During my last session I found that the Borg F/4 refractor was poorly collimated - not really very surprising since I hadn't done any adjustments after assembling the scope parts. In the past, I have not been able to satisfactorily collimate the optics of this scope - there just wasn't enough travel in the main objective adjustment screws. So, I was a bit worried whether I could do it this time. If not, then I'd have to ditch this configuration and go with the main objective at F/6.4 without field reducing optics.
Last evening I set up for collimation using an 'artificial star' on a tripod 70 meters away. With a 13mm eyepiece and a 5x Televue Powermate it was easy to see the in-focus diffraction rings - they were indeed way off as the previous test session had suggested. This time I had no problems getting good collimation; although it was at the extreme end of the refractor's possible adjustment range.
Right after doing this I attached the DSLR and aimed the scope at Vega, focusing in liveview with Imagesplus like before. Then I ran the Eclipse Orchestrator script, running through various exposure settings. The image below shows three identical shots before and after collimation (exposure time 1/320 sec, ISO 100). The improvement is dramatic: FWHM has decreased from 4.3 to 1.8 pixels - great!!!! The next question is: how stable is the collimation? I need the scope to be able to take some handling and still retain excellent collimation, since it won't be possible to fiddle with that on eclipse day. I'll do some scope handling and then repeat this test a week later to see if collimation holds.
|Close-up images of Vega before and after collimation of the Borg F/4 scope.|
A longer exposure is shown below - the blue halo seen last time is still very apparent. This reveals a limitation of my optics: they are not true apochromatic; 'only' extra dispersion (ED) corrected. This will reduce contrast of fine coronal details and produce a blue ring where the black moon meets the bright inner-corona. However, the problem can be alleviated with the 'Fringe Killer' filter which reduces the blue/violet component of the spectrum. I ordered the 2" version which will fit inside my existing adapter, roughly 2" in front of the DSLR sensor. Can't wait to test this out!
|Longer exposure reveals a blue/violet halo, as expected from ED optics. I want to avoid this on eclipse day!|
A final thing I learned this evening was just how long I can expose without experiencing image smear due to diurnal motion. The image below is a one second exposure; it is fairly clear that half a second will have no image smearing.