Astronomy Now - January 2021

I

Nik Szymanek

The Sky Imaging Edition: Part 3

n the previous two issues, we assessed e Sky Imaging Edition’s (TSIE’s) capabilities for creating an

equatorial-mount pointing model using ‘TPoint’, and tested its functionality during autofocusing

and autoguiding. TSIE comes with TPoint included, in addition to a ‘Camera Add-on’ that allows

full control of CCD cameras along with autoguiders, focusers, lter-wheels, rotators and dome

control. So in this nal part of my exploration of TSIE’s capabilities, I’ll assess how it performs

during an imaging run, and cover some of the software’s new features.

Easy connections

After ring up TSIE on my observatory laptop, it took only a minute or so to connect all of my

imaging devices to it and to cool my QSI 683wsg CCD camera to an operating temperature of –15

degrees Celsius. I chose a nice, easy rst target, M27, the famous Dumbbell Nebula in the

constellation of Vulpecula. Figure 1 shows the zoomed-in eld after slewing onto the target. e

central star was offset by a tiny amount and I didn’t feel the need for any further accuracy. I’d called

upon the @Focus2 autofocusing routine to focus the image and it did a great job rst time (see last

issue for details on both @Focus2 and @Focus3).

After downloading a dark-subtracted image, which was taken with my Starlight Xpress Lodestar

guide camera, I was able to choose a suitable star for guiding. e sensitivity of the Lodestar coupled

with the QSI camera’s integral off-axis guider port means that it’s always possible to nd a guide star

with exposures of around ve seconds. I shot a few two-minute exposures with a luminance lter to

ensure that all was well and by that point M27 was nearing the meridian. After a full meridian ip,

M27 appeared with just a tiny offset from the crosshair centre, so I shot a 10-minute exposure to use

as a luminance le and I combined it with the earlier two-minute exposures.

Next, I planned to take a sequence of images through red, green and blue lters. When imaging the

deep sky, most astrophotographers take long sequences of exposures that will be calibrated, aligned

and stacked to produce a master light-frame. e exposure sequence can be automated and in TSIE

it’s done using the ‘Take Series’ tab in the ‘Camera’ menu. Figure 2 shows the menu with the Take

Series tab selected. At centre is where the sequence is set up. Clicking on any of the elds, such as

the ‘exposure duration’ shown in blue, allows a particular preference to be inserted, in this case 300

seconds for the exposure time. Below that are binning, type of frame (i.e. dark, light, at, etc.),

lter, how many frames to take (repeat), and a choice of calibration frame to be used. I left this on

none, to apply my own calibrations later. Clicking on ‘Add Series’ does just that and, for Series 2, I

left everything the same but changed the lter setting to red. I then created two additional series,

set up for the green and blue lters. ere’s a choice to use ‘Per series’, which in our case would

capture and save twelve 300-second exposures through the luminance lter, or ‘Across series’, which

would take a 300-second exposure through the luminance lter and then change to the red, green

and blue lters consecutively. I left this on Per Series.

Below that are options to set up dithering. is is an important procedure to use. Basically, the

mount is offset by a few pixels at the end of every exposure so that when the images are aligned,

noise in the images is also offset rather than stacking up in registration. During the stacking process

(using stars for alignment), a reference frame is chosen and usually this is the frame with the best

tracking and FWHM values. After comparison with the reference frame, noise in the remaining

images is more easily subtracted and replaced with average-value pixels, producing a much cleaner

image.

As seen in Figure 2, when setting up the dithering procedure I used a shift of three pixels. It’s also

critical to use an exposure delay to allow the guiding to settle after dithering has occurred, and for

this I chose thirty seconds.

File names

e ‘Automatically save photos’ box must be ticked, and then clicking on the ‘AutoSave’ button

opens a menu where you can choose a le location for the images to be saved to. I found this menu

to be a bit awkward because without user-intervention the le name for each image was long and

clunky. Fortunately, under the ‘Customize AutoSave File Names’, you can include useful descriptions

in each saved-le name. e ‘Abbreviations Key’ information at the bottom lists available options

that can be used (see Figure 3). I rst chose the subject title from the ‘File name prex’, in this case

M27. en under the ‘Light frames’ eld I typed :b :e :f :l and this inserted, respectively, binning

status, exposure time in seconds, lter name, and image type (a light frame in this case). A resulting

le name in this sequence would be, for example, M27 2×2 Blue 300 L 00000001.t.

