PhotoDemon; free, open source Photo editing software

Every so often I come across some software that, as far as I am concerned, has been hiding in plain sight. Such a program is PhotoDemon.

PhotoDemon is developed by Tanner Helland.

It was first released in 2012. It is open source and freeware, although donations can be made to the project. At the time of writing PhotoDemon is on version 2025.4.

PhotoDemon is not installed, it is an executable along with its required dependencies and requires no administrator privileges. Aside from a temporary folder – which you can specify in the Tools > Options menu – PhotoDemon leaves no trace on the hard drive. PhotoDemon can be run  from a USB stick or SD card.

It is lightweight, fast, completely portable and works with 8 bit, 16 bit and 32 bit images; unlike many photo editing programs which reduce higher bit depth images to 8 bits before they can be used.

Due to its portable nature, PhotoDemon is only available as a 32-bit application. This means it cannot load or save images larger than  about 2 GB in size.

Complex editing actions can be recorded as macros. A built-in batch processor lets you apply macros to entire folders of images. 32 bit 8bf plugins can be installed and used. The 32 bit 8bf, Hasta La Vista, Green, HLVG, from DeepSkyColours installs and works perfectly in PhotoDemon.

PhotoDemon is very intuitive and usable. Small touches like real-time effect previews, save/load presets on all tools, unlimited Undo/Redo, customizable hotkeys, mouse wheel and X-button support, and descriptive icons make it fast and easy to use. For those who know something about Photoshop, many of the keyboard shortcuts work the same way.

PhotoDemon has very extensive file format support, including Adobe Photoshop (PSD), Corel PaintShop Pro (PSP), GIMP (XCF), and major camera RAW formats, but at the time of writing, does not support the Fits file format.

Advanced multi-layer support, including editable text layers and non-destructive layer modifications

Colour-managed workflow includes support for embedded ICC profiles.

Its on-canvas tools include digital paintbrushes, clone and pattern brushes, advanced selection tools, interactive gradients, and more.

Adjustment tools include levels, curves, HDR, shadow/highlight recovery, white balance, and many more.

Filters and effects include perspective correction, edge enhancement, noise removal, content-aware fill called ‘Heal selected area’, and resize, unsharp masking, gradient and palette mapping, and many more. More than 200 tools are provided in the current build.

Screenshot of PhotoDemon using Curves on a Star layer that has been Screen blended with the starless layer.

Screenshot showing the adjusted stars layer

PhotoDemon is right up there with Photoshop, Gimp and Affinity Photo. It will be one of the tools in my workflow for processing astronomical images. I can recommend it to any astrophotographer who wishes to use a powerful yet intuitive program to contribute to the processing of her/his images.

I was unable to get it to work in Wine in Fedora or Ubuntu Linux. If it can be made to work in Wine a special Wine ‘bottle’ may have to be developed.

PhotoDemon can be downloaded from https://photodemon.org/

The Lobster claw nebula region

AstroDMx Capture was used with an SV605CC OSC colour CMOS camera and an Altair Starwave ASCENT 60ED doublet refractor with 0.8 reducer/flattener and a Pegasus Focuscube v2. An Altair 2” magnetic filter holder version 2 containing an Altair quadband filter was placed in the optical train.

The equipment was mounted on a Celestron AVX GOTO mount. An SVBONY SV165 guide-scope fitted with a QHY-5II-M guide camera was mounted on the imaging scope. An INDI server was running on the imaging Linux computer indoors. The guide camera was connected by USB to another Linux computer indoors running PHD2 autoguiding software via the INDI server. The mount and the focuser were controlled by AstroDMx Capture via the INDI server.

AstroDMx Capture slewed the scope to the star Altair and plate-solved to centre it. A Bahtinov mask was used to enable Altair to be brought into sharp focus.

Screenshot of PHD2 autoguiding

Then AstroDMx Capture plate-solved and sent the scope to the star HD 240247. This star was chosen so that the image would frame a region containing five objects: SH2-157, the Lobster claw nebula, NGC 7635, the Bubble nebula, NGC 7538 (emission nebula), NGC 7510 and M52 (open clusters).

AstroDMx Capture was used to capture 26 x 5-minute RAW exposures of the region.

Screenshot of AstroDMx Capture saving RAW FITS files. Live stacking was used to improve the preview of the captured image.

Negative preview

26 light frames, 5 dark frames, 50 bias frames, 50 flatfields and 50 dark flats. To capture the flats and other calibration frames, the scope was slewed to ther zenith and tracking stopped. The flats were captured using a device designed to avoid lateral light leakage and using a stack of 3mm thick acrylic white A5 sheets to evenly diffuse the light from a light panel and attenuate it sufficiently to get Flats of 1.5s duration.

