by 320mm. It does an excellent job as a dew shield and also as a light baffle. Ian indicates the tube should extend past the secondary by at least 1 x the scopes aperture. That is consistent with my thoughts as well being that the tube should extend somewhere between 1 and 1.5 x the scopes aperture past the secondary. My secondary only dews under the most unfavourable conditions and even then it takes an awfully long time for the secondary to dew up. I actually forgot to take my dewshield with me to the April mini star party, you could have had a look at it and judged how effective it is. I really think it a better option than a secondary heater for several reasons including those that Ian mentioned.
In addition to the dew shield there are sound practices you can follow that also delay the onset of secondary dewing.
1. Return the scope to the horizontal position when its not in use.
2. Cap the focuser when you remove an eyepiece. A 35mm film cannister works well for this. Keep the cannister in your pocket and drop it in when you remove the eyepiece.
Originally posted by Thiink Excellent, thanks Anthony! I was thinking of building something similar for my 10".
A question for you dew experts, would it be better mounting a fan under the mirror mount as in the above pics, or mounting the fan onto some form of board/"plug" (which would require bigger mirror-locking bolts) to force the fan to draw air from inside the tube/around the mirror, as in stringscope's setup on the first page.
Simon,
Anthony's is a factory fitted blowing fan. There are 2 schools of thought as to whether the fan should blow on the primary or draw air down the tube. The majority of factory installed fans including the GSO dobs "blow" on the primary. I have an older GSO 10" dob to which I have installed a fan and it blows on the primary. My primary mirror cell has a steel cover plate and I have attached the fan to this. The newer scopes have a different mirror cell without the steel protection plate. I removed the plate to allow the scope to cool faster when I didn't have a fan but used the plate as a convenient method to mount the fan when I decided to stick a fan on it. I used a 12V DC 90cm brushless ball bearing fan from DSE ($20).
I am also in the process of fitting a 12V "air pump" to the scope to gently blow air around the face of the primary to "lift the boundary layer" I probably won't have this finished for another 2 weeks or so. I will post some pics of the entire setup when I have it finished. There has been a large amount of literature over the years about "removing the boundary layer" in tubed newtonians and I am convinced the results can only be positive.
If you care to post some digicam pics of the mirror end of the scope I can advise you about the best way to mount a fan to it.
Thanks for the reply ausastronomer. I've never noticed it before, but there is already mounting holes for an 80mm fan. The original question is still valid though, should I just mount a fan to that and blow onto the mirror, or should I surround the fan and draw air from the tube.
Originally posted by Thiink Thanks for the reply ausastronomer. I've never noticed it before, but there is already mounting holes for an 80mm fan. The original question is still valid though, should I just mount a fan to that and blow onto the mirror, or should I surround the fan and draw air from the tube.
Thanks.
I would get an 80mm Fan from DSE and just mount it to those mounting holes and blow air onto the mirror. The idea of this is to cool the mirror faster so that it reaches thermal equilibrium and the images stabilise more rapidly. This wont help the dew issue and to alleviate that i would mount an extension tube/dew shield to the front of the scope as Ian and I both discussed in an earlier post.
What some have done is to fashion a ring which seals against the inner wall of the OTA and with a hole with a diameter to just cover the primary mirror clips. This can give three possible advantages.
1: Channeling of the air from a fan behind the mirror to scour the warm air barrier layer from the mirror surface.
2: Act as a mask to hide any possible turned edge on the primary mirror.
3: Remove diffraction noise contributed by the mirror clips.
(None of this has anything to do with secondary dewing).
I have 1 fitted to my scope so I can show you exactly what it is. The only difference being, mine has a slightly larger aperture than the baffle Geoff is talking about. I installed mine purely as a light baffle hence the slightly larger aperture. I previously had installed a mask exactly as Geoff described but subsequently removed it as it caused an issue with tube currents but at the time I did not have the cooling fan installed. I may have a fiddle there again and see what effect the sub aperture mask has in conjunction with the cooling fan and the boundary layer air pump, that may just all work nicely actually with 2 air sources helping to move the tube currents.
Originally posted by ausastronomer I would get an 80mm Fan from DSE and just mount it to those mounting holes and blow air onto the mirror. The idea of this is to cool the mirror faster so that it reaches thermal equilibrium and the images stabilise more rapidly. This wont help the dew issue and to alleviate that i would mount an extension tube/dew shield to the front of the scope as Ian and I both discussed in an earlier post.
CS-John B
I'm going for a drive to DSE tomorrow (I have to go to Orange, they closed our DSE down here in Bathurst.. ). Whats the best way to feed the fan the required 12v? I could use one of the DSE 8x AA battery holders and use rechargeable batteries, but that only gives off 9.6v. I know the fans work on lower voltages, albeit at slower speeds, but is that going to be enough to cause enough CFM to be useful? What size/quantity of batteries does the factory fan use? I noticed the Orion branded fan/battery packs run 8x D sized batteries, ouch!
The only other thing I am going to need is a plug so I can disconnect the fan so I don't eventually crush the wires when I sit the tube on the floor inside the house while I am packing up.
Simon, you can get a 12 volt 4 ah Gel Cell,(jaycar part#sb-2484 @ $35.00),which will power the fan for days. It's a small compact battery. The fan runs on about 150 ma., but of course you don't use it all night. As for plugs etc, use a DC plug/socket. eg; part # pp-0511 (2.5 mm female plug) $1.55, & part # ps-0528 (2.5 mm male inline socket). $2.45. or if you want to mount the socket on a bracket, use > part #ps-0524 Bulkhead male. $2.65.
HTH. L.
I previously had installed a mask exactly as Geoff described but subsequently removed it as it caused an issue with tube currents but at the time I did not have the cooling fan installed. I may have a fiddle there again and see what effect the sub aperture mask has in conjunction with the cooling fan and the boundary layer air pump,
CS-John B
John I recall a discussion on one of the yahoo groups about this and I raised the concern about having to run the fan all the time without a choice due to tube currents entering the light path.
The reply was that a sub aperture mask isnt required and just needs to be big enough to direct air toward the upper edge of the mirror to create turbulence and it will give the desired effect.
Originally posted by Starkler A sub aperture mask isnt required and just needs to be big enough to direct air toward the upper edge of the mirror to create turbulence and it will give the desired effect.
Geoff,
I am aware of that but I want to try the sub aperture mask to eliminate the diffraction noise off the mirror clips, I already have the problem solved for the boundary layer.
CS-John B
Last edited by ausastronomer; 30-04-2005 at 09:54 AM.
here's my fan on the back of my 12" gso scope. I then put a round piece of "foamie" on the back to keep stray light out.
I tested the fan about 2 weeks ago & it ran on USED batteries for 7 hours!!!!!
--------------------------------
Mark
12" gso scope
). Whats the best way to feed the fan the required 12v? I could use one of the DSE 8x AA battery holders and use rechargeable batteries, but that only gives off 9.6v. I know the fans work on lower voltages, albeit at slower speeds, but is that going to be enough to cause enough CFM to be useful? What size/quantity of batteries does the factory fan use? I noticed the Orion branded fan/battery packs run 8x D sized batteries, ouch!
L.
