Spectral Pro 30 Minute Integration Analysis - Navi IC59 IC63 & NGC281

To date laboratory testing has proven the bands are correct, there is high transmission (>98%), and the transmission is maintained for emission wavelengths for fast optics to F/2. Field testing by multiple users have produced beautiful images (qualitive results) with the Spectral Pro. What is also necessary is produce field testing results that stress test the product goals such as halo suppression, f/2 compatibility, and target type performance. 

These results are made at F/2 the the Samyang 135 mm lens so we can test other performance effects of the lens such as Halos and star bloating evident from use with other filters. Note that while this lens is generally considered excellent for astro imaging, you may note that in uncropped images there are optical artifacts such as coma. These can be easily resolved by stopping down the lens. This is intentionally left at f/2 for these tests to analyse the filter performance at f/2.

Navi, IC59, IC63, & NGC281 nebula were chosen as the target for this test as it was visible from he observatory horizon, has Ha signal, and most importantly, a bright star in the FOV to test for halo's. Test images were made to set the stretched histograms to peak at ~1/3 along the x-axis. Clipping was then examined in NINA to ensure the exposure was correct for both filters. As such the no filter setup required a 4s exposure and and the Spectral Pro required 60s at gain 800. Typically in this environment a filterless image requires 15-30s exposure before the sensor is swamped with light pollution (see discussion in the datasheet). However the light pollution environment (Bortle 8/9) from the observatory was severe reducing no filter exposures from 30s to 4s and Spectral Pro exposures from 240s to 60s.

Observations have been extremely challenging this winter with zero cloudless nights from September to November. The only opportunities come from gaps in clouds. As such on the night of 20/11/25, light exposures of 30 minutes were acquired for no filter and with Spectral Pro configurations.

Acquisition Details:

 Exposures No Filter Spectral Pro
Lights 459 x 4s = 1836s 31 x 60s = 1860s
Flats  42 x 2s 50 x 2s
Darks 14x 4s 14 x 60s
Biases 10 x 1/4000s 10 x 1/4000s

 

The OSC preprocessing script in Siril was used on both sets of data. Background extraction and auto-stretching were performed about image sets. Using auto stretch is useful here as a repeatable function is applied to each image. However note that GHST may be even more successful at bringing more detail and contrast out of the filtered image.

OSC Preprocessing time no filter  OSC preprocessing Spectral Pro
1 h 34 min 10 s 18 min 39 s

 

No filter

Spectral Pro

A zoomed tight crop of Navi and IC59 and IC63. This image shows the filter is causing no halos. The filter is controlling star bloating due to its IR cut properties when used with an astro-modded camera and it is effectively blocking light pollution while transmitting the Ha from IC59 and IC63.

  • No star halo on Navi/Gamma Cassiopeia imaged at f/2.
  • Spectral Pro eliminates light pollution while transmitting Ha effectively
  • Sharp stars with Spectral Pro with an astro-modded camera (no filter has bloated stars)
  • Gradients easy to remove effectively with Spectral Pro, challenge gradients remain without a filter
  • The Samyang 135 mm shouldn't be used at f/2 due to aberrations at the edge of the sensor; however, it will continue to be used in this configuration for field testing purposes.

The other product that can be tested in this configuration was the clip in adapter for 1.25" lenses. This adapter is being design so that Samyang 135 mm users with a Canon DSLR may install a standard 1.25" filter. This is easy with astro cameras but not easy with DSLR's. While a standard 2" filter would be preferable from a vignetting point of view, testing is indicating the effects are minimal with this adapter. The vignetting is there but is likely a easy trade off to make for light pollution suppression. The other factor is that Leviathan mounts and retaining rings are designing with wider clear aperture (CA), further minimising the effects of vignetting.

Here is a photo of the Spectral Pro with a prototype clip in filter adapter for a Canon DSLR. Nikon and Pentax are planned for development, if this is of interest, please get in contact and sample adapters can be sent with a purchase.