HAC125DX astrograph with SynCam 585C CMOS camera

A fast, compact astrograph paired with a small-format CMOS camera promises rapid deep-sky imaging at modest cost. Chris Woodhouse puts the HAC125DX and SynCam 585C combination to the test.

Chris Woodhouse

At a glance

Ape HAC125DX astrograph

Aperture: Focal length: Effective focal ratio: Imaging circle: Field of view: OVL code: Price:

125 mm 250 mm f/2.2 16 mm 3.7° 10282 £600 (OTA)

SynCam 585C CMOS camera

Sensor: Back focus: OVL code: Price:

Sony IMX585 12.5 mm 20141 £299

Bundle HAC125DX + SynCam 585C

OVL code: Price:

10282/20141 £898

My astronomy club has an annual light-hearted debate at our Christmas social. Last year’s question was whether the golden era of terrestrial astronomy lies in the past, present or future. I argued for the present: modern manufacturing and electronics have made practical astronomy more accessible than ever, even as light pollution and unpredictable weather continue to worsen. It is against this backdrop that I was asked to review the SkyWatcher HAC125DX, a fast f/2 astrograph designed for deep-sky imaging, paired here with the SynCam 585C camera. With its compact design and short focal length, it promises to capture faint nebulosity far more quickly than more conventional systems.

For the last few years, my own imaging setup has relied on patience rather than speed, using dual refractor rigs on a single mount driven by NINA’s synchronisation and scheduling tools. Some of my images exceed 1,200 five- minute exposures. That approach works, but it is demanding on both time and conditions, and it makes the appeal of a genuinely fast optical system obvious.

For the HAC125DX, an APS-C or full-frame deep-sky camera is too large for front-mounting and overkill for the small imaging circle. An optimal coupling is a small one-shot-colour camera like the new SynCam 585C, or its square brother, the 533C. Both cameras are available separately or as part of an imaging bundle with the HAC125DX.

With its compact design and short focal length, it promises to capture faint nebulosity far more quickly than more conventional systems.

SynCam 585C

This uncooled one-shot-colour camera (and its larger sibling, the 533C) has 12.5 mm back focus and is ideally suited for front-mounting. Like the SkyWatcher, they are keenly priced at £299 and £499 respectively. To keep things compact, these cameras are not cooled and are powered via their USB 3.0 connections. Exposure times are typically shorter with a fast f/ratio instrument and dark noise is less of a concern. Both cameras feature high frame rates; the 585C sensor has a video-like aspect ratio of 11.2 × 6.3 mm, while the more unusual 533C is 11.3 mm square, both within the 16 mm imaging circle of the HAC125.

The sensors in these cameras are quite distinct; the 585 has a 12-bit ADC, while the more expensive 533C is 14-bit. They both have several operational ‘modes’.

For deep-sky images with faint nebulosity and stars, I chose HDR mode, which optimises dynamic range, with a modest gain of 200, resulting in a sensor gain of 0.2 e /ADU, 1.79 e noise, 13 stops of dynamic range and a 13,000 FWC. Both cameras are fitted with an IR-blocking window in front of the sensor. The SynCam manual recommends using the ToupTek ASCOM driver available, which worked flawlessly throughout the review.

HAC125 DX

This unusual catadioptric instrument (which simply means a combination of mirrors and lenses) features a 125 mm diameter Mangin primary mirror to minimise spherical aberration. Together with the closed tube design, the silvering is protected from the environment. The front lens is doughnut- shaped, with a central tubular baffle and field- flattening optics behind a camera coupler. The imaging circle is small, at just 16 mm in diameter, allowing the instrument to image up to a 3.7-degree field of view.

The central obstruction introduces its own diffraction and nominally reduces the effective aperture to about f/2.2. At the rear of the main mirror are three sets of opposing push/pull collimating adjusters. These are pre-adjusted and, unless there is a good reason, best left alone. The camera position remains constant, and the large smooth focuser knob in the middle of the rear plate moves the primary mirror. (Autofocus operation is possible by usinga third-party autofocus module and a pulley/ toothed-belt coupling.)

The camera coupler extends out front and, with the 585C attached, the clip-on dew shield just clears the rear face of the camera (without its USB cable). When fitted, the USB cable folds back and passes through a rubber grommet on the side of the dew shield to keep things tidy. The main optical tube has two standard finder shoes, and the entire unit, with its sturdy tube rings and Vixen dovetail bar, weighs about 4.5 kg.

In Practice

Fast f/ratio instruments are sensitive to sensor spacing and tilt. This package includes three adaptors: a standard 1.25-inch and two 42 mm (T2) adaptors, which assume 6.5 mm and 12.5 mm camera back focus, with the aim of placing the sensor plane 30 mm from the rear optic. The camera adaptor terminates in a circular dovetail secured to the telescope by a single thumb screw. I noted a little camera tilt and wobble and would have benefited from a more secure retention.

After fitting the camera via the short adaptor, I loaded the system on a small harmonic drive mount and polar-aligned it. I attached a short guide scope and the HAC125 on a side-by-side saddle. While the optics acclimatised, I calibrated the guider system. Although an f/2 instrument facilitates short exposure times, harmonic drives are known for large peak-to-peak tracking errors and, more significantly, periods of rapid error change that require rapid correction through guiding (or integrated optical shaft encoders). Autoguiding with 0.3-second exposures kept tracking errors around 0.5 arcseconds, much less than the 2.4 arcseconds/ pixel scale of the 2.9 μm sensor.

