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.