Focal Length: 2,032mm (f/10)
Supplier: Altair Astro
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Aperture fever describes when someone has a burning desire to increase the light-gathering power of their telescope.
For night-time instruments, larger apertures mean more light collection, but it is even more valuable for solar observing, as increased aperture also improves resolution, allowing finer detail to be seen.
Consequently, solar observers regularly get the desire to upsize too.
However, upgrading to a large hydrogen-alpha setup can be very costly, certainly far more expensive than moving from a small night-time instrument to a large one.
The Airylab 203mm Ultra High Resolution Hydrogen-Alpha Chromosphere Telescope, or Airylab ‘HαT’, tackles this issue.
It is a Celestron EdgeHD800 CG5 that has been modified to be a front end for a rear-mounted hydrogen-alpha filter.
The filter is not included.
The HαT works optimally with the Daystar range of hydrogen-alpha filters up to a field diameter of 44mm.
For our tests we mainly used a 0.6-Angstrom Daystar Quantum PE filter, but also tried it out with the less expensive Daystar Quark.
A high-end filter will cost you several thousand pounds on top of the HαT, but the Quark currently comes in at around £850.
It’s also possible to use the etalon from a Coronado PST or – with suitable neutral density filters – observe or image the Sun in white light through the HαT.
With the Quantum PE filter fitted, the HαT is an impressively simple system to use.
It is lightweight, and the button-press tuning of the filter means that everything is kept at the eyepiece end of the scope where it’s easy to reach.
Of all the solar setups we’ve tested over the years it is among the easiest to use, despite the fact that it is aimed at serious solar observers.
Seeing and the Sun
Our first efforts to view the Sun were a little blurry.
Like all Schmidt-Cassegrains, the HαT can go out of collimation, something we confirmed by observing an almost full Moon.
We decided to recollimate using a star.
Finding one through the coated corrector without a finder initially proved bothersome – though we did manage it.
We then fitted the supplied 2.7x telecentric amplifier and tweaked the collimation again to get it perfect.
The next day we had another go and the story was completely different. Now, despite mediocre seeing, we saw what all the fuss was about. Active regions showed superb fine detail, which we were also able to record in our images.
The setup was fantastic on the chromosphere and for prominences alike.
Again, we witnessed tantalisingly fine detail in these huge features as they extended off the edge of the Sun.
A minor gripe is the lack of fine focusing, the standard Schmidt-Cassegrain focuser of the EdgeHD800 being a little coarse for high-resolution observing.
The chromosphere is a blanket layer of hydrogen covering the Sun’s visible surface to a depth of several thousand kilometres.
Seen at the edge of the Sun, it takes on an appearance that looks like fine fur.
The high-resolution capabilities of the HαT revealed the spicules, or jets of gas, that create this impression really well.
Scaling for success
The telecentric amplifier mentioned above increases the effective focal length of the scope to around 5.6m.
This gave us an image scale that was magnified but still relatively easy to work with.
The contrast of the view looked low compared to other systems we’ve used but this is fairly easy to correct when imaging.
Delighted with the results we got with the Daystar Quantum PE, we decided to replace it with the less expensive chromosphere optimised Daystar Quark.
This has its own 4x amplifier built-in so we didn’t use the telecentric amplifier supplied with the HαT.
Here the extra amplification pushed things a little too high for our observing conditions but it has to be said that this also produced reasonable results.
As with any large aperture telescope, the key to obtaining the best results is to use the scope when the seeing is good.
Sadly in the UK this can take a long time to occur but when it does, the Airylab HαT can prove itself to be worth every hard earned penny spent on it. It can reward you with views of the Sun that will take your breath away.
A Cure for aperture fever
An 8-inch aperture hydrogen-alpha scope is something of a dream machine if you’re into solar observing.
Coupled with a suitable hydrogen-alpha filter, the Airylab HαT breaks the confines that have dogged solar observers and imagers for many years.
Though other options are available, they tend to be large, heavy and expensive refractor-based solutions.
The full aperture energy rejection filter-coated corrector on this scope has a 120nm bandpass (full width at half maximum) with a transmission of greater then 80 per cent at the hydrogen-alpha wavelength.
The extra resolution provided by the HαT’s 8-inch aperture works best if the seeing is good to excellent.
To make the most of good seeing, Airylab also offers an optional solar scintillation monitor, which attaches to the HαT and reports on the quality of the sky; with certain cameras, it can even automate imaging to only capture when the seeing gets good.
The corrector plate is coated with a hardened dielectric layer.
This layer, visually blue-green, provides the energy rejection filter properties that make it safe to point the scope at the Sun.
The filter’s red transmission also means you can use this scope for red-filter planetary imaging or recording long exposures of the deep-sky in hydrogen-alpha.
The HαT has a simple but effective slip-on white plastic cap fitted over the secondary housing to help keep the normally dark assembly cool while pointed at the Sun.
The cap did turn out to be a little loose though, and had to be pushed back on frequently.
Celestron EdgeHD telescope
The Airylab solar modifications are made to a standard Celestron EdgeHD800 CG5 8-inch – and its 9.25-inch and 11-inch cousins can be modded too.
According to Airylab, particular instruments are benchmark tested for better than one-quarter wavelength performance at the 635nm wavelength.
This means that the telescopes are optimally suited for high-resolution hydrogen-alpha solar work.
Celestron CGE dovetail
The standard EdgeHD800 comes with a Celestron CGE dovetail that will fit Celestron CGEM, CGEM DX and CGEM Pro mounts.
The dovetail is also Losmandy compatible, a standard fitting found on many highend mounts.
As the 8-inch EdgeHD is quite light, you shouldn’t have trouble using it on most mid-range mounts.
2.7x telecentric amplifier
The converging rays coming from the telescope need to be virtually parallel when they enter a rear-mounted hydrogen-alpha filter etalon.
This is achieved by fitting the custom-designed telecentric amplifier between the scope and etalon.
The amplifier increases the effective focal length by 2.759x – in this case 5.6m.
This review originally appeared in the October 2015 issue of BBC Sky at Night Magazine.