Tuesday 15 January 2013

Unexpected Tuesday Sun & Testing a New Scope - 15th January

Sometimes solar opportunities just present themselves when you're least expecting it, and today was no exception.  I went to work this morning and upon arrival we were all sent home as there was severe icing on all outside surfaces.  Subzero temperatures and clear blue skies could only mean getting the solar scope out and grabbing some images:  The shot above was taken with the DS-PST and DMK31 camera with a 0.5x focal reducer.  Normally I don't advocate the use of focal reducers, but they allow me to image the full disk in one go, and seeing as this was mounted on an undriven camera tripod this allows me to capture 2000 frames before the sun drifts out of the field of view - a nice amount given the poor seeing associated with the low angle sun.   Monster active region AR11654 is still plainly visible in the centre of the suns disk.

I also had enough blue skies to do a visual test of a prototype scope i'm currently making.  This is going to be a 70mm f6 PST mod, using my workhorse 70mm refractor pictured.  I really like this scope as the 420mm focal length allows easy full disk imaging.  The jump from 40mm PST to 70mm aperture is quite significant and allows alot more structure to be seen on both the disk and also with prominences on the limb.  It also allows a much higher magnification to be used visually.  The scope is very portable and collapses down into next to nothing size wise when travelling.  From my location 70mm seems to be an aperture I can use regularly without suffering the detrimental effects of seeing. 

The scope uses a 75mm Baader D-ERF as the primary filter as can be seen in the shot on the right.  This is mounted in a custom designed and made acrylic cell that sits with an interference fit inside the dew shield of the scope.  This is easily removeable by simply unscrewing the locating ring on the dew shield when the scope is needed for CaK duties.  For the scope to work at it's optimum I have had to replace the collimating lens on the PST etalon assembly from its native f10 lens, to a f6 alternative.  Edmunds Optics provided the necessary glassware in the form of a 40mm x -120mm fl negative focal length achromatic lens.  This will be stopped down to 20mm to match the aperture of the etalon of the PST assembly, and, in the process will take the focal ratio to f6 to match that of the donor scope OTA.  Whilst I am currently awaiting delivery of this lens, I tried in it's place a 40mm x -80mm fl lens from a barlow that I had lying around.  While this was not going to give a collimated beam going into the etalon assembly I was keen to see what it would show:  promienences were very apparent as was some disk detail in the centre of the  field of view.  The image very quickly went off band away from the centre of the field of view and sweetspotting was very apparent, however, this is exacttly what would be expected from using a collimating lens of the  wrong specification!  What it did show well was a good indication of image brightness and also what magnification I can expect to use with this setup.  All in all i'm very pleased with how it is developing and are looking forward to first light properly hopefully in a couple of weeks time.