Making the most of an old Coronado CaK PST (Ca K-line Personal Solar Telescope).

Coronado was a premier manufacturer of solar telescopes and filters and ceased operations in July 2024.

Coronado was owned by Optronic Technologies, the parent corporation who also owned Orion Telescopes and Meade Instruments. Optronic Technologies ran out of capital following a costly antitrust lawsuit in 2019 and severe supply-chain issues as a result of the COVID-19 pandemic.

In July 2024 the parent company shut its California offices, laid off its entire staff and ceased all manufacturing. The remaining physical assets were liquidated at auction. In early 2025, the remaining inventory was acquired by retailers like High Point Scientific.

Coronado produced two versions of the PST: The H-alpha PST introduced in the early 2000s and the CaK PST released in the mid 2000s and discontinued it after a short production run due to practical and physiological problems. 

These telescopes were relatively low cost making them affordable to many amateur astronomers, but their cheap designs were a severe burden. They have severe back focus problems meaning that the H-alpha PST was primarily a visual instrument. In order to bring a camera to focus a Barlow lens or a lens such as the Magnimax could be placed at the end of the camera’s nosepiece to increase the focal length and allow the image to be brought to focus on the camera sensor. The CaK PST was slightly easier to bring to focus on some astronomical camera sensors, and due to the physiological problems associated with the scope had to be marketed as a photographic scope. 

The problem being that any person over the age of about 30 would be unlikely to see anything through an eyepiece when the CaK PST was pointed at the Sun, whilst a much younger person would be able to see a bright blue image of the Sun. This is because as people age, the cornea and lens of the eye naturally take on a yellowish tint which blocks near UV light and renders the Sun invisible to the eye when viewed through a CaK PST.

Notwithstanding these problems, both types of PST suffered from degradation of elements of the optical train due to the effects of atmospheric moisture. The H-alpha scopes suffered from the oxidation of ERF coatings and the Induced transmission filter (ITF) producing dim and blurred images rendering the scope unusable. Much like the H-alpha PST, the internal blocking filters and ITFs of the CaK PSTs were highly prone to the effects of moisture and edge-delamination over time. One of the consequences is that due to continuum light ingress, the image as seen on a camera sensor is totally lacking in contrast and impossible to focus. Due to the fact that the manufacturer no longer exists, it is very difficult to get such compromised telescopes repaired.

Our H-alpha PST, which was among the first of its kind to be imported into the UK, has developed these problems. With respect to the CaK PST, which was also one of the first of its kind to be imported into the UK, there has been little, if any degradation. Nevertheless, we  placed a Ca K-line narrowband solar filter on the nosepiece of the monochrome camera being used. This is to take advantage of the multiplicative nature of the transmissions of a given wavelength passing through two filters. Moreover, if there is any continuum leakage reducing the contrast, the additional filter will remove it.  A fact that is noteworthy is that for our H-alpha PST, the change from a perfectly functioning scope to a severely compromised one took place over the relatively short time of about a year.

The equipment used

A CaK PST mounted on a Skywatcher Solar Quest solar finding and tracking mount and an Altair 678M camera with 3840 x 2160 pixel resolution was for these tests and fitted with a Baader stacked Ca K-line filter on July 2, or a 3nm FWHM Ca K-line filter on July 6. AstroDMx Capture was used to capture a 500 frame SER file on July 2 and a 1000-frame SER file on July 6.

The best 95% of the frames in each SER file were stacked in Autostakkert!4, wavelet sharpened in waveSharp3 and further processed in Gimp3.

Results

The results are presented as colourised images in two common hues plus a monochrome image

July 2, 2026, Baader stacked Ca K-line filter

July 6, 2026, 3nm FWHM Ca K-line filter 

The images obtained by having the additional Ca K-line filter in the optical train had high contrast and were very satisfactory.

