[This is just one of many articles in the author’s Astronomy Digest.]
My first ‘large’ telescope was a Celestron C9.25 SCT having an aperture of 235 mm and a focal length of 2,350 mm (f/10). The 9.25-inch optical tube is widely regarded as something of a jewel in the crown of Celestron’s Schmidt-Cassegrain range. Its primary mirror has a larger focal ratio than the other models, so the secondary magnification factor is less, resulting in a flatter field of view with less coma – a distinct advantage for imaging. This does, however make the optical tube longer – almost as long as the C11. At greater cost (£2,995 rather than £1,895) Celestron produce an Edge HD version which includes some optical elements in the baffle tube to give a wider, very well corrected, field of view. I was interested to find out if the Edge HD version would give better images when imaging with a Micro 4/3 sized sensor. Inspecting the star images towards the corners of the field, I found that their shapes were still good so I think that it wouldn’t have made a significant difference when the standard 9.25 SCT is used at f/10 – but I suspect would do so if an APS-C sized sensor were used.
I have equipped the telescope with a Starlight Express steel track focuser. This very largely removes the SCT problem of ‘image shift’ when focussing. The camera sensor needs to be at a distance of 146 mm from the rear of the telescope tube so I set the focuser to give this distance to the sensor and used the normal focus control to get to close focus before making any small final fine adjustments using the Starlight Express focuser.
I was hoping to image Saturn and Jupiter when they came into view and, in the meantime, decided to take one hours total exposure on the Dumbell Nebula, M27, which was at a high altitude. The transparency was good and, without doing any star alignments, used the ‘Plate Solve’ feature of SharpCap to centre the field of view on M27. [This is effectively doing a ‘1 starfield’ alignment very close to the target.]
I was aiming to use 20 second exposures, but taking test exposures found that some frames were suffering from wind gusts so reduced the exposures to 10 seconds so that less of the total imaging time would be lost as frames had to be removed from the stack. The camera temperature was set at -15C. Clouds ended the imaging after 56 minutes. I went through all frames and had to delete quite a few, three due to satellites with the rest due to wind gusts, before aligning and stacking the remainder in Deep Sky Stacker. [Deep Sky Stacker will remove those frames where the star images are not good and, if the Sigma-Kapper stacking mode is employed, will remove the satellite trails – so my weaning of the frames may not have been necessary.]
The Altair Astro 294 mc colour one shot camera has a pixel size of 4.63 microns, with each pixel subtending 0.4 arc seconds on the sky when used with the 9.25 SCT at f/10, so suitably sampling an image limited to ~1 arc second resolution. The resolution of the scope is 0.6 arc seconds so it is the atmosphere that will limit the achieved resolution. However, each captured frame must have sufficient stars to enable them to be aligned and I usually bin 2×2 giving effective pixels of 9.24 microns which reduces the nominal resolution to about 2 arc seconds – commonly the limit due to atmospheric turbulence in any case. To try to get back some of this loss, I select a 2x drizzle when stacking in Deep Sky Stacker. I doubt that this makes a significant difference but it cannot do any harm.
The output from Deep Sky Stacker had, as usual, a green cast due the fact that astro-cameras do not compensate for the fact that there 2 green pixels for each red and blue.
Processing the stacked image
I tend to use my (now quite old) academic version of Adobe Photoshop CS4 for image processing and will go through the steps I took using it, but Affinity Photo, Siril and even, now, the free program Glimpse could be used. I will refer to some aspects of these when applicable.
Removing the sky background
A technique that I have used for many years works very well for this image. The image was duplicated to give a second layer. Using the clone tool, the nebula region was over written from close by to give the following image.
Then the ‘Dust and Scratches’ filter was used with a radius of ~60 pixels when the stars are removed to give an image of the sky background. [In Glimpse, the Median filter is used instead of the ‘Dust and Scratches’ filter.]
The two layers were then flattened using the ‘Difference’ blending mode and the sky background is removed.
[In Affinity Photo, the selection brush can be used to select the nebula region and then, under ‘Edit’, the ‘Inpaint’ tool used to remove it from the image before applying the Dust and Scratches filter. Affinity Photo also has a specific ‘Sky Background Removal’ tool which could be used as does Siril (which is even better) and these could be used instead.]
Stretching the image
In Photoshop, I opened the ‘Levels’ tool and moved the central slide to the left – this brightens up the fainter parts of the image more than brighter parts. Two or three small applications are probably better than 1 large one. [Siril has two very good stretching tools; ‘Asinh Transformation’ and ‘Generalised Hyperbolic Transformation…’. One can also use the levels tool in Affinity Photo.]
Enhancing the image
I then used the Hue and Saturation tool to increase the saturation somewhat (as can be done in Affinity Photo whilst Siril has a dedicated saturation tool) and then used the ‘Smart Sharpen’ filter to make the image a little crisper.
To reduce the impact of the stars, I went into the free program ‘Images Plus’ which, in its ‘Special Functions’ has a star size reduction tool.
This then gave the final result
The Ring Nebula, M57.
On the following night, whilst again waiting for Saturn and Jupiter to rise, I imaged and processed M57 exactly as I had done for M27. Its angular size, as can be seen from the two images, is far smaller. Pleasingly, the central White Dwarf, at magnitude 15.75, was visible.
It is informative to submit the image to astrometry.net. This gave an image size of 28.5 x 19.4 arc minutes. [Not surprisingly, exactly the same as for the M27 image.]
I normally use the ‘Blackwater Skies Imaging Toolbox to see what focal length would best encompass an object. For the Celestron 9.25 SCT and Micro 4/3 sensor camera this gave a field of view of 28.2 x 19.2 arc seconds – a good agreement. The annotated image from Astronomy.net above brought my attention to the galaxy IC 1296 – which I had not initially spotted. Having a magnitude of ~15, this is just visible as seen in the crop below.
I was very pleased with the stellar images across the whole of the Micro 4/3 frame. A further image taken the following evening of the Globular Cluster M13 in Hercules was even better as I think I has managed to get a slightly better focus. No obvious coma was apparent which is why I suggested above that the standard version will be fine when imaging at f/10 using a Micro 4/3 sensor. I need to do further experiments when using the f/6.3 focal reducer (See updated article ‘The Celestron 9.25 inch Schmdt-Cassegrain telescope’) . I believe the Edge HD version works very well indeed with up to APS-C sized sensors but less so on full frame sensors.
Many celestial objects sit nicely within the Micro 4/3 sensor (which is squarer than APS-C sensors) and so it seems to me that buying a standard Celestron SCT and the smaller sized camera would save considerable cost and have few downsides. [Note: Meade now only produce their ACF models which also give a larger corrected field of view than the original versions.]
I hope that this gives some encouragement to imagers who have to image from light polluted locations as mine which is Boortle Class 5. Reasonable results can be obtained even using an f/10 telescope with less than one hours total exposure.
Everything I did in Photoshop can also be done in the new, and free program, Glimpse so I believe that most astronomical image processing can now be done using free software.