Tright here has been a radical change in the best way that astrophotographers picture the evening sky. Step again ten years and the bulk had been utilizing monochrome CCD cameras fitted with a number of LRGB and narrowband filters housed in an digital or guide filter-wheel. Though producing tremendous outcomes, nice dedication was required by way of the time wanted to safe sufficient information via the filters, and a willingness to put money into costly {hardware}.
With the astonishing inflow of high-quality cameras outfitted 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 provide color pictures, or DSLR cameras which might be low cost and straightforward to make use of, with some limitations.
One other revolution has involved the usage of further filters for these cameras. The filters are capable of 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.
Further benefits are price financial savings, via utilizing only a single filter, and time financial savings. For those who’re imaging a vivid object you may safe a fairly respectable picture in only one evening, making this technique appropriate for folks with restricted time, particularly if you’re not outfitted 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 contains a bandpass of 5nm centred on the hydrogen-alpha and oxygen-III traces. After I was requested to assessment this filter I hesitated, as a result of I nonetheless use CCD cameras outfitted with a number of filters and don’t personal a cooled one-shot digicam with which the Antlia filter would work finest. In fact, this filter can be utilized with a monochrome CCD or CMOS digicam 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 via 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. This can be a good function and the standard casing appeared prefer it ought to have housed an costly piece of jewelry somewhat 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 equipped in two-inch format, so it was simple to put in on my cameras utilizing a normal two-inch nosepiece. The filter cell is threaded on each 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 digicam and Pentax 75mm (three-inch) SDHF apochromatic refractor. I hooked up a two-inch nosepiece to the digicam after which hooked up the ALP-T filter to that. The CCD digicam has an inside 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 digicam to permit an unobstructed mild path to the sensor. I wasn’t certain if this might work with out inflicting vignetting on the edges of the body, however fortunately this didn’t look like an issue.
Advantageous particulars via 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 attention-grabbing goal that needs to be imaged in a particular technique to get the very 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 vivid nebulosity surrounding them to grow to be overexposed with a specific imaging set-up. I discovered that 180-second sub-exposures labored very effectively, preserving the celebrities seen but additionally recording numerous 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 file rather more of the faint outer extremities of the nebula. I then processed the 2 datasets individually and blended them collectively in Affinity Picture to get the very best of each worlds.
The ALP-T filter did an awesome job, capturing superb particulars all through the nebula. The intense cluster stars at centre-right had been halo-free, an indication of high-quality coatings on the filter. I used to be pleasantly stunned by the quantity of construction recorded in NGC 1977, the well-known ‘Working Man’ Nebula, proven at decrease centre-left. This can be a reflection nebula, finest imaged utilizing LRGB filters or color cameras, so it was good to see it recorded so effectively with a 5nm narrowband filter. The intense stars embedded on this nebula had been additionally freed from any haloes.
Whereas the sky was nonetheless clear I eliminated the QSI digicam 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 vivid 78 per cent waxing Moon lurking in Taurus. This would supply an excellent check of the filter’s means 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 a normal Canon digicam 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 concept if this mechanical configuration would work, however it turned out to be fairly profitable. I managed the DSLR in the course of 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 assessments performed with the Canon 5D Mk II, I selected an ISO ranking of 1600. I needed to cut 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 had 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 an excellent 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 known as upon Affinity Picture 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 tip 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 had been attributable to the spacing of the focal reducer. To check this conjecture, on the subsequent 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 direction of the Pleiades and taking a single 600-second publicity. The picture downloaded and a fast perusal confirmed that the star pictures had 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 varied size and blended collectively datasets consisting of 120 seconds and 300 seconds. To this point, the filter had carried out very effectively, however I wished to attempt 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 subsequent clear evening with none moonlight. The Rosette is a reasonably large nebula, so I reinstalled the 0.67× focal reducer and, after focusing and establishing the autoguider, I began a sequence of thirty-one 300-second exposures. To calibrate and stack the pictures 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 Picture, I neutralised the background and was then in a position to make use of the ‘Color Stability’ software 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 phase and, regardless of the fickle climate and moonlight, it was an pleasing expertise. The Antlia ALP-T filter carried out very effectively with my monochrome and color DSLR cameras underneath far lower than good skies. The filter’s means to dam harsh moonlight was admirable. Regardless of being a CCD ‘dinosaur’, I actually appreciated having only a DSLR digicam on the telescope with out having to take hours and hours of triple-filtered exposures and every little thing felt a lot less complicated. Customers of recent, cooled, color CMOS cameras will be capable of get some nice outcomes with the ALP-T filter.
At a look
Dimension: 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
Obtainable from: 365astronomy.com