Within the menu, under the ‘Sample AutoSave le name’ eld, you can see a preview of the le

name with the chosen parameters before starting the series capture. Each series records the

appropriate lter used, making it easy to differentiate the images when applying calibrations and

stacking at a later date. Clicking on the ‘Take Series’ button at top left initiates the imaging

sequence. Although I used a very small imaging run (hence the choice of a bright target), the

sequence worked awlessly, producing the LRGB image of M27 shown in Figure 4.

As the sky conditions were so good, I slewed onto NGC 6946, which is a lovely face-on spiral galaxy

in Cygnus. Since it is a fairly bright galaxy, I initiated another short imaging run using LRGB

lters. e resulting image after processing is shown in Figure 5. After a couple of nights using

TSIE, I found that the interface became quite intuitive to use and navigate. ere was a lot of

switching between tabs to access different devices, but it all became quite natural to use and it was

great to have all aspects of the imaging and telescope control within the same program. Also highly

commendable was the fact that at all times TSIE was completely stable, with no crashes or hanging,

or any form of delay when switching between devices.

Collimation

TSIE has some new features that are worth mentioning. e rst is a collimation tool to assess the

alignment of mirrors in a reecting telescope. e tool is located in the Camera menu under

‘Focusing Tools’ (the same location as @Focus2 and @Focus3). Clicking on the ‘Collimation’ button

launches the view shown in Figure 6. At the centre is a defocused image of the star Alpheratz (alpha

[α] Andromedae). At top right of the screen is where the exposure, binning and lter are chosen.

Clicking on the ‘Take Sample’ button takes an image and displays it on the screen. Clicking on the

‘Loop’ button runs a continuous series of images. I used my Paramount’s hand paddle to gently

nudge the star images to the centre of the screen and then adjusted the size of the red rings to match

as closely as possible the inner and outer edges of the defocused star. is helps with assessing the

circularity of the star image and whether the secondary mirror is positioned at the centre. e ring

dimensions are adjusted using the two sliders at the bottom of the screen. When a star is selected,

you can zoom in and create a sub-frame that matches the zoomed view, speeding up download

times. It’s also possible to save the focuser’s current position, defocus the star for assessment and then

return the focuser to its starting position. e menu also incorporates autofocusing using the brilliant

@Focus3 routine. is is denitely a handy tool to do a quick check on the collimation of your

mirrors.

Into the corners

ere are three buttons at lower right: ‘Inspection Mode’, which shows the star without the

crosshairs; ‘Crosshairs’, which gives the view shown in Figure 6; and ‘Four corners’, which splits the

view to show the corners of the sensor to allow you to check for image planarity. I slewed my

telescope onto the Double Cluster in Perseus and took a seven-second exposure with a luminance

lter. at placed plenty of stars in the eld of view (Figure 7). e user manual suggests defocusing

star images and then adjusting the image plane if possible (some cameras have tip-tilt adjusters for

this purpose) until all the out-of-focus stars appear the same size in each corner. e stars in this

image looked pretty similar in size but I noticed a few non-circular outliers. I suspect these were

caused by my focal reducer but they could also be the result of the quality of the main telescope’s

optics tailing off in the extreme corners.

Live stacking for public events

Another nice new feature is the live-stacking tool, which is accessed under the Camera menu.

Clicking on the ‘Take Photo’ tab opens another menu and then clicking on ‘Live Stack’ brings up

the screen shown in Figure 8. e purpose of this tool is to take multiple exposures that are aligned

and stacked on-the-y to build up a strong image. To test this I slewed the telescope onto M31, the

magnicent Andromeda Galaxy, and from the options given at the top right of the screen I selected

an exposure time of 30 seconds with the camera binned 2 × 2 through a luminance lter. I’d taken

suitable dark and at-eld images before starting the live stack, so I loaded them using the

‘Calibration Frames’ button at top right. Clicking on the big ‘Start!’ button initiated the procedure.

As the rst frame downloaded, it appeared on the screen looking quite good. As the second image

downloaded, it was automatically aligned and stacked, and so on. At upper right is an ‘Images’

readout, where the number of images taken is displayed and also how much the noise component

has been reduced.

e whole purpose of stacking images is to reduce noise. e signal component of stars, galaxies,

etc., adds in a linear sense, whereas the noise component only adds as the square root of the total

number of images taken, so taking lots of images results in a great signal-to-noise ratio. I shot

thirteen images and was informed that there was 72. 26 per cent less noise compared to a single 30-

second exposure. When the sequence nished, I saved the image as a FITS le. You can also elect to

save each of the individual FITS images. I think this tool would be great for observatory open

evenings, where many people can see the image building up on screen. It’s possible to take images

with a one-shot colour camera and see a colour image continuing to improve as more and more sub-

frames are taken. Objects like the Orion Nebula would work well in this context.