An un-cooled, high resolution image of the Eagle nebula

An un-cooled, high resolution experiment was carried out using a Stella Mira 66mm ED APO refractor with a field flattener. The scope was fitted with a ZWO EAF motor focuser and an Altair V2 magnetic 2” filter holder containing an Altair Quadband filter. A 12 bit ZWO ASI585MC OSC uncooled camera was attached. AstroDMx Capture for Linux x86_64 running on an Ubuntu Linux mini computer was used to capture 78 minutes worth of 3 minute exposures of the Eagle nebula (centred on the star HIP89743) as FITS images with  matching darks, flats and dark-flats.

An SVBONY SV165 guide scope with a natively connected QHY-5II-M guide camera was used for PHD2 multistar pulse auto-guiding via an INDI server running on a Fedora Linux mini computer. The auto-guiding was controlled by a separate Linux laptop indoors. AstroDMx Capture sent the scope/mount to the star Arcturus which was used to focus the scope with a Bahtinov mask. The ZWO EAF was controlled by AstroDMx Capture via the INDI server. AstroDMx Capture then sent the scope to the star HIP89743 within the Eagle nebula to optimally place the nebula in the field of view.

The equipment used

The ZWO ASI585MC uses an IMX585 sensor which is BSI, and built on Sony's STARVIS 2 technology. It has a full well depth of 40k to 47k electrons and a Read noise of 0.7 to 5.5 electrons depending on gain. It has a high QE of 91% at 500nm and 80.9% at 656nm.

The ADC bit depth does not determine the physical dynamic range which is calculated as: 

Dynamic range = Full well capacity / Read noise

A 12-bit ADC offers 12 stops  212 = 4,096 of digital container. If one shoots at the optimized HCG gain setting where dynamic range increases and read noise falls to extremely low values every digital step represents genuine, photon data.

The ADC bit depth determines how much charge CAN be stored, but as long as the full well capacity is not met by the incoming photons producing charge, a 16 bit ADC depth has no advantage over a 12 bit ADC.

What the 12 bit ADC does limit is the exposure time. The exposure must be limited so that the full well capacity is not reached or the centres of stars will be saturated and blown out.

Stacking Nullifies the 12-bit Limitation. While a single 12-bit sub-exposure might technically show minor step-like gradients in very smooth nebulae, stacking images permanently breaks the 12-bit ceiling. When deep-sky stacking software averages many 12-bit sub-exposures the mathematical integration recalculates the data into 32-bit floating-point files. The fractional differences created by the noise floor create very smooth gradients that can equal native 16-bit files. The advantage of the 16 bit ADC is that much longer exposures are possible before the stars become saturated.

This thinking is behind the practice of EAA which frequently uses cameras such as the ZWO ASI585MC and short exposures with live-stacking, to build up an acceptable image, often for outreach but also as an observational technique not using an eyepiece.

Another point to note here is that if a number a short exposures are captured with the aim of preserving the fidelity of the star colours, then the stars from the stack of these images can be used to replace possibly-saturated stars in the longer exposures. These short exposures might ideally be made through a UV/IR cut filter so that the star colours cannot be influenced by a filter such as a quadband or dualband. This is a technique that we shall explore in the future and is analogous to the standard technique of using RGB stars in narrowband images.

AstroDMx Capture saving 3 minute exposures of the Eagle nebula

Negative preview

The data were Stacked and part processed in PixInsight and further processed in SetiAstroSuitePro, and Gimp3. 

The Eagle nebula

The experiment showed that it is possible to get acceptable deep sky results from a 12 bit OSC camera such as the ZWO ASI585MC which has an 8.29 MP sensor of dimensions 11. x 6.3 mm and a resolution of 3840 x 2160. When the ZWO ASI585MC is paired with a Stella Mira 66mm ED APO refractor with a field flattener, it produces high resolution detailed images.

We look forward to working more with this setup and exploring the use of short star exposures as discussed above.

Meanwhile Nicola continues the work on Version 3 of AstroDMx Capture whilst still maintaining the code-base of Version 2.