Back After a Long, Cloudy Hiatus
I I keep a log of all astronomy nights, starting with my first night on July 10, 2015! Astronomy nights include astrophotography, visual observing, outreach events –any night I pull out my telescope or astro binoculars, even if the clouds roll in.This is entry #170.
It has been a month since the last time I was out at my astronomy club’s observatory due to recent cloudy weather. But my first night back out was a good one --no moon, pretty good transparency, and the clouds stayed on the horizon.
The new rig my club put inside one of our backyard domes--a Celestron CGX-L mount with a Meade 127mm f/9 apo refractor -was roughly polar aligned by another club member, but still needed some fine-tuning.
So I hooked up my ZWO ASI1600MM Pro on its filter wheel to the telescope, ran all the cables and got everything connected, and started up SharpCap to run its polar alignment routine.
Polar alignment is important to get right for astrophotography --you need the mount pointed directly at the north celestial pole in order to have accurate tracking.SharpCap reported that the mount was off by about 43 arcminutes, which is close enough for visual observing, but is certainly what was causing all the drift I saw last time I was out.
SharpCap tells you whether you need to move the mount up or down and left or right, so I adjusted each direction until it reported an error of only 2-6 arcseconds, which varied every frame due to the atmosphere. Pretty good!
After rebooting the mount so that I could put in new alignment stars afterI changed the polar alignment, I had it go to Vega first, and its first guess was fairly close --it put the star within the camera's field-of-view, at least.After doing the two alignment stars plus four calibration stars, I told it to slew to M1, the Crab Nebula.I had a tough time choosing a target --we're not far enough into winter yet to have Orion and allits goodies be high enough to image in the first part of the evening, but all of the summertime goodies were off to the west, which I try to avoid because of the light pollution.I had originally expected polar alignment to take longer, however, so when the mount finished slewing, the telescope was pointing toward the trees!I slewed up to a higherstar to focus themaincamera and guide camera, and then I left it alone for a bit while I set up my other rig for the evening: my Nikon D5300 on my Vixen Polarie.
I just bought a new tripod so that I could have a removable head to replace with the Polarie's Fine Adjustment head. I was also thinking about myupcomingtrip to Chilethis July, where I will be imaging the solar eclipse and deep sky targets using my Polarie. I boughtone that folds up smalland is lightweight(5 lbs), yet still sturdy. This one is made of carbon fiber, and it has three leg joins to collapse the legs, which fold upward back along the center post so that the whole thing is less than two feet long.
But if you extend the legs and the center post all the way, it goes up to 63 inches tall.
Without the center post extended (which you want to avoid for a stable astrophotography platform), it is still tall enough to comfortably look through the viewfinder, even at my height of 5’9”.It also has a hook in the center column to attach a sandbag or something to weigh it down, and you can remove one of the legs and turn it into a monopod for daytime photography.The tripodwas $130, which as far as nice tripods go, isnnottoo bad!Unfortunately, the bolt that you attach mount heads to was a different size than I thought, so I still couldnotattach the Polarie Fine Adjustmenthead directly, and had to use my 3/8-to-1/4inchbushing and attach the Fine Adjustmenthead to the ball mount that came with the tripod. Soon I'll get all the right screws!
I got everything connected and pointed thePolarie north, or at least as north as I can tell by aiming my eye through the sight hole on the edge of the Polarie.The Fine Adjustment headknobs help a great dealto move it more precisely. There were a lot of moving parts in this rig --first I needed to make the tripod level, but the bubble level is on the ball mount head, so I had to get that approximately upright first.Then Ihad the Fine Adjustmenthead screwed onto that, and the Polarie screwed onto that, and another ball mount head on the Polarie, whose rotating section is tightenedwith thumbscrews, and then finally the DSLR attached to that.Sometimes I'd go to adjust the ball mount that the DSLR was on andwould instead accidentally turn one of the thousand other rotatable things that I hadn't screwed down tightly enough.
I got a little rail that attaches to the camera shoe (the same size as for gun scopes) and attached the red/green dot sight that came with my Oberwerk binoculars to it. Since itisperfectly aligned with the binoculars, I didn't want to mess with it, so it was
aligned well with my camera, but I have ordered another one just for the camera!It should make aiming it mucheasier.
While I adjustedthe polar alignment, I saw a bright flash just above, and a slow-moving green-blue meteor splashed across the sky.Behind it was a sputtering tail of smoke.Beautiful sight!Possibly an earlyGeminid, since it was moving from east to west.
