Shooting Astro Narrowband with Medium Format Sensor (Phase One IQ260 Achromatic)

It took me sometime to gather all the small components necessary to adapt the IQ260 for some narrowband, in-city astro imaging.  This is a term I coined because I feel that photography is a term reserved for terrestrial targets that are typically of short exposure.  Astro imaging is always conducted at infinity focus involving celestial targets and of long exposure.

In the first installment with the IQ260, we learned that a large sensor readily reveals all the optical aberrations that are present in the field periphery as opticians bias their prescriptions to optimize the central regions.  This is particularly true in telescopes because the human eye has terrible peripheral resolution in the dark with an unusually high density of rod cells (and very few cone cells) in the fovea or center part of the retina.  And as you age, the ability of the pupil to dilate is impaired and older people see even less in the dark and through a telescope at night.

The Astro Physics Traveler is a 105 mm aperture, 610 mm focal length f/5.8 refractor made of three spherical elements with the center one made of ED glass and instead of being air spaced the gaps are filled with mineral oil.  Spherical lenses are the easiest to grind accurately and polish well with the oil filling in any residual surface roughness promising an image of sharpness and high contrast – which is what the Traveler delivers.  But this design does result in significant peripheral field curvature and coma which in the 1990s would only bother photographers using medium format film, really a small niche group.  However Astro Physics is not a company to ignore any deficiencies in its products and came out with a screw on field flattener (AP 67PF462).  People who have them clearly want to hang on to them because they are impossible to buy as Astro Physics long stopped making them.  I would have to come up with an alternate solution otherwise astro imaging with a digital medium format sensor would offer no advantages if I ended up cropping away large distorted regions of the image.

There are many 2″ diameter field flatteners available since this is all that’s necessary for visual and full frame digital sensor work but such an accessory would vignette badly on the medium format sensor.   There are several refractor telescope companies that make larger scopes for the amateur market with 3″ diameter focusers, as does Astro Physics but they do not recommend using field flatteners for their bigger scopes with the diminutive Traveler.   I discovered Stellarvue marketed a reasonably priced 2 element field flattener (SFXX-1) with an optimal operating distance to the sensor of approximately 98 mm for my scope specifications.  Since the Phase One flange to sensor distance is fixed at 63 mm (the same spec as the film borne Mamiya 645 system) that gives me less than 40mm to cobble together connections that will also allow the incorporation of imaging filters in the optical train.  (Astro Physics does market a current field flattener for the new reissue of the Stowaway refractor that can work on the Traveler (92FF) but it only has an imaging circle of 50 mm and a very limited backfocus spacing of 64 mm, which will simply not work with a Phase One body).

Stellarvue advised me that it might be months before they hand assemble a new batch of field flatteners and they source all the components outside of the US.   Which explains why a Chinese company named Sharpstar Optics markets something that looks identical to the Stellarvue product (FF25L).  Intellectual theft in China is common and factories manufacture surplus quantities of products outsourced by a foreign company in order to market it as their own product while severely undercutting the price.
The Stellarvue field flattener shown on the left is different from the Sharpstar clone with the inclusion of positioning screws that allow fine positioning tuning of the two optical elements relative to each other. I really preferred to purchase the Stellarvue product, even if it cost nearly a $100 USD more. But it was simply unavailable. The Chinese copy was available locally and I bought it.
The myriad of connectors needed, left side is the camera side and right side is the telescope side.                               1.   Mamiya 645 flange to female M65x1 thread adaptor   2.  home made 93 mm diameter aluminum disc x 1/8″ inch thickness with male M65x1 threaded insert (for optical filter)   3.  Astro Physics 6×7 outer bayonet adaptor (AP 67RT) to connect to Pentax 6×7 camera   4a.  custom made male M68x1 to female 2.7″ Astro Physics thread adaptor  4b.  Baader Planetarium male M68x1 to female 2.7″ AP thread adaptor   5.  Sharpstar Field Flattener    6.  Baader Planetarium female M68x1 to male 2.7″ AP thread adaptor
Phase One body connected to AP Traveler focus draw tube (with AP extender in black).  The use of the Pentax 6×7 to Mamiya 645 adaptor is not possible with the field flattener because it consumes too much backfocus distance.
(Right) How the AP 6×7 outer bayonet works as a clamshell to secure around a Pentax 6×7 lens flange    (Pentax 6×7 to Mamiya 645 adaptor shown).                  |                                                                                                              (Left) Home made insert made to house custom made 65 mm Hydrogen Alpha filter with 90% transmission and FWHM of 3 nm, courtesy of Rapid Spectral Solutions  (formerly known as Omega Optics).
MTF curves (MTF10 in red and MTF30 in green) from top to bottom: no field flattener, field flattener with backfocus distance of 99 mm, field flattener with backfocus distance of 108 mm.  Clearly, backfocus distance is highly critical to field flattener performance and the specifications based on focal length, aperture and f number of the Traveler dictated an optimal distance of between 98 and 101 mm.