The Flat field equipment

The data were calibrated, debayered, stacked and part processed in PixInsight and completed in GraXpert, Seti Astro Suite Pro Cosmic Clarity and Gimp. A Pixinsight Cosmic Photons script was used to Generate a Hertzprung Russel diagram of the stars in the image.

Hertzprung Russel diagram of the stars in the stacked image

The Lobster claw nebula region. All images with RGB stars.

RGB channels linked

RGB blend of linked and unlinked channels

Annotated image

HOO rendering

Hubble palette rendering

Classical SHO rendering

Meanwhile Nicola's mammoth refactoring of AstroDMx Capture is progressing nicely, although with the decision to make a number of functional and efficiency changes in addition to making the software Wayland compliant with Qt6, the process has taken longer than was initially anticipated. The ground is being prepared for the introduction of new frameworks such as Alpaca as well as new functionality.

Seti Astro Suite Pro; A relatively new image processing suite for astrophotographers

Seti Astro Suite Pro

A different and relatively new image processing suite for astrophotographers

Official SASP logo

Link to the SASP website

Until recently the main goto software for image processing have been Deep Sky Stacker, Siril, PixInsight, Sequator, AstroSurface, Astro Pixel Processor, Astronomy tools, Autostakkert!, Affinity Photo and a handful of lesser known programs. These range from expensive paid-for software, to freeware-donationware. Some, such as Sequator and Autostakkert! are mainly stacking software, whilst others have various post stacking capabilities. For a variety of reasons, financial among them, each program has its own following of loyal users.

Seti Astro Suite Pro (SASP)

SASP is being developed at a prodigious rate with a couple of versions being released in a week being quite common. At the time of writing the version is 1.3.18

SASP runs on Windows, Linux, MacOS x86-64 and Apple silicon.

Seti Astro Suite Pro is developed by Franklin Marek. It is written in Python with Qt6 and is open source donationware. SASP grew out of Seti Astro Suite, its predecessor, as more advanced functionality was added. 

SASP has comprehensive functionality and has integrated GraXpert and Starnet++ functionality as long as the software is present and the binaries are available. SASP also has integrated Aberration Correction (AI) and Franklin Marek’s own software; the Cosmic Clarity suite plus other processes.

Rather than list the processes available we shall use a series of screenshots showing the drop down menus that give access to the various processes.

One process that sets SASP apart from other image processing software is that it offers Multi-Frame Deconvolution Stacking (Image MM) in addition to offering regular stacking.

Multi‑Frame Blind Deconvolution has been developed by a mix of international academic research groups, national observatories and institutes, defence laboratories, and industry teams collaborating to develop algorithms into high‑performance systems.

Image MM works in a completely different way to regular stacking and so in SASP provides choice in how a final stacked image is produced.

Multi-Frame Deconvolution (Multi-Frame Blind Deconvolution (MFBD)) is a class of image-restoration methods that jointly estimate a single high‑quality latent image and the set of blurring kernels (point‑spread functions, PSFs) that produced a sequence of separately blurred frames, then combine that information to produce a single de-blurred, higher‑signal image.

Regular stacking (alignment and ‘averaging’) improves signal‑to‑noise ratio by summing information but does not remove blur introduced by the PSF. SNR (Signal to Noise ratio) improves as √(n) where n = the number of frames stacked, but resolution stays limited by the worst blur.

Multi-Frame deconvolution explicitly models and removes the blur by estimating PSFs and inverting the convolution process, recovering higher spatial frequencies and therefore improving effective resolution in addition to SNR.

Regular stacking assumes the object is unchanged and that noise/statistical rejects are the main issues.

MFBD leverages frame‑to‑frame PSF diversity as information, so it can recover detail lost to variable blur.

PSF estimation per frame measures  PSFs  or estimates them blindly within the optimisation loop.

Iterative reconstruction alternates updates of the latent image and the PSFs until convergence, using regularisation to avoid noise amplification.

Final combination output is a single deconvolved image with improved resolution and SNR compared with any single frame or a simple ‘average’ stack.

Some stages in the Muti-Frame Deconvolution Stacking of a group of images

The final Image MM stacked image loaded into SASP for further processing

SASP offers regular stacking with Drizzle if required. This is computationally less demanding than Multi-Frame Deconvolution Stacking and executes quicker, particularly if GPU hardware acceleration is not available on the users computer. Multi-Frame Deconvolution may not execute properly if a non NVIDIA GPU or GPU co-processor such as RADEON graphics is present. However, it is possible to set the stacking suite not to use hardware acceleration and stacking then works, albeit without the speed advantage of hardware acceleration.

We have tested this successfully on 7 computers with various operating systems and SOCs. and found the speed without hardware acceleration to be quite acceptable.

It is clear that Seti Astro Suite Pro is set to be one of the main contenders in the league of astronomical stacking and image processing software.