Focus Fun

Focusing is best done with an imaging app set up to display a camera frame every second. The camera’s USB cable obstructs any focus mask, but it is easy to find the best focus by noting the star diameter readout in the app. In this instance, I used the free NINA imaging app, and it was straightforward to locate the focus position with the lowest HFD readout (a measure of star size). NINA makes an average assessment by measuring multiple stars across the entire image. This is helpful if there is a focus variation across the image, for instance caused by image tilt or field curvature. Out of the box, the first attempt achieved an HFD of 2.7 pixels (6.5 arcseconds). This improved to around 2 pixels (5 arcseconds) after some adjustment.

But the most revealing optical test is to take a brief exposure of a dense star field. If it can cope with that, anything else just requires patience and correcting any focus drift. The first test exposure of a loose cluster showed evidence of some tilt and mild field curvature, suggesting under-correction. Since the power of a field flattener usually increases with sensor distance, it made sense to try again with a greater camera spacing.

Collimation and Spacing

Fast telescopes are notoriously tricky to focus and collimate. An f/2 optic has a depth of focus of about ±2 μm and is extremely sensitive to the slightest mechanical adjustment. So, I decided to accept the factory alignment and a small amount of tilt.

As supplied, the SkyWatcher telescope has a fixed sensor spacing on the camera mount. Manufacturing tolerances on the camera and optical assembly are to be expected, but with a little ingenuity it is possible to adjust the focus plane by a millimetre or so in either direction. Inserting a slim 42 mm ring onto the camera mount extends the camera spacing. If the camera is too close, however, the addition of a 1.5 mm glass filter within the camera adaptor has the effect of extending the focal plane by about 0.5 mm. I temporarily retrofitted the camera adaptor thumb screw with an Allen-headed pointed grub screw and tried both adjustments. As expected, adding a few thin spacers improved field flatness, while inserting a clear filter made things slightly worse.

Inserting a narrowband, or dual narrowband, filter within the camera coupler is useful for imaging faint nebulosity. For best results, choose a model compatible with f/2 optics and consider adding a spacer to the camera coupling to maintain field flatness.

The HAC125DX is a curious beast, with fast f/2 optics and a short focal length of 250 mm. It has no direct competitors at this price point, but at a much higher cost a 6-inch RASA is not too dissimilar in its field of view and light-gathering capability. So too is a much longer refractor with a similar aperture and angular field of view. These systems share the same theoretical diffraction-limited resolution, set by their physical aperture, at about 1.1 arcseconds.

Although the HAC125DX’s f/2 ratio grabs the headline, the real standout feature is its system cost. In approximate terms, this telescope and sensor bundle is around £900, a fraction of a RASA or a long refractor with a large imaging circle and a sensor to match. Like many short focal-length systems, this SkyWatcher system is undersampled, as the sensor pixels are larger than the theoretical optical resolution. The 585C’s 2.9 μm pixels yield a system pixel scale of 2.39 arcseconds per pixel. It takes about 3.3 pixels to resolve a star pair, so the overall system resolution is more than 8 arcseconds.

But the most revealing optical test is to take a brief exposure of a dense star field.

Conclusion

With a little experimentation, this dinky reflector delivered an acceptable star field. Finding the best focus is a lesson in compromise; it is tempting to focus accurately in the middle, but this can make defocus or aberrations more obvious towards the edges. Usefully, NINA’s aberration inspector shows image corners at 1:1 magnification and makes it possible to find a focus position that gives the best overall balance across all nine regions. I also measured the test exposure with the venerable CCDInspector utility, which showed slight field curvature and a small amount of tilt. On the test image, two corners at 1:1 magnification show some star elongation, which is less obvious at normal viewing distances and even more so after running the BlurXTerminator deconvolution plugin.

Finally, before the clouds returned, I imaged a deep-sky target for an hour. I slewed to Caldwell 49 (the Rosetta Nebula) and settled on a 10-second exposure duration. Even then, a few pixels saturated at the centre of the brightest stars. The image of the Rosetta combines 360 10-second exposures, simply calibrated with dark frames. The evenly illuminated field and clean sensor made flat-frame calibration unnecessary. Image processing was kept to essentials and comprised deconvolution, noise reduction, sky-gradient correction, colour calibration, and separate stretching for the nebulosity and stars. A rewarding result in such a short time.

This HAC125 bundle sits alone in the small astrograph market. It is an enticing, affordable entry into deep-sky imaging over a modest target area with a unique short focal length, small imaging-circle combination. Its compactness and simplicity are commendable, and first-time imagers should be satisfied with the results out of the box. The SynCam 585C is ideally suited to this application, with no wasted sensor space, small pixels and excellent red sensitivity.

As with most systems, irrespective of cost, it requires a little experimentation to achieve the very best results. In the case of the HAC125, my recommendation is to embrace its simplicity, optimise the camera spacing, but resist the temptation to tinker with collimation adjustments which, at the best of times, are not for the faint hearted.

Chris Woodhouse is a fellow of the Royal Astronomical Society.