An experimental imaging session with an Astronomik UHC-E filter

The Astronomik UHC-E filter is quite a gentle filter with two quite wide pass bands that include the wavelengths of H-beta and OIII in the blue/green pass band and H-alpha and SII in the red pass band. The filter also passes the strongest emission line typically seen in comets at about 520nm, so in theory might be useful for observing and imaging comets. The filter is intended for visual and photographic work and transmits quite a lot of light. It is an effective light pollution filter cutting out the wavelengths between 540nm and 630nm.

Transmission curve of an Astronomik UHC-E filter

An ALTAIR STARWAVE 60 ED Imaging Refractor with an 0.8 reducer/flattener at f/4.8 was mounted on an AVX mount. An Astronomik UHC-E filter was placed in the optical train in an Altair magnetic filter holder. An SVBONY SV605CC OSC cooled CMOS camera was mounted on the scope.

An extension cord was used to connect the mount to the Hand controller which was indoors with the Windows imaging computer. The hand controller was connected to the imaging computer via USB to connect to the INDI server on the virtual Linux machine running on the Windows machine.

As usual, the mount was placed on marks on the concrete base which give a reasonable polar alignment. AstroDMx Capture passed the time, altitude and location coordinates to the hand controller via the INDI server. The hand controller which now contained all of the correct information was set to its previous alignment and was unparked by AstroDMx Capture.

AstroDMx Capture for Windows was used to send the mount/scope to the star Rigel for focusing using a Bahtinov mask, and then to the Satellite cluster at the centre of the Rosette nebula. The field was plate solved and the cluster centred.

We were limited to capturing 5 x 3min exposures during our available time, because an overhead cable started to intrude into the field of view. Nevertheless a total of 15min of exposure was sufficient for the purposes of the session.

Here we shall show the complete workflow to produce the final image of the Rosette nebula.

AstroDMx Capture for Windows capturing 3min FITS images of the Rosette nebula

The images were registered and stacked in the Open Source,  cross-platform software ASTAP (Astrometric Stacking Program and fits viewer). ASTAP also has limited image stretching capabilities and can export stretched images in various formats, or just the unstretched, stacked FITS image.

Stacking and stretching in ASTAP

The stacked image of the Rosette nebula exported from ASTAP.

Starnet++ v2 was used to remove the stars from the image, leaving just the nebula.

Starless image of the Rosette nebula

In the Gimp, the starless image was subtracted from the original image with stars to produce an image of the stars. The star image was reduced in saturation, and curves adjusted slightly to make the stars less prominent.

Stars image

The background of the starless image was extracted in Siril

After the background extraction a little histogram stretching in Siril

The starless, histogram-stretched image was noise reduced in Neat Image. 

The starless image after noise reduction

The denoised image was then further processed in the Gimp 2.10.

Denoised, starless image following some levels adjustment

Using the Gimp 2.10 the stars were added back into the image

Stars added back

The gimp was then used to post process the image to produce the final image.

The Rosette nebula with an Astronomik UHC-E filter

The star removal technique is variously used by astrophotographers. It allows the processing of an image to bring out and balance dimmer parts of an image without causing the stars to become bloated. It also allows for the correction of star colours introduced by the filter used.

The Astronomik UHC-E filter is a challenging filter to use and should be used in the absence of moonlight and under reasonably dark skies. The skies under which these experiments were carried out are Bortle 4, but on this occasion, the full moon had just risen so there would have been more blue sky light than would have been ideal for this filter. Nevertheless, an acceptable result was produced with only a short total integration time.

A similar workflow was used to stack and process 25 x 2min exposures of the Seagull nebula to produce the final image.

Seagull nebula

These should be regarded as guidelines rather than a rigid set of procedures. For example is it best to extract the background in Siril or to use auto colour correct in ASTAP which produces similar results? The point is that a set of procedures such as these presented here can produce the results required. We have used free, open source software wherever possible and where inexpensive paid-for software has been used, free, open source alternatives are available for example, for noise reduction. we have chosen to use Neat Image, but alternative software such as Free Photo Noise Reduction, a free subset of the low cost PT Photo Editor are available. The removal of artefacts left over from the removal of very bright stars can be done with healing tools in the Gimp, or clone tools such as that provided in Free Clone Stamp tool, another free subset of PT Photo Editor.