Tright here has been a radical change in the way in which that astrophotographers picture the evening sky. Step again ten years and the bulk have been utilizing monochrome CCD cameras fitted with a choice of LRGB and narrowband filters housed in an digital or guide filter-wheel. Though producing tremendous outcomes, nice dedication was required when it comes to the time wanted to safe sufficient information by means of the filters, and a willingness to spend money on costly {hardware}.
With the astonishing inflow of high-quality cameras geared up with CMOS sensors (each color and monochrome), a fast transition has occurred, realistically sounding the death-knell for CCD cameras. Most beginner imagers select to work with ‘one-shot’ color cameras that require no filters to supply color pictures, or DSLR cameras which are low-cost and straightforward to make use of, with some limitations.
One other revolution has involved the usage of extra filters for these cameras. The filters are in a position to isolate particular emission from deep-sky objects, permitting efficient narrowband imaging with color cameras. The preferred are the Optolong L-eXtreme and L-eNhance twin/triple bandpass filters that isolate hydrogen-alpha, oxygen-III and hydrogen-beta wavelengths from emission nebulae. With the ever rising scourge of sunshine air pollution, having a filter that may block most home emissions, in addition to moonlight, permits high-quality imaging even from cities and cities.
Extra benefits are value financial savings, by means of utilizing only a single filter, and time financial savings. When you’re imaging a shiny object you may safe a reasonably first rate picture in only one evening, making this technique appropriate for folks with restricted time, particularly in case you are not geared up with a devoted observatory and must arrange from scratch each clear evening.
A flexible filter
A brand new providing is the Antlia ALP-T dual-band filter. Additionally designed for use with color cameras and DSLRs, it includes a bandpass of 5nm centred on the hydrogen-alpha and oxygen-III strains. After I was requested to assessment this filter I hesitated, as a result of I nonetheless use CCD cameras geared up with a number of filters and don’t personal a cooled one-shot digital camera with which the Antlia filter would work greatest. After all, this filter can be utilized with a monochrome CCD or CMOS digital camera and the 5nm bandwidth of the filter matches my very own Astrodon filters, so I used to be intrigued. I additionally personal an ageing Canon 5D Mark II DSLR, which is superb for daytime and twilight pictures however being unmodified is much less succesful for astrophotography. With the above limitation in thoughts I got down to put the Antlia ALP-T filter by means of its paces.
The filter arrived in a really good, sq., plastic casing that had 4 small magnets fitted, one in every nook that allowed the case’s lid to snap into place. It is a good function and the standard casing appeared prefer it ought to have housed an costly piece of jewelry relatively than an astro filter. Additionally included is a filter scan report acquired with a high-quality spectrometer that certainly confirmed the beforehand talked about bandwidths. The filter was provided in two-inch format, so it was simple to put in on my cameras utilizing an ordinary two-inch nosepiece. The filter cell is threaded on either side, permitting additional equipment to be hooked up if required.
I made a decision to begin off by testing the Antlia ALP-T filter with my monochrome QSI 583wsg CCD digital camera and Pentax 75mm (three-inch) SDHF apochromatic refractor. I hooked up a two-inch nosepiece to the digital camera after which hooked up the ALP-T filter to that. The CCD digital camera has an inner filter-wheel and off-axis autoguiding port built-in, so to make use of the two-inch filter I needed to take away one of many 1.25-inch filters within the digital camera to permit an unobstructed gentle path to the sensor. I wasn’t certain if this is able to work with out inflicting vignetting on the edges of the body, however fortunately this didn’t look like an issue.
High quality particulars by means of the filter
My first goal was M42, the magnificent Orion Nebula, which was conveniently positioned close to the meridian throughout my testing in mid-January. The Orion Nebula is an fascinating goal that needs to be imaged in a selected approach to get one of the best outcomes. The primary plan of action is to find, by trial and error, what the longest publicity time is that doesn’t trigger the Trapezium stars and shiny nebulosity surrounding them to turn into overexposed with a selected imaging set-up. I discovered that 180-second sub-exposures labored very effectively, protecting the celebs seen but additionally recording loads of the fainter nebulosity. I shot six of those after which an additional 13 300-second exposures. These longer exposures would definitely overexpose the core space however the plan was to document rather more of the faint outer extremities of the nebula. I then processed the 2 datasets individually and blended them collectively in Affinity Photograph to get one of the best of each worlds.
The ALP-T filter did an awesome job, capturing tremendous particulars all through the nebula. The brilliant cluster stars at centre-right have been halo-free, an indication of high-quality coatings on the filter. I used to be pleasantly shocked by the quantity of construction recorded in NGC 1977, the well-known ‘Operating Man’ Nebula, proven at decrease centre-left. It is a reflection nebula, greatest imaged utilizing LRGB filters or color cameras, so it was good to see it recorded so effectively with a 5nm narrowband filter. The brilliant stars embedded on this nebula have been additionally freed from any haloes.