I experienced three hugely enjoyable nights putting e Sky Imaging Edition through its paces. As an

integrated package, it worked awlessly, and it was easy to switch between controlling the imaging

equipment and controlling the telescope. As mentioned earlier, the product is stable and efficient. Is

it worth its steep $595 price tag? For sure it’s a considerable outlay, especially given that it is in

competition with free programs such as N.I.N.A. and APT (Astro Photography Tool), but considering

the power of TPoint for mount modelling and polar-alignment assistance, complete hardware

functionality and a brilliant planetarium package, I think it is denitely good value for the money.

Nik Szymanek is an astro-imager based in Essex and is the author of Shooting Stars and Shooting Stars II, which are Astronomy

Now books about modern astrophotography techniques.

At a glance

Minimum system requirements

macOS

2GHz Intel Core Duo or faster

macOS Sierra (10.12), High Sierra (10.13), Mojave (10.14) or Catalina (10.15) 512MB RAM, 64MB video RAM,
2.5GB disk space

Windows

1.5GHz or faster

Intel Pentium 4, Pentium M, Pentium D or better, or AMD K-8 (Athlon) or better, Windows 10

512MB RAM, 128MB video RAM, 2.5GB disk space

Linux

A computer running 64-bit x86 Linux Ubuntu 12.04 LTS or later

Ubuntu GUI and OpenGL

512MB RAM, 2GB minimum disc space

Raspberry Pi

Figure 1: At the start of the third night’s imaging run, the author slewed onto M27, the Dumbbell Nebula in Vulpecula. Shown here

is the placement of the target on the sensor, just fractionally offset from the crosshair marker and requiring no further centring. All

images: Nik Szymanek.

Nik Szymanek wraps up his deep dive into the capabilities of the new imaging edition of

Software Bisque’s The Sky by taking the program on several full-blown imaging runs.

Figure 2:To set up an automated sequence of exposures, click on the ‘Take Series’ tab (found under the ‘Camera’ menu). Here you

can set up multiple sequences using various lters and exposure times. Below-centre is where dithering is set up and an exposure

delay to allow the mount to settle between frames. Below that is where the directory location is chosen to automatically save the

images.

Figure 3:The ‘AutoSave Setup’ menu. In this you can customise the le name by choosing any of the terms found in the ‘Abbreviations

Key’ section in the lower half of the menu. I chose four options to display the binning mode, the exposure time in seconds, the lter

name and the image type (i.e. light, dark, at, etc.).

Figure 4:An LRGB image of M27, the Dumbbell Nebula in Vulpecula. This was taken as part of a test of TSIE’s AutoSave menu to

automatically take and save images as well as dithering at the end of every exposure. The image was taken through a 254mm

Ritchey–Chrétien telescope with a QSI 683wsg CCD camera, Astrodon LRGB lters and a Paramount ME equatorial mount.

Figure 5:An LRGB image of NGC 6946, the ‘Fireworks’ spiral galaxy in Cepheus. This was another image taken as part of a test of

TSIE’s AutoSave menu to automatically take and save images as well as dithering at the end of every exposure. The image was

captured through a 254mm Ritchey–Chrétien telescope with a QSI 683wsg CCD camera, Astrodon LRGB lters and a Paramount ME

equatorial mount.

Figure 6: TSIE’s ‘Collimation’ tool, showing a heavily defocused image of the bright star Alpheratz (alpha [α] Andromedae) against a

red reticle with concentric circles that help assess collimation issues. The dark central area is the shadow of my 254mm Ritchey–

Chrétien telescope’s rather large secondary mirror. The red circles can be enlarged or reduced using the two sliding scales at the

bottom of the screen.

Figure 7: The ‘Four Corners’ option of the Collimation menu shows the extremities of the sensor, allowing a check of image

planarity. This defocused image of stars in the Double Cluster in Perseus shows quite uniform stellar diameters, though some

optical aberration is apparent in the stars at the extreme edge of the eld.

Figure 8:An image of M31, the Andromeda Galaxy, obtained using TSIE’s ‘Live Stack’ menu. This consists of thirteen 30-second

images that were taken and stacked on-the-y before being combined into a single exposure. This feature could be useful for club

observatories and open evenings.

View

The Sky Imaging Edition: Part 3

January 2021

Astronomy Now