Some time ago I decided to try using my old Quantaray zoom lens, once used on a Minolta film camera, for astrophotography. The experiment worked reasonably well, but at f/5.6 at best, the Quantaray lens is fairly slow. In addition, that lens was designed for autofocus use. This presents some problems for astrophotographic applications because moving the focus manually just a little makes a big difference and the manual focus is very sensitive to just a slight touch.
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| Tamron 135 mm f/2.8 lens with T-adapter glued to the top |
I have seen a number of nice images on Astrobin taken by John Leader using a Tamron Adaptall f/2.8 camera lens (in fact, this is what originally inspired me to try the Quantaray). Seeing a few more images motivated me to look into how much the Tamron lens might cost, and I found that used lenses are available for low cost on e-Bay from time to time. Knowing that Tamron invented the T-mount to allow their lenses to be mounted on a variety of cameras, I assumed the lens would come with T-threads, which fit my astrophotography cameras. I found a Tamron 135 mm f/2.8 lens with a Canon adapter of some sort on e-Bay for $15. Its condition was not specified, but pictures of the lens looked good and the price was so low I figured it was worth a try so I ordered it. When it arrived I found its condition was excellent, but my assumption that I could just pull off the Canon adapter and expose T-threads was ill-founded. I decided I could do something similar to what I had done earlier with the Quantaray, which was to just glue a T-adapter to the back of the lens. I used a hot glue gun and a liberal amount of glue to attach the adapter, then wrapped the joint in black rubber waterproof tape to secure the joint and discourage light leakage. A spring-loaded toggle switch enables the f/stop adjustment, but for my initial experiments I left it free so the lens remains wide open at f/2.8 (later on, I can easily shim the spring-loaded toggle so I can use the f/stop ring to stop down the lens if I want). The attached picture shows the adapter on the lens. I got a mounting ring for my ASI1600MM-Cool camera with electronic filter wheel. The mounting ring screws onto a dovetail bar, which also holds a finder/guider so I can put the whole thing on my mount and do autoguided LRGB and narrowband astrophotography.
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| Lens, filter wheel, & camera on dovetail bar with guide scope |
I chose a very challenging target, Sh2-240, aka Simeis 147, the Spaghetti Nebula, to test the setup. This target is about 3° in diameter, much larger than I can image with any other optics I have, but it is quite dim and benefits greatly from narrowband filters, especially Hα. Therefore, this target would present a good challenge for this lens. The image was a first in many ways. It was a first attempt at this large, dim, difficult target. It was my first use of the Tamron 135 mm f/2.8 lens. It was my first try at using high gain with the ASI1600MM-Cool (gain set to 300—much greater than unity—and exposures of only 45 sec for LRGB and only 180 sec for Hα), and it was first light for the Tamron 135+ASI1600MM-Cool combination. I didn’t get any blue data (the neighbor’s darn tree got in the way again), so I cheated: I used the Hα data both for luminance and for R, and I put the R data in the B channel. All things considered, it could have turned out worse, and the lens passed with flying colors. It was easy to focus and seemed to stay focused. I won’t say that the stars looked perfect all the way to the edge, but they looked pretty good especially in the region I cropped to for the attached image. I did the cropping primarily so I wouldn't have to deal so much with the extensive vignetting produced by my 31 mm diameter unmounted filters on the ASI1600MM-Cool imaging chip. I had to process the heck out of this to get a result that was anywhere near reasonable. The supernova that produced this remnant is reported to be about 3000 light years distant and to have gone off 40,000 years ago!
Date: 7 Jan 2017
Subject: Sh2-240 (Simeis 147) Spaghetti Nebula supernova remnant
Scope: Tamron 135 mm f/2.8 lens
Filters: ZWO 31 mm RG, ZWO 7nm Hα
Mount: EQ-6 (EQMOD)+PEC
Guiding: Finder/Guider+DSI IIc+PHD 2.6.2.4 (Win 10 ASCOM)
Camera: ASI1600MM-Cool, -20 °C, acquired Hα 1x1 RG 2x2, Gain 300 Offset 50
Acquisition: Sequence Generator Pro 2.6.0.3
Exposure: 80x180 s Hα, 55x45 s R, G
Stacking: Neb 4.1.2, flats & darks, trans+rot align, Nebulosity 1.5σ stack. Processing: too extensive to detail