Whereas the sky was nonetheless clear I eliminated the QSI digital camera and changed it with my Canon 5D Mk II DSLR, however the whopping 242 × 162 arcminute fieldof-view decreased the Orion Nebula to minuscule proportions, so it was on to Plan B.
Together with a focal reducer
The subsequent clear evening was chilly and crisp however faintly hazy, with a shiny 78 per cent waxing Moon lurking in Taurus. This would supply a great check of the filter’s capability to dam moonlight. By now I had put in the Canon 5D Mk II DSLR onto my 254mm GSO Ritchey–Chrétien telescope. When imaging with this telescope and my QSI CCD cameras I often make use of an Astro Physics 0.67× focal reducer. On this configuration the focal size of the telescope is decreased from 2,000mm to 1,340mm. I opted to make use of this now on the Canon DSLR and to connect it I employed an ordinary Canon digital camera adaptor coupled to a two-inch nosepiece. The focal reducer screwed onto the entrance of that and on the finish of the chain I put in the ALP-T filter. I had no thought if this mechanical configuration would work, however it turned out to be fairly profitable. I managed the DSLR throughout the imaging run utilizing the fantastic Yard EOS Premium Version.
As soon as extra, it was essential to assess the optimum publicity lengths to have the ability to mix a number of pictures. For all the checks performed with the Canon 5D Mk II, I selected an ISO ranking of 1600. I needed to scale back the publicity time to only 60-seconds to protect the Trapezium stars. These shorter exposures appeared paying homage to bi-colour pictures, which isn’t too shocking provided that they have been taken with a dual-band filter. The 60-second dataset consisted of simply 5 sub-exposures after which I went on to seize an additional fifteen 300-second exposures. As all the time, I used to be cautious to take a great quantity of calibration frames (darkish, flat and bias) earlier than ending the imaging session. To calibrate and stack each datasets I used Astro Pixel Processor software program and it carried out effectively.
No haloes
Once more I referred to as upon Affinity Photograph to create a twolayer picture and blended the 2 units collectively. The picture scale was very nice and, contemplating that the sky was a bit hazy and flooded with moonlight, the top end result was fairly good. I famous some unusual reflections across the brightest stars however at this level I used to be 99.9 per cent sure that these have been attributable to the spacing of the focal reducer. To check this conjecture, on the following clear evening when the Moon was much less troublesome, I eliminated the focal reducer and took some additional pictures working at f/8. I began off by pointing the telescope in the direction of the Pleiades and taking a single 600-second publicity. The picture downloaded and a fast perusal confirmed that the star pictures have been completely freed from haloes. All of these bizarre reflections had certainly been attributable to the focal reducer.
After this, I returned to the Orion Nebula another time. As earlier than, I shot a sequence of exposures of assorted size and blended collectively datasets consisting of 120 seconds and 300 seconds. Up to now, the filter had carried out very effectively, however I needed to try to picture a goal that might produce a much less monochromatic view, and for this I selected the Rosette Nebula in Monoceros. There was a little bit of a delay ready for the following clear evening with none moonlight. The Rosette is a fairly large nebula, so I reinstalled the 0.67× focal reducer and, after focusing and organising the autoguider, I began a sequence of thirty-one 300-second exposures. To calibrate and stack the photographs taken with the DSLR I as soon as once more used Astro Pixel Processor. It did an awesome job of making a grasp color picture, though at this stage it was missing a bit in color variation once more and the sky background was tinged in a bluish–inexperienced.
Throughout additional processing in Affinity Photograph, I neutralised the background and was then ready to make use of the ‘Color Stability’ instrument to begin to tease out refined variations inside the nebula, producing a view very paying homage to a threefilter narrowband picture.
This concluded the testing part and, regardless of the fickle climate and moonlight, it was an fulfilling expertise. The Antlia ALP-T filter carried out very effectively with my monochrome and color DSLR cameras beneath far lower than excellent skies. The filter’s capability to dam harsh moonlight was admirable. Regardless of being a CCD ‘dinosaur’, I definitely appreciated having only a DSLR digital camera on the telescope with out having to take hours and hours of triple-filtered exposures and the whole lot felt a lot easier. Customers of recent, cooled, color CMOS cameras will be capable of get some nice outcomes with the ALP-T filter.
At a Look
Measurement: two-inch (M48 × 0.75 thread)
Filter thickness: 2mm
Central wavelengths: 500.7nm (O-UII) 656.3nm (H-alpha)
Bandwidth: 5nm
Peak transmission: 90% (H-alpha), 82% (O-III)
Worth: £393
Out there from: 365astronomy.com
