Review - Optolong SHO Narrowband Filters
Posted: 22 December 2019
Updated: 10 October 2020
HA SII OIII Narrowband Telescope Filter Kit (2")
At the "First Annual David H. Levy Arizona Dark Sky Star Party", October 2019, my wife Laurraine won a door prize: an Optolong 2" HA SII OIII Narrowband Telescope Filter Kit.
The frequency specs for the filters are:
H-Alpha: 656.3nm, 7nm
SII: 671.6nm, 6.5nm
OIII: 500.7nm, 6.5nm
During my initial focus tests I had difficulty getting as precise a focus as I wanted (even when using a Bahtinov Mask). Part of the problem was that I had to remove the camera to swap filters. That resulted in a potential focus change as well as a likely slight field-of-view orientation change. After researching filter wheels and sliders, I decided to go to Starizona in Tucson and check out their new Filter Slider System.
As I had not previously used individual filters for my astrophotography, there is a rather steep learning curve I had to begin to climb in order to use them and do some basic image post-processing. I first learned that for Deep Sky Object (DSO) imaging through SII, HA, and OIII filters you need to use different exposures for each filter in the ratio of 3:1:2, respectively. My first DSO images were of NGC7635 (Bubble Nebula) in the constellation of Cassiopeia. The following three images are not processed to show the unedited images. All images are StarLock autoguided, ISO 6400, White Balance 5560K.
SII filter, 6 minutes
HA filter, 2 minutes
OIII filter, 4 minutes
Now that I had images through each of the three filters I needed to edit each individual image (including "stretching" using Levels) and convert each image to a narrowband greyscale image. I first did my normal image processing in Lightroom and GraphicConverter. Each image was then copied and pasted into corresponding RGB channels in Adobe Photoshop to create a full color (yet false color) image. I decided to use the "Hubble Pallete" which maps SII to red, HA to green, and OIII to blue. Here is the "final" result. I say "final" because as I learn more about post-processing SHO images I may come back to these images to try to improve them.
Well, not bad for a first attempt. The image has more digital noise than I expected; longer or more exposures with each filter should help reduce the noise. But of course, processing this image took a LOT MORE time than was required for the following single 6 minutes exposure image (ISO 6400, White Balance 5560K) with no filter (shown for comparison). The color difference is from using the Hubble Pallete for the SHO image.
My second imaging with the filters was delayed by poor weather and a period when the electrical outlets in the observatory had failed. Once the outlets were replaced and the weather improved I imaged M1 (Crab Nebula). This is a brighter object than NGC7635 and longer exposures were used. For this test I used the same exposure for each filter (StarLock autoguided, 5 minutes, ISO 6400, White Balance 5560K) to see if I could determine what ratio might work for me.
Here are the individual image files.
SII filter, 5 minutes
HA filter, 5 minutes
OIII filter, 5 minutes
The SII image had more noise than the HA and OIII images at the same exposure length. Conclusion: SII filter imaging will need a longer exposure time, possibly two to three times as long. I will use a longer exposure time on my next use of the filters.
I used slightly different post-processing steps for the images of the Crab Nebula:
Edit each image in Lightroom, crop if needed, save as TIFF.
Resize and edit (levels, sharpness) in GraphicConverter. (images above)
Open each narrowband image in Photoshop.
Copy each image to a layer in a new document.
Set blend mode to "Screen" for each layer.
Tweak alignment as needed.
Merge visible layers. Check whether to merge all three images or just two.
Save image as JPEG and do final edits as needed in GraphicConverter.
This image is a merge of all three filter images with no adjustments in colors.
This image is a merge of only the HA and OIII filter images with no adjustments in colors. I think it looks better.
Which filters to merge to give "pleasing" results vs "scientific results" will sometimes depend on the object being imaged. But I found this technique quicker and simpler.
Experienced Deep Sky Astrophotographers will appreciate the quality of the Optolong filters. For beginners at using multiple filters (like me), the Optolong filters will provide lots of capabilities that you can grow into as your experience increases.
I plan to do more Deep Sky Object imaging with the Optolong HA SII OIII Narrowband Telescope Filters and will show the results on my Cassiopeia Observatory Reports.
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