A very small, wide-field, guided imaging rig Part 3. Second tests

The equipment in operation

This time a Mars-C II camera was used which has an optical window with very good, multi-layer anti-reflection coatings.

There was no moonlight during this session but there is a streetlight close to the imaging spot that has to be shielded by the occultation board visible on the right.

B142-43 dark nebulae centred on the star HD185898, captured by AstroDMx Capture (pre-release-version3) through a GS-100 PAPO quadruplet ED f/4 (F=100mm, A=25mm) mini scope with a Player One Mars-C II camera (with a SONY IMX662 1/2.8" CMOS colour sensor). 60 minutes of 60s exposures with a Pegasus Astro L-UV-IR cut filter. Captured by AstroDMx Capture pre-release version 3. Processed in PixInsight, GraXpert, SASPro and Gimp3.

B142-43 dark nebulae

Also the North America and Pelican nebulae centred on the star HD199479, captured through the same scope and camera. 60 minutes with L-UV-IR cut, 30 minutes with quadband, 30 minutes with HaO3 and 15 minutes with S2O3 filters. Captured by AstroDMx Capture pre-release version 3. Processed in PixInsight, GraXpert, SASPro, Gimp3 and CS2, blending the various filter results.

North America and Pelican nebula

The stars are from the Pegasus Astro L-UV-IR cut filter because they are true RGB stars and did not produce halos.

During this session we were able to observe that different filters introduce various gradients across an astronomical image? some produce large gradients that have to be removed whilst others produce no gradient on the same object at essentially the same time.

Screenshot of AstroDMx Capture saving 5 minute exposures of the North America and Pelican nebulae through the Pegasus Astro L-UV-IR cut filter,

 

It can be seen that there is a huge gradient across the image.

Screenshot of AstroDMx Capture saving 5 minute exposures of the North America and Pelican nebulae through the Altair quadband filter

It can be seen that the gradient has gone but some of the brighter stars now have halos. A similar result was obtained when using an Askar  HaO3 filter, and also with an Askar S2O3 filter.

Screenshot of AstroDMx Capture saving 5 minute exposures of the North America and Pelican nebulae through an Asker S2O3 filter

Animation showing the gradient produced by the Pegasus Astro L-UV-IR cut filter vs the absence of the gradient produced by the Altair quadband filter but the presence of star halos

Different astronomical filters introduce vastly different gradients because of the type of light they let pass and how they interact with wide angles of incoming light. When shooting the exact same target at the exact same time, a broadband filter like the Pegasus L-UV-IR cut filter can show a massive sky gradient, while a narrowband filter like the Altairquadband filter can appear completely gradient-free. This is driven by two main factors: the nature of environmental sky glow and the physics of interference filters. 

The primary driver of gradients is sky glow, which includes artificial light pollution, moonlight, and natural atmospheric airglow. During these tests there was no moonlight.

Broadband filters pass wide windows of light (often 100nm or more). Because light pollution (especially from LEDs) and moonlight scatter across the entire visible spectrum, they create a thick background glow. Because the telescope points at an angle through the atmosphere, one side of the image frame is always closer to the horizon or a light source than the other. This creates a steep, highly noticeable brightness gradient across the sensor. 

Narrowband filters isolate an incredibly tiny sliver of light (usually between 3nm and 7nm) centred precisely on the emission lines of nebulae. They block up to 99% of all scattered sky glow.

H (656nm) and SII (672nm): Moonlight and urban light pollution contain very little energy at these deep-red wavelengths. Thus, the background stays completely dark and gradient-free across the whole sensor.

OIII (501nm): Unlike H, Oxygen-III sits in the blue-green spectrum where moonlight and LED scattering are very strong. Even with a narrow 3nm filter, if the Moon is out, OIII will often suffer from severe gradients while an H-alpha frame taken at the exact same moment remains completely clean. As we had no moonlight and the fluorescent streetlight nearby, the S2O3 filter didn't produce a gradient.

Some gradients are not in the sky at all; they are generated inside the imaging equipment itself. Modern astrophotography filters are interference filters, meaning they use microscopic chemical layers to reflect unwanted light and pass specific wavelengths.

When light rays hit an interference filter at an angle (which happens frequently at the edges of a wide-field telescope or fast f-number optics), the physical distance the light travels through the filter layers increases. This shifts the filter's passband toward shorter wavelengths. With broadband filters, this shift doesn't matter because the window is wide enough. But with ultra-narrowband or complex multi-band filters, a bandpass shift means the edges of the sensor stops capturing the target's light and starts capturing background light noise, creating colour gradients.

Some filters have poor off-band blocking performance. When a filter fails to block out-of-band light completely, that light enters the filter glass and creates intra-filter reflections. The light bounces back and forth between the front and back surfaces of the filter glass before finally escaping toward the camera sensor. This reflected, slightly offset pool of light is what produces a halo around bright stars. The halos we observed are most likely due to this cause.

In conclusion the GS-100 mini scope can be used successfully as a wide-field imaging scope in conjunction with the appropriate camera and filters.

We have used these tests to simultaneously test pre-release version 3 of AstroDMx Capture.

A very small, wide-field, guided imaging rig Part 2. First tests

 

The equipment

The scope and guide-scope mounted on the AVX mount

Although a number of components can be added to the rig, such as a filter drawer or filter wheel there are very limited degrees of freedom to develop a setup that is truly flexible. For example, there is such limited back-focus with probably any camera, that it is impossible to fit even a very thin camera rotator to facilitate the framing of astronomical targets. There is room to fit a very small number of spacer shims so by varying the number and thickness of these and tightening up the camera, it may be possible to approximate the desired framing, or not!

