Checking collimation and killing fringes

After many cloudy days and nights I finally had the chance to test two things:

1. does collimation of the F/4 refractor hold up against time and handling?
2. how much can the blue fringes of my ED optics be improved by filtering?

Ten days ago I succesfully collimated the optics of my refractor - operating at F/4 is quite unforgiving in terms of alignment. However, it is not enough just to be able to collimate the scope and take images. I must be able to collimate the scope the evening before the eclipse, transport it for many hours in a bus over bumpy roads - and then get sharp images after setting up in the field. It has now been 10 days since the first collimation and I've transported the scope a couple hundred kilometers by car and handled the scope quite a bit.

Testing the scope on the artificial star showed that collimation had remained good! I did not even tweak it before continuing. Next, I aimed at Vega, focused using liveview in Imagesplus and ran the Eclipse Orchestrator script that will be used during the eclipse. This gave lots of shots, from which three closeup images at three different exposure times are shown below. The stellar FWHM is around 2-2.3 pixels - a bit more than last time but that could be due to different seeing conditions. The image scale is 2.86 "/pixel, so my angular resolution is around 6".

Closeup views of Vega at three different exposure times.

The new 'fringe killer' filter was installed for the images above, but to really see the effect I have compared saturated images of Vega (1/2.5 sec, ISO 200) to identical exposures taken an earlier night without the filter. As seen below, the filter clearly reduces the intensity of the blue halo. I'd say that anything the size, price and convenience of this filter, that will help improve coronal contrast, is very welcome in my setup!

Saturated exposures of Vega showing how the Fringe Killer filter helps reduce the intensity of the blue halo.

A more quantitative analysis of the filter performance is shown below. The graphs were made by averaging all lines of the images above. The red/green channels are unaffected by the filter, while the blue channel is reduced by ~35%. Definitely worth the effort!

Line averages from the Vega images above. Red, green, blue lines correspond to each color channel of the image; dotted lines are without the filter while full line is with filter.

Getting aligned...

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.

Smearing of stellar image during 1 sec exposure due to diurnal motion.

Putting the pieces together....

The new tripod and alt-az mount has arrived and look very nice - great workmanship and finish. The compactness of the tripod and dual-scope capacity of the mount will do wonders in reducing the amount of gear I need to haul along. I am using a simple aluminium plate to connect the Borg refractor to the mount - it needs to be offset pretty far from the mount to ensure proper balancing.

Berlebach tripod and TS AZ5 mount with the two refractors that I'll be using for the upcoming total solar eclipse.

Next step is to connect all the parts and do some field testing of the imaging setup. I need to find out if the setup suffers from vibrations and investigate if the optical quality of the 4" f/4 refractor is OK. Doing this is really simple: just set up like it's a total solar eclipse, point the scope at Vega and run the Eclipse Orchestrator script. This script runs through a number of exposures, ranging from 1/800 to 1.3 second. The short exposures will primarily sample the optical quality while the longer ones will be sensitive to vibrations.

In-focus stellar images taken with Borg 4", f/4 refractor. Scope was not collimated prior to this test.

I focussed the scope using a zoomed liveview display on my laptop. A bright blue halo appeared around Vega at focus and the highly out-of-focus image was not perfectly round. This is not indicative a excellent optical performance! I was in a hurry, so I pressed on running the script. Below are shown six images from the run; three at 1/320 second exposure and three with 1.3 seconds. Each series shows the same star which is near the center of the field. From these shots I conclude:

1. The short exposures are generally sharper and more variable - both effects are due to atmospheric seeing.
2. The short exposures reveal an asymmetrical halo - this is likely due to optical misalignment within the scope.
3. The long exposures show no signs of vibration

The refractor used for this test consists of a Borg 4" f/6.4 ED lens coupled with field corrector and reducer lenses to yield f/4. Such a fast configuration does require careful attention to mechanical alignment of the optical elements - and I did none prior to this test. I have previously done tests at f/6.4 where I could reach FWHM=3.2 pixels.

Next step is to try and improve the collimation of the f/4 configuration - if I can't get it satisfactory I'll go for f/6.4 and the sharper images. Perhaps it would also be beneficial to use a filter to reduce the blue wavelengths - this is known to improve contrast with achromatic and ED refractors.