A Year's Experience with the ZWO ASI1600MM-Cool and a Year's Progress

I bought my ZWO ASI1600MM-Cool a year ago, and after using it for a year I still love this camera. It produces far better images than I could ever get with my old Meade DSI IIc. Some of this performance is no doubt because of the smaller pixels and the fact that it's a monochrome camera so doesn't suffer from the limitations imposed by a Bayer matrix. It also true that this better performance comes at a cost; not just the higher cost of the camera, but also in much greater effort required for processing the data (at least in my hands). The much higher number of pixels alone means that processing time is much longer (I'd estimate a factor of 3-5 times at least) and data storage requirements are much greater (more than a factor of 30--my primary drive is filling up fast!). In addition, it has been a real adventure (and actually a lot of fun) learning how to do LRGB, SHO, HSO, and various other combinations to produce color images. I've still got a lot to learn about that!

I've now used this camera with my AT8IN 8-inch imaging Newtonian, with my Orion ST-80 very-non-apochromatic refractor, and with an old Tamron 135 mm camera lens bought for $15 on e-Bay, and it has done a good job with each. The much larger imaging area does mean that the optical flaws in my systems are much more apparent than they were with the DSI IIc; its tiny sensor only sampled the nearly flawless center of the image plane, whereas the ASI1600MM-Cool finds lots of flaws out near the edges of the field of view and I see plenty of effects from vignetting that I never saw before. I've got one dust mote on the sensor that I either have to correct with flats or correct in software, but I've pretty much figured out how to do that.

I've experimented with various gain/offset settings, and find myself gravitating back toward the "default" unity gain (-139) offset 21. I really don't have a scientific reason for this. It's just what I've found most comfortable with the exposure times and dynamic range expectations I have. I think higher gain combined with shorter exposures is an approach with a lot of promise, as it seems like it would allow better statistical treatment and would minimize any frame-to-frame drift from differential flexure (I see a lot when using my finder/guider with the Tamron lens, and even see some when using an off-axis guider). A future goal is to get a bit more scientific with these experiments.

The only persistent problem I've had with this camera appears to be in the USB-3 cable connecting the camera to a hub located at the telescope mount. Occasionally the connection to the camera and filter wheel drops, with the result that the next attempt at downloading an image hangs. I've had quite a few sessions end prematurely because of this. Often it happens while doing a meridian flip or right after a filter change. I'm sure it's not the hub or the USB cable back to my living room, because the guide camera is also on that same hub and never has problems. When I originally set up the ASI camera I noticed it only seemed to work well with the cable that shipped with it, so it seems to be a bit picky about the cable. I might try getting another cable to see if it makes a difference. Usually, jiggling the cable and doing an unplug/replug fixes the problem.

All of that said, I've learned a lot over the last year. As evidence, I offer the following two images. The first was done a year ago, and was the first time I had used the ASI1600MM-Cool with the Tamron 135 mm lens. Because it was a "first time" in many ways, maybe this isn't exactly a fair comparison. At any rate, I was delighted with the image at the time, because there was no way I could have done anything like that with my prior setup.

This image was a first in many ways. It was a first attempt at this large, dim, difficult target. It was the first use of the Tamron 135 mm f/2.8 lens I bought for $15 on e-Bay. It was my first try at using high gain with the ASI1600MM-Cool, 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. I had to process the heck out of this to get a result that was anywhere near reasonable. The supernova is reported 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 2.6.0.1

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.

The second image is my most recent, just finished today. In the last year I've learned a better way to mount the camera on the lens, and I've built, tested, and used an automated stepper-motor-based focuser (based on Robert Brown's designs and using his software) for the lens. I've learned to do bicolor combinations and have benefitted greatly from Annie's Astro Actions for Photoshop, which make this easy. I've gotten better at picking good stars for doing alignment and stacking in Nebulosity (I tend to use dimmer stars that have good shapes, and I always magnify the image when I choose them, avoiding ambiguity in which star is picked). I'm trying to be a bit more subtle in my processing, so my images probably are not as stark from trying so hard to pick up the really dim stuff. To me this newest image is far better than the one from a year ago. One of the things I really love about astrophotography is that it's easy to see progress!

One motivation for this image was to compare with the image I took a year ago, which was the first using the Tamron 135 mm lens; I wanted to see how much my technique and experience had changed over the last year. The new image suggests these have changed a lot. I had intended this to be HSO/SHO, but the session failed while trying to focus the SII filter (which was the last in the sequence) long after I had gone to bed, so it ended up being only bicolor. It looks like the weather is turning stormy at last (winter is coming pretty late this year in Utah) so I’m going with the data I have rather than wait for more clear skies. These data were tough to process because this target is quite dim, hence the original data required very extensive stretching and were consequently pretty noisy. Nearly all the structure shows up only in Hα (only the barest hint of the filaments in OIII), though I’m curious whether anything would have been visible in SII. I continue to have trouble with my Tamron 135 flats (the current ones overcorrect), so I ended up not using them and just let StarTools’ Wipe vignetting module handle things. I went back and forth a few times about how dark to make the background, and ended up leaving it fairly dark. I understand this nebula is thought to be a supernova remnant. The structure in the filaments is amazing—it looks to me more like a wad of fine spun gold with the color choices in this image, rather than spaghetti—leading me to wonder what caused it. Most likely it was just random turbulence at the time of the supernova explosion, but it is fun to speculate that maybe it was somewhat shaped by a planetary system around the star, destroyed at the time but imprinted on the remnants (now that we know that most stars do have orbiting planets). Of course I have no evidence for such romantic speculation!

Date: 19 Dec  2017

Subject: Sh2-240 (Simeis 147), Spaghetti Nebula

Scope: Tamron 135 mm f/2.8 lens stopped to f/4

Filters: ZWO 31 mm diameter unmounted Hα, OIII (7 nm bandpass)

Mount: EQ-6 (EQMOD 2.000j)+PEC

Guiding: Orion 9x50 Finder/Guider + DSI IIc +PHD 2.6.4.5 (Win 10 ASCOM) using predictive PEC algorithm

Camera: ASI1600MM-Cool, -20 °C, Gain 139 Offset 21 

Acquisition: Sequence Generator Pro 3.0.0.4

Exposure: 40x300 Hα, 36x300 OIII

Stacking: Neb 4.1.6, darks only, trans+rot align, Nebulosity 1.5σ stack and align.

Processing: StarTools 1.4.328: Used StarTools’ “Wipe” module to correct for vignetting, then stretched and deconvoluted each channel separately in StarTools. Aligned the processed layers in Nebulosity then combined in Photoshop using Annie’s Astro Actions’ HO Bicolor module. Multiple rounds of Photoshop Curves and Levels, combined with several rounds of Carboni’s Make Stars Smaller, then Deep Space and Space Noise Reduction and a few rounds of Less Crunchy More Fuzzy. AstroFrame.