The Seti Astro Suite Pro website can be accessed by using the link under the SASP logo at the top of this page, or click HERE.

  

Folded optics in refractors

The majority of refracting telescopes have straight through optics and the physical length of the telescope is dependent on the focal length. This helps when the objective lens is acromatic rather than apochromatic. With an acromatic objective, the red and blue light are brought to the ‘same’ focus whereas with an apochromatic objective all red blue and green light are brought to the ‘same’ focus, reducing or even eliminating chromatic aberration. With a long focal length, an acromat brings the different wavelengths of light to a closer focus than with a short focal length. 

Other types of telescopes such as Schmidt Cassegrains, Maksutov Cassegrains and a number of other types of telescopes use folded optics to shorten the physical length of a scope of long focal length. There are some folded optics design telescopes that are not intended to have an eyepiece, but which are designed to have a camera at the position normally occupied by the secondary mirror in say, a Schmidt Cassegrain. Such a scope is the recent Sky-Watcher HAC125 DX Minigraph which is very fast at f/2 and F=250mm and is very suitable for Electronically Assisted Astronomy, EAA.

However, we are dealing here with a class of refractors that have folded optics and we shall use 4 examples:

Example 1

The most familiar example of folded optics refractors is Porro prism binoculars.

The folded optics using Porro prisms shortens the length of the binoculars as well as placing the objectives further apart, enhancing the 3D viewing experience.

Although historically there have been a number of folded optics telescope made by scope manufacturers such as Unitron and amateur astronomers such as Dave Trott, we shall only look at a selected few:

Example 2

The Zerochromat Refracting Telescope

Peter Wise, from Pensarn, Conwy, Wales, UK, designed and made the 10" Zerochromat Refracting Telescope for the Custer Institute and Observatory in Long Island, New York State. It is Located in the main observatory dome and is the largest of its kind in the United States. Designed by award-winning optician Peter Wise and manufactured in Pensarn. Its dialyte lenses make it apochromatic. A dialyte lens is a compound optical lens where the individual lens elements are separated by a significant air space, allowing for the correction of optical aberrations like chromatic aberration. This air-spaced design offers more "refractive surfaces" for correcting distortions. 

Peter Wise demonstrating his 8" Zerochromat folded optics refractor to the Swansea Astronomical Society meeting in Swansea University, UK on March 14, 2014.

Some of Peter Wise's publications:

The Dialyte refractor revisited. Journal of the British Astronomical Association. vol.127, 6, p.350-353, 2017

The retrofocally corrected apochromatic dialyte refracting telescope Wall, J. & Wise, P.  Journal of the British Astronomical Association, vol.117, 1, p.29-34, 2007

Example 3

This example is a beautiful folded optics refractor made by Berger Astrogeräte. 

https://www.astrogeraete.de/

The mechanical part was made and designed by Andreas Berger. The optical path was designed by Dr. Georg Dittié. Georg Dittié is the programmer of the program "Giotto".

The photographs of the telescope were provided by Andreas Berger.

One side removed to show the folded optics with the objective, two reflecting mirrors and the eyepiece holder.

This telescope is of a very compact design and demonstrates how it is possible to build a refractor into a small space by the use of folded optics.

Example 4

The Seestar S50 smart telescope by ZWO

The telescope is controlled by the Seestar app on an Android tablet/phone; or an iPad/iPhone.

It has a 50mm objective and a focal length of 250mm. The camera sensor is a Sony IMX462. This produces an image with a field of view of 0.73° x 1.29°.

The Seestar S50 in AZ mode

The Seestar S50 in EQ mode

Zwo have not published schematics for the Seestar but the closest they come is a promotional image representing the interior with an exploded view of the apochromatic triplet objective. This picture is derived from that image:

The folded optics are clearly visible showing the light rays converging and being reflected onto the camera sensor. The folded optics are the reason why Seestars are such compact telescopes. The design, whilst not being directly related, is very reminiscent of Andreas Berger's telescope.

It would be very interesting to see astronomical images produced by folded optics refractors other than the smart telescopes.

Further use of AstroEdit on Seestar stacked and saved results

Workflow processing an image downloaded to the Seestar S30-controlling iPad upon termination of 62 minutes of live-stacking:

Click on an image to get a closer view

The image loaded int AstroEdit has the watermark at the bottom which needs to be cropped out. This is why it may be better to set the Seestar not to save with a watermark

The image is set for cropping

The watermark is cropped out

The cropped image ready for processing

Removing the stars

The removed stars

Contrast increased on the starless image

The stars put back into the image

The stars reduced in AI mode (maybe a bit too much but to emphasize the effect)

The Original image

The processed image

A blink comparison of the original and processed images

There are more things that could have been done to the image, but this simply illustrates some of the things that are possible with AstroEdit in iOS.

Steve Wainwright