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.

Over the years I've hauled many different setups to solar eclipses - and this year is no exception. Each time I have some different ambition, requirement or constraint.

To photograph totality I want to use a 15 year old Borg 100ED F/4 refractor. This instrument isn't too heavy and is fairly compact. The camera will be a Nikon D300 - not state of the art, but this is what I have. I do have lightweight and heavy-duty tripods and mounts which I've used before. However, these are either too little or too much - resulting in either non-optimal images or very heavy luggage. So, this time I'll be optimizing the setup by buying a new tripod and telescope mount - here's what I just ordered yesterday from a German shop:

The Berlebach Tripod UNI 8 is a transportable tripod for astronomical mounts - best quality "made in Germany". The low height makes it ideal for Newtonian telescopes, 323 euro.

• Weight 6 kg - 13.2 lb.
• Minimum height 60 cm - 24 inch
• Maximum height 83 cm - 33 inch
• Transportation length 67 cm - 26 inch
• Load capacity 60 kg (121 lb) with very good vibration damping
• Stable accessory tray (37 cm)

The leg lock offers an even spread and prevents the legs from slipping, 58 euro.

• Suitable accessory tray included
• The legs are locked in a defined angle of 23°
• The spread stopper comes readily installed with the UNI tripod

Berlebach tripod bag 24cm / 70cm length, 42 euro

TS AZ5 Alt-azimuthal Mount with fine adjustment in both axes. A very stable alt-azimuthal mount. Accommodates two different instruments in parallel. Features:

• maximum load 13 kg, at full stability
• fine adjustment with clamping in both axes
• telescope mounting with robust quick coupling with area clamping
• further attaching with 90° offset for binoculars or spotting scopes - up to 5 kg can be kept in balance
• counterweight shaft included in delivery, can be attached in two positions
• with versatile tripod connection

Berlebach Tripod Adapter from UNI to Vixen GP, Skywatcher EQ5 and similar. This adapter is necessary for using Vixen GP and SkyWatcher EQ5 mounts with the Berlebach UNI tripods.

For viewing the partial phases I have a 70mm refractor with a H-alpha filter - this can go on the same mount as the photography equipment since the mount can accept two scopes. Thereby I only have to bring a single tripod and mount instead of two!

I'm baaack.....

Hi world,

it has been three years since the blog was last updated. About time to get back to it! Since last time I have been on three eclipse expeditions to Ethiopia, Indonesia and Madagascar. Check out this movie we made from totality in Indonesia!

Ingrid and the corona - Indonesia 2016

Now I am warming up to the next total solar eclipse which will occur in the U.S.A. on August 21st, 2017. I am leading a group from Viktors Farmor to Oregon. In addition to providing a great experience for the guests I'll also attempt to photograph the eclipse using a 4-inch refractor and a DSLR camera. I have done almost all kinds of astrophotography over the years and a total solar eclipse is - by far - the hardest project to undertake. I have failed three times and succeeded once. Even with the one where I finally got the images the post-processing of these to yield a final result has proven so daunting that I haven't yet done it!

Still, I'm trying again. Follow this blog in the coming months to see how it goes.

Autostakkert / Avistack - a casual comparison

In this post I will compare the output from two of the best programs for sorting, aligning and stacking numerous exposures: Autostakkert v2.3 and Avistack v2.0. The subject is solar H-alpha images. The latest Avistack version is from July 2010 so it is not exactly undergoing a hectic development! Autostakkert is newer and is in a more dynamic development phase with minor updates occurring many times pr. year.

I did not spend a lot of time tweaking the processing parameters with Autostakkert - instead I deliberately did a 'quick run' using most settings at their default values. Avistack I know quite well and have been using for solar image processing for the past two years. During this time I have tweaked the settings to best suit my data. Hence, in a way the comparison is not 'fair' - that's why I called this a 'casual comparison'! See the cropped images below for the results, figure 1 is a prominence and figure 2 is an active region around a sunspot.

Figure 1 was made using 1500 frames from a Coronado filter on a 70mm f/7 refractor. The best 60 were used for the final image which then had wavelets applied to sharpen it (same settings for both images of course).

Figure 2 was made from 700 frames taken with a Daystar filter on a 6-inch f/6 refractor. Again, the 60 best frames were used but this time no wavelet processing was applied.

Fig. 1: comparison on a prominence.

Fig. 2: comparison on a sunspot.

The results are not dramatically different; both Autostakkert and Avistack do a very good job. However, the Autostakkert result is generally slightly better with just a little more 'omph' to it. Details of the settings are given at the end of this post - notice that Avistack has many more parameters to set. I have tried to choose settings for the programs that were similar to each other, but of course I cannot rule out that some Avistack settings exist that would allow matching or even surpassing Autostakkert. However, I do know Avistack quite well and have tweaked its parameters extensively in the past - I find that the results do not depend super critically on the chosen parameter settings.

In conclusion: Autostakkert gives a slightly better result and has much fewer parameters that require setting - a clear advantage, in my opinion. Some will balk at this 'lack of control' but sometimes there are just too many things to adjust and you can never be sure that the correct settings have been found. I find that Autostakkert is simple to use and works very, very well 'right of out the box'.


Some of the parameters used for the comparisons:

Autostakkert: image stabilization: surface, noise robust: 3, AP size 50, HQ refine: on, Drizzle: off, number of frames to stack: 60.

Avistack: quality analysis: on, frame cutoff: 60, area radius: 35, search radius: 28, alignment type: surface, ref. point min. distance: 13, structure threshold: 44, lower cutoff: 0.23, noise reduction: 0, quality area size: 32, ref. point alignment area radius: 24, ref. point alignment search radius: 2

Throwing out the focuser

Every summer - where the Danish nights are too bright for deep sky imaging - I have fun doing Solar imaging instead. Being able to enjoy astronomy on a nice summer day without the cold and fatigue of a winter night is great!

I do high resolution imaging using a 6" f/6 achromatic refractor and a Daystar Quantum SE 0.5Å H-alpha filter. A Baader TZ4 tele-extender increases the magnification four-fold. When building such a setup the imaging train on the rear side of the telescope becomes really long and this leads to significant problems with flexure. Over the last few years I have optimized my setup to employ threaded connections everywhere, thus reducing flexure to a minimum. The imaging train currently looks like this:

Imaging train for high-res Solar imaging in H-alpha light (click to enlarge).

A special feature of this setup is that the telescope's original focuser has been replaced by a fixed length tube. Instead, focusing is done with a small helical focusser just before the camera. This way only the camera is moved and not the entire imaging train. This makes for much less flexure and focus slippage while shaving off nearly a kilogram from the total weight on the rear side of the scope.

Yesterday was my first time out this year with this setup and it was also first-light for the fixed-length telescope adapter. I had measured out very carefully how long it should be, but it was still scary-exciting to see whether or not I could get a sharp view using the little helical focusser at the camera. Fortunately, it worked just fine!

I only had a few minutes before having to go out but I managed to shoot active region 1748 which just a few days before unleashed four X-class flares in quick succession. More flares are likely but nothing happened during the two minutes I shot it.

AR1748, best 60 out of 2200 frames (click to enlarge).