This is a tricky target from my back yard, I only have about a 50-minute window between some trees to capture this. Imaged on a windy night the stars are not a great shape but I can live with that. This an Ha only image cropped and focused on the horsehead itself:
And here is the full frame:
Imaged using the ASI1600 on the OO250mm Newt, just 22 x 120sec Ha. Accidentally imaged at 0 gain! It’s tricky to get more data at this focal length due to the small window of opportunity so I’ll likely go portable for the classic a wider view with my short focal length refractor.
This is the bright 7-Sisters star cluster in Taurus visible high in the winter northern skies. This is a target that has been long on my list of failures – results have previously been poor with not having the right focal length and not taking enough sub-frames. It’s a broadband object so it is also at the mercy of light pollution.
Whilst the stars are bright the reflection nebula surrounding this cluster is quite faint and easily swamped. Bringing out the reflection nebula without saturating the stars requires choosing an exposure length that does not clip the stars and then take lots of exposures.
I decided that it was a good target for my un-modded, uncooled Canon 550D, matched to my 70mm APO refractor to give me a good level of detail with some surround star field for context. The result above took 479 x30sec exposures (4hrs). It was processed in Pixinsight and finished off in Paintshop Pro as a full frame resized to 30%. If I took another 4 or 8 hrs(!) I might be happy to display this at native resolution.
The HEQ5-Pro continues to be an excellent mount for this scope. Basically, its sub-arcsecond tracking accuracy with this load means it’s rare to lose a frame due to mount errors.
Earlier this year purchased a ZWO ASI1600mm Pro + 36mm 7-position filter-wheel complete with the LRGBNB filter set.
The ZWO filters represent very good value and I have had a lot of fun with them. However, of the set the Oiii is not the best performer, it produces distracting halos around bright stars and rings around dimmer ones.
I have learnt a technique to reduce the impact of halos during image processing but it is better not to have them in the first place if they can be avoided. Its asking a lot of budget filters so I had a look around, prepared to spend a lot more. But I noticed that the Baader filters had recently been updated with improved anti-reflection halo reducing layers (even claiming halo free). At £143 from FLO they are a similar cost to the ZWO filters when bought individually. I decided to give the Baader filter a trial to see how it lived up to the claim.
I fitted both the Baader and ZWO Oiii filters to the filter wheel for a side-by-side comparison. The optical train was ASI1600mm Pro, Filter-wheel, Lacerta KomakorrF4, Orion Optics 250mm F4.8 Newtonian. Autofocusing between filter changes was done with a ZWO EAF.
The camera gain was set @ 139, sub-frames were 120s. I took 34 subs with the ZWO Oiii and selected the best 10 frames to stack. I took over 100 frames with the Baader. Using the same quality selection criteria I had 38 frames, from which I took the first 10 for stacking for this comparison. I used the N.I.N.A autofocus routine with each filter change. All subs were taken during the same one-night session. Conditions during the night varied but because I restarted a few times, the conditions evened out for the two filters.
To conduct the test, I chose a new target for me but one with a reasonably bright star in the ROI. I chose the Bubble Nebula. Below are the stacked images with automatic stretch applied:
ZWO 7nm Oiii Narrow Band Filter
The ZWO filter gives us a very obvious halo around the bright star.
Baader 6.5nm Narrow Band OIII Filter – CMOS Optimised
The Baader filter also has a halo but it is very faint. Additionally, the star appears better defined suggesting less scatter overall. The fainter stars appear brighter than in the ZWO image. (More testing would be needed under stricter conditions to verify this).
Finally for comparison here is a closeup of the ZWO Ha subs (80 minutes integration). There is no halo apparent.
The new Baader 6.5nm Oiii filter offers a significant improvement in halos over the ZWO 7nm. There are still halos present in my particular setup, but the Baader halo is over 4 times fainter that the ZWO.
There is also the suggestion of less light being scattered, giving brighter stars and better definition in distributed objects like galaxies and nebula.
This is the Wizard Nebula. I’m told it is supposed to look like a wizard in a pointed hat and gown with open arms. I imaged this over two nights under very bad weather conditions and a bright moon.
I attempted and abandoned some S2 – the signal was weak and too many subs were trashed. At the end of the two days I applied more than my usual rigor to the frame selection process and ended up with just 100mins each of Ha and O3.
Stacking just the best subs didn’t change the star sizes but the fainter ones were sharper, over-all contrast, range and noise was improved (so spending the time up-front to select good frames is really worth it). O3 was still weak but I decided not to push it too hard. I applied the halo reduction technique for the O3 described here it works surprisingly well.
I’ve combined the signals as R=Ha, G= 0.3*Ha +0.7*O3, B= O3. The central blue area of the nebula was enhanced using an O3 range mask and just increasing the blue saturation a little. This was quite tricky to process and easy to overdo.
After imaging this nebula, I’ve purchased a replacement O3 filter. For future narrow-band projects I’ll be swapping out the notorious-for-halos ZWO 7nm O3 filter for the Baader 6.5nm (new for 2021) O3 filter that is supposed to have much improved antireflection and halo reduction coatings. I plan to do a side-by-side comparison in a mini-review just to see if the Baader filter is worth it in my setup.
After cleaning and correctly(!) fitting my LRGB filters I decided to go a for a bright broadband target. Clouds, moon and poor seeing did not help but I managed about 45 mins each for RGB and Ha – shot with the ASI1600mm. This is tight cropped as I combined Ha from two sessions and the framing wasn’t very well repeated. (I really must work out a way to index my camera rotation.)
This is the first full imaging session since changing the RA and DEC boards of the CEM70G. The resolving power (Dawes-limit) for the scope is about 0.45 arc seconds. At this focal length the ASI1600mm pixel size is 0.65 arc seconds. This camera is basically working at the limits of the scope. If you pixel-peep this image even with the poor seeing you can tell the CEM70G is now getting the most out of the combination. An OAG may give me fewer reject subs but I am very happy using my 60mm guidescope +ASI290 guide camera.
This was trickier for me to process than the Heart nebula. Although it was clear, conditions were not great and I had few gremlins to deal with. Overall, it meant fewer hours of data so far captured (7hrs total, 2hrs each for Ha and O3, three hours for S2). This shows in the noise and processing artifacts visible in the enlarged image as I attempt to get something close to the Heart image in detail and vibrancy.
Details: ASI1600mm pro, Altair Astro 70mm EDT-F on the HEQ5.
Here is another attempt at processing the data, this time I just used the subs from the second night as they were better framed, so this represents about 3.5hrs of S2, Ha, O3, processed as SHO in a pseudo-Hubble palette.
This is my first imaging season using narrow band filters on a mono camera. I’m enjoying the freedom to image and not fight moonlight or light pollution from the city.
The Heart nebula (IC1805) is in the constellation of Cassiopeia. This is the result of 6hrs of 120s subs in S2, Ha and O3. Imaged using the ASI1600mm pro with a gain of 200. Processed in PixInsight with the Hubble palette SHO.
Visible at the top of the heart image is what looks like a small blue galaxy or planetary nebula. At bit of research reveals this is WeBo 1 (PN G135.6+01.0) a planetary nebula. It has a good signal in Ha and Oii. Perhaps imaging with my 250mm Newt will reveal more of its structure.
This image is the result of 13 hrs of imaging. I still have much to learn about image processing but I am quite happy with this result. The S1, Ha and Oiii subs were 120s @gain 200 to avoid burning out too many stars.