It should be possible to build in a camera rotation system into the scope’s design without reducing the back focus. This is of no consequence if the scope is used as a guide scope, but is of huge importance for an imaging device for which it is heavily marketed and hopefully Touptek will consider this.

Considering the components that we used for the first test, a number of important lessons can be learned.

The GS-100 is a fast scope that may cause problems with larger sensors due to the angle of incidence of some of the rays hitting the sensor. The faster the scope, the steeper these can be. This can increase crosstalk between pixels resulting in poorer colour fidelity and contrast.

Probably the biggest problem with using the ZWO ASI585MC camera concerns reflections back and forth between the sensor and the filter. The camera used does not have anti-reflection coatings on the sensor window glass which means that the reflected light is not inhibited and passes back and forth between the sensor and the filter wheel, passing through the quadruplet lenses on its journey. The shifting of the originally parallel light rays leads to the development of Newton’s rings interference patterns which are particularly noticeable towards the ends of the long axis of a rectangular sensor. There are limited options for changing the distance between the filter and the sensor, which would probably have little effect anyway. There is no tilt plate built into this camera so no way of using tilt to mitigate the problem. The filter itself has inadequate anti-reflection coatings which could have reduced the impact of the interference patterns. After consideration, the quadband filter may not have been a good choice. I am not sure whether ours is V1 or V2. If it is V1 then this filter was renouned for poor anti reflection coatings and producing bad halos around stars. We clearly had reflection problems and some stars did have bad halos which were unsatisfactorily cosmetically treated during processing. A high quality UV/IR filter may have been a better choice.

Unlinked channels stretched image showing the concentric interference bands

The interference bands are more evident at the left hand side of the image where the star density and nebulosity are lower.

Careful post processing can reduce these artefacts but not completely eliminate them

North America and Pelican nebulae test

Sadr region test

The images were improved by cropping out the central area where the interference was less evident. However, this partly defeats the object of using the ZWO ASI585MC camera.

Cropped and reoriented images

North America and Pelican nebulae

Sadr region

For a future test our Player One Mars-C II IMX662 Colour Astronomy Camera will be used. Learning from our previous experience, this camera seems to have a number of advantages over the ZWO ASI585MC. It has Diameter 21 * 1.1 mm High Quality AR Plus (Anti Reflection) Multi-Layer Coating. Instead of a single layer of anti-reflective material, this optical element has multiple microscopic layers cooked onto the surface. Multi-layer coatings are superior because they cancel out reflections across a much wider spectrum of light wavelengths (colours), rather than just one. It also has a built in front tilting plate that allows for adjustments to avoid interference patterns if required.

Further camera details:

SONY IMX662 1/2.8" CMOS sensor (color). 2.1 Mega Pixels.   Maximum Resolution  1936×1100

2.9μm square pixels.   Sensor Size  5.6mm×3.2mm

If we centre on the star HD198597 the field of view is such that much of the Cygnus Loop can be framed.

Stellarium simulation of the field of view framed by the GS-100 with the Player One Mars-C II camera. The framed area is within the red frame lines.

The Player One Mars-C II camera will be used in conjunction with a Pegasus Astro L/UV-IR cut filter which offers a transmission range of >95% between 410–680nm. This blocks the ultraviolet and infrared bloat while letting the crucial visible spectrum (including Hydrogen-Alpha at 656nm) pass cleanly through to the sensor. It intentionally leaves a gap between the green and red bands. This blocks the primary Sodium light pollution line at 589nm, resulting in cleaner colour separation. 

The glass  features anti-reflection and anti-halo coatings which is crucial for UV/IR filters. It has a 1/4 wavefront polish.

The Player One Mars-C II camera on the test rig

It is hoped that this setup will allow capture without interference banding and star halos.

Again, the testing will be done with a pre-release version 3 of AstroDMX Capture, so we will be testing not only the hardware but also the software.

The Soap Bubble nebula (PN G75.5+1.7)

The Soap Bubble nebula (PN G75.5+1.7) from 1 hour of narrowband data (Ha and O3). 5 minute exposures (30 minutes through each filter) captured by AstroDMx Capture through a William Optics 81mm APO refractor with a QHY Minicam 8 camera/filter wheel. Processed in PI, SASPro and GIMP3. Rendered as HOO. 'RGB' stars were constructed in Siril from narrowband data using the Standard Continuum Mapping pixelmath procedure. 

If we include the 30 minute worth S2 data in a SHO/NBN palette (giving 1.5 hours total exposure), the Soap Bubble is much less pronounced. This is because most of the information on the Soap Bubble is in the Ha and O3 channels. A blend of HOO and SHO gives a reasonable compromise, with the Soap Bubble being visible along with some of the hues associated with the SHO.

Click on any image to get a closer view

HOO NBN

HOO NBN SHO blend

SHO NBN

This session was revisiting an object that we imaged in September 6, 2023 in narrowband with the same scope but using a 14 bit SVBONY SV605MC using a total of 5 hours of accumulated exposure.

The QHY Minicam 8 camera/filter wheel camera used here is a 12 bit camera and a considerably shorter total exposure time was used. Nevertheless, we were able to make a good capture. The best image was the HOO image with just 1 hour of total exposure time.

Steve Wainwright and Nicola Mackin