The Sharpstar field flattener advertises the exact same specifications and screw on accessories as the Stellarvue, which is why I spent $200 USD to have a custom made adaptor (4a) with male M69x1 threads going to female AP 2.7″ thread on the camera side of the field flattener.  Turns out the machinists in a Chinese factory can be a lazy lot since they decided it was easier just to cut M68x1 threads on both the nose and tail end of their field flattener.  And my local machinist was kind enough to recut the male thread to M68x1 at no cost.  But I could’ve saved even more money because Baader Planetarium sells the very adaptor and it’s much thinner to boot (4b)!

So here’s first light with the IQ260Achromatic and the Omega Optics 65mm diameter Ha filter set up on my front lawn in urban Toronto. The target is NGC7000 (The North American and Pelican Nebula) using 30×2 minute unguided subexposures that used the Phase One’s in body noise reduction that subtracted a true dark frame from each exposure by exposing the 2 minute frame twice each time. The camera has a ISO1600 ceiling so ISO800 was chosen. Clearly this sensor is not well suited for astro imaging applications, it has very strong electronic noise signal that may only be tamed and eliminated with strong thermoelectric cooling and that is very typical of older style CCD sensor designs of that era. As a result our nebula signal is barely recognizable as its data cannot be distinguished from the background noise signal and to be fair this camera was designed to function optimally as a standard camera and not as astro imaging camera.  It’s disappointing, but I don’t think I will be doing any more astro imaging with this rig.
To demonstrate that the astro imaging deficiences lie with the camera body and not the Ha filter, I reimage with my thermoelectrically cooled Canon T2i through the same scope and for the same total duration, 30×2 minute unguided subexposures.  A dark frame was successfully able to remove all thermal noise with the sensor operating at 13°C on a very warm summer night of 23°C.   The APS sensor on the T2i has about a third of the FOV of the Phase One and only a partial crop of the the North American Nebula is visible but the quality of the image is far superior as it should be with this narrowband Ha filter.   This is despite having to image through the Bayer layer of the T2i with only one quarter of the pixels (the red ones) able to record any data.  Ignore the elongated star shapes, I failed to carefully balance the scope and camera on the equatorial mount.  To learn more about thermoelectrically cooling your DSLR, see my earlier blog entry

I will be returning the 65 mm diameter Rapid Spectral Solutions Ha filter because unfortunately I won’t be doing any more astro imaging with the IQ260A.   My understanding is that they will be pricing this filter north of $500 USD and it will probably be competitive to the $700 USD 3.5 nm Baader Planetarium 65nm square shaped filters used with SBIG’s STX family of full framed sensored cameras.  In the urban conditions of Toronto there is a distinct advantage to using 3nm vs the more typical 7-12 nm Ha filters in punching through light pollution.  The advantage with dealing with Rapid Spectral Solutions is that they are particularly attuned to the amateur astronomy community and are willing to make custom sized filters and filter applications with all manufacturing based in the US.  In this era of supply chain issues and the belligerence of China, it’s good to be able to source materials at home.

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