Fast imaging

Is Fast Imaging a Solution to Limited Clear Skies?

Over the past couple of years, the recent run of poor weather here in West Wales has led me to look for an economic way of collecting data faster. This will provide the opportunity to make the most of the available clear skies.

The majority of my deep sky imaging takes place on my annual winter trips to Portugal. Here over 10 weeks I can expect to record upwards of 100 hours of high quality data. This allows for a deep dive into many objects and also narrowband imaging.

However, after six years of practice, the data collection process has become “easy” as I have my astrophotography rig and software running without issues for the vast majority of the time.

To some extent I have lost the fun and immediacy of capturing images. With data runs of 12-24 hours it is the processing that is the enjoyable part. In addition here in West Wales, the lack of prolonged clear skies means that the ability and motivation to capture long data sets doesn't really exist.

Fast Imaging Telecopes

For these reasons, I have looked into fast imaging telescopes with an f-ratio of f/2. For context, an f/2 telescope is 6.25 times faster at collecting light than an f/5 telescope. That means that a one minute exposure with an f/2 scope is equal to 6.25 minutes with an f/5 scope.

There are a number of telescopes available, however, the traditional 8” and 11” Celestron RASA scopes were outside my budget. These are very good scopes with good optics but with a price tag of £2600 to £4800. The newly launched Celestron 6” RASA is £2000. If you can afford one and have a mount to handle them then they are a great way to get into fast imaging.

Last year Skywatcher launched a new version of their HAC 125 f/2 (Honders Advanced Catadioptric) reflector in the UK and USA market. At £599, the scope met my budget and early reviews seemed positive. As a route into fast imaging I purchased one of the first batch into the UK. Given the central obstruction I calculate this scope is five times faster than my SW

ED80/400mm refractor.

Skywatcher HAC125DX

The telescope is a 125mm diameter reflector with a focal length of 250mm. The build quality is good with major components manufactured in Aluminium. The focus knob is on the rear cell and is smooth in operation. There are adapters provided for various camera makers. The scope can accept cameras with an image circle of 16mm. This includes the IMX533; IMX585 and IMX183 sensors in cameras from ZWO; Altair and QHY. 

Collimation is achieved by three sets of push-pull screws in the rear cell. A dew shield is provided along with two finder scope shoes. The scope has a Vixen dovetail. The telescope weighs 4kg so can be mounted on a small GEM; Strainwave or Star Adventurer GTI. If you have a mount already, use that!

Click on an image to get a closer view

Set-Up and Imaging

The set up of this scope is quick and easy. The camera is mounted in the appropriate adapter and then fitted into the holder with a thumbscrew. See below.

The camera cable is fed through a hole in the dew shield. The green focus knob at the rear is easy to use. A guide scope can be fitted into the holder. Guiding with PHD2 is recommended to take advantage of dithering to prevent walking noise.

To capture images the camera, guide camera and mount need to be connected to a laptop running image capture software such as Sharpcap or AstroDMx Capture. Once polar aligned and focused, imaging can begin. I recommend taking 15-20 second exposures and using thelive stacking feature of the software. The captured RAW .FITS files can be saved for later processing. Using live stacking capture is a great way to enjoy this scope. Due to the amount of light-gathered in each exposure, visible results are obtained within a few of minutes for bright objects such as Orion Nebula and Andromeda Galaxy.

Twenty to forty-five minutes of exposures stacked and processed can produce very nice images. In addition data for mosaics of 4-6 panes can be collected in around two hours. In an evening with five hours of clear skies it should be possible to collect a good level of data for six to eight objects.

Example Images

I have taken a number of photographs of celestial objects with this telescope and I am generally happy with the results. The scope has enabled me to use the clear skies that I have in a productive way, rather than being frustrated with not getting enough data with my f/5 and f/8 telescopes.

NGC1499 California Nebula 20mins of 15s exposure processed in Pixinsight

Orion Nebula Stack of 20sec images as seen on screen during live stack.

M42 

Bodes Nebula (M81 & M82) 30mins of 30sec exposure processed from live stack

NGC7000 

North American Nebula 3x2 Pane Mosaic. Each Pane 20mins of 15sec exposures.

Total of 2 hours of exposure time.

A Great Scope for EAA (Electronically Assisted Astronomy)

While in Portugal I held a couple of EAA Sessions for non astronomy friends on the campsite. It was a great way to engage people as they are able to see amazing objects after a few minutes of exposure using the live stacking software. In an hour on a very cold night we viewed six objects and there was genuine surprise that it was possible to see such clarity and detail in only 10 mins of exposure time.

Conclusions

Overall I have been pleasantly surprised with this telescope. It is well made, robust, light and easy to set up. The optics have been designed by a European firm of optical designers and work well. The focus is smooth and offers a generous throw to enable accurate focus. A 3-D printed  auto-focus kit is available from Starizona and can be fitted with a ZWO Focus Motor. I have this upgrade and it works well providing a more accurate focus. There are a couple of issues that need to be mentioned. The first is that a single thumbscrew holds the camera in place. It has a clamp ring but three screws would be better. It is vital to ensure the camera is clamped firmly to prevent tilt. Secondly the optical quality in the image corners is acceptable but not great. For a scope of this type and price point this is forgivable and the quality in the majority of the frame is good. You only notice poor star shapes when zoomed in. The scope does punch above its weight for the price and offers a budget way to access fast f/2 imaging to make most of the rare clear skies in West Wales.

Paul Howat​

March 2026