Wednesday, 20 July 2016

Hot on The Sun - 19th July (Mods and Developments in Solar Animations)

Temperatures may have been up in the low 30's on the hottest day of the year, but things are much hotter on the sun.  This animation represents several weeks of trial and error and modifications of the 0.2m Airylab HaT.  

Firstly, this animation was captured at the same cadence as the AIA imaging rig on the solar dynamics observatory, ie 1 frame every 13 seconds.  Straight away this demonstrated the importance of short exposure runs, as at this image scale temporal distortion can be a real problem in blurring images.  Indeed, I may try and increase the cadence still further and achieve 1 frame every 10 seconds in an animation.  This was done using the PGR Blackfly GigE IMX249 camera; however using the Chameleon 3 and 2x2 binning should give a much faster exposure, frame rate and cadence, and the original image size will make capture of the frames and post processing easier in terms of computing power.  This incidentally is currently the biggest hurdle to overcome in terms of pushing the animation envelope further.

Secondly, and more minor, was the relocation of the Hutech solar guider directly by the aperture of the HaT, and this along with tightening backlash in the gears on the HEQ6 mount has resulted in much better tracking, with fewer errors and drift.  

Thirdly retrofitting a Starlight SCT microfocuser in the place of the original focuser on the HaT has made focusing, both visually and for imaging an absolute dream and doddle and is whole heartedly recommended for all users of the Airylab HaT.

Finally the introduction of the Tempest SCT Cooling Fan system to the HaT has made a real improvement in image stability.  Just to come back to the start of the post, this animation was made with a 0.2m scope on the hottest day of the year.  Normally observing with such large apertures would be impossible, restricting use to smaller scopes, however the cooling fan makes a real difference.  The principle of this product originates for night time astronomers, where as the ambient temperature cools down in an evening / night this is not matched by the cool down of the SCT telescope which then introduces thermal gradients within the scope as different parts of it cool down at differential rates. The converse principle operates during the daytime, in so much as the ambient temperature is often rising and hotter than the temperature in the scope.  The exit beam from the HaT is 'warm' too, so there will be some heating of the air (and also components) within the tube assembly.  Regardless of whether it is hotter or cooler the point is it is not in equilibrium, and this is what introduces convection of the air within the OTA and it is this which affects the seeing conditions in a negative way.  It was quite clear with the TEMPest that it was having an effect, turning the fan on it was apparent on the laptop screen that the image was noticeably steadier, turn the fan off and within about 30 seconds the instability started to get greater.  Turning the fan back on and after about 30 seconds it would calm down again.  There was a clear correlation with fan off / fan on.  The system works by having one fan draw air into the OTA and the other fan drawing air out of the OTA.  This introduces a flow of air over the mirror that prevents thermal gradients forming over the mirror or around the walls of the OTA.  The effect is not subtle, and draws the question of whether this principle could help other solar scopes, especially where a full aperture ERF is not used and there is heat build up in the scope.  A cooling fan on the Tal100R for CaK work may be a future project to look at!

This timelapse is over 10 minutes and was taken with the Airylab 203.2mm HaT telescope, a double stacked Daystar etalon at 5.6m focal length using the PGR Blackfly GigE IMX249 camera.  The link to the full size animation can be found here.