Finally, I got the DSLR pointed in the vicinity of the California Nebula (there's a string of three bright stars that make that very easy) and set my 55-200mm zoom lens to100mm. After a few focusing and test frames, I remembered that I had a much more important target to image --Comet 46P/Wirtanen! How could I forget! I looked up its position in SkySafari, and it was nice and high just underneath Cetus, with a bright star nearby for reference.
SKySafari reported its brightness at magnitude +4.1, although this seemed optimistic to me. I found it easily, although I had to take about 10 test frames before I got it centered exactly where I wanted.Finally,once that was complete,I took some test frames to see how long the Polarie would track for, since it is entirely dependent on being very well polar aligned.Two-minute exposuress how edstarstreaks, and1 minute 30 seconds showed streaks too. I set it down to a minute and was still getting small star streaks.I adjusted the polar alignment a bit, and finally got reasonably small star streaks.Then I hit the “go” button on my intervalometer and let it click away. The shorter exposure time was probably a smart idea anyway, since comets tend to move quickly against the background of stars.
By the time this was done, it was a little after 7:30PM, and the Crab Nebula had cleared the trees up to the 20 degree mark, which is the absolute minimum altitude at which I will image because the atmosphere is too “mushy”below that.Since it was still low.
By the time the colors channels would finish, I figured, the Crab Nebulawould be nice and high and in the good part of the atmosphere for the luminance frames later.I calibrated PHD (Push Here Dummy autoguiding software) for guiding, and the calibrationlooked good.I set the camera cooler on -40C, which it reached, and everything else looked good to go, so I set Sequence Generator Lite to take 15x180s frames on the blue filter, and I snuck back inside where it was warm.
A little later on, I came out with a pair of handheld binoculars from the equipment room and went hunting for the comet.It wasn't difficult to find, although it looked more like a splotch than a comet.If it wasnotso cold, I might have set up a larger pair of binoculars on a tripod, but it was cold!I hurried back inside, where I had the warm room warmed up to a nice 70 degrees.
I periodically went out to the dome to check on things, and the blue and green filter images looked slightly out of focus. When I changed to the red filter, I slewed to a nearby star and re-focused --I must have pushed the focuser in a little when I was rotating the filter wheel. My filter wheel is very stiff, especially when it's cold.It will not be much longer now before I just give up and get an electronic one,and 2-inch filters...although I'm going to need to start plugging things into the ZWO's two back USB ports because I am out of ports on my 4-port USB hub!My Surface 3 tablet, despite how awesome it is for astrophotography and many other applications, has only one USB port.
At least it is USB 3.0!
Guiding looked okay, but not great.The declination axis looked good but right ascension bounced all over the place. I thought at first it was the seeing, but since declination looked fine, I wasn't sure. I zoomed in on the stars, and they were slightly skewed up-down, which I think was along the RA axis. I wondered if I needed to apply some different settings in PHD for the CGX-L mount, since it is belt-driven, unlike the other mounts I use. The agressiveness was already turned down to 70, but maybe that is still too high. More experimentation and research are required!
I stayed out until midnight, but then packed up and drove home, since I needed to be up for work the next morning. The nice thing about winter nights is that they are much longer, allowing me to get a full summer night’s worth of imaging in before midnight. This of course comes with the downside of freezing temperatures.
It took about a week tofinally find some timeto process the final Crab Nebula image.The blue and green channels were slightly out-of-focus, but the red and luminance were better (the seeing was not great that night). I was able to apply a deconvolution algorithm to the luminance and get some really nice detail back from the not-fantastic guiding.
Rather than exhaustively go through every step of my PixInsight processing chain (I have a few blog posts on this, and Light Vortex Astronomy has a wonderful series of tutorials), I will simply list the steps I took, in order. I am fairly new to PixInsight, having just bought the license in September, but it has been really fun to learn so far, and I have been achieving some amazing results!-Used BatchPreprocessto generate master darkand master bias, and calibrate & register light frames (linking the different exposure timesof the dark framesforthe RGB vs the L)-Stacked each channel with Light Vortex tutorial settings recommendations.
- Combined RGB channels
-AppliedDynamicBackgroundExtractionto the RGB and L images to remove the light pollution background
-AppliedPhotometricColorCalibrationto properly color-balance the RGB image(I lovethis algorithm!!)
-Dust spot removal using theCloneStampon L and RGB (my attempt to clean the objective of the refractor didn't work very well when it was below freezing -the cleaning fluid didnotwant to evaporate, so I had to use a hair dryer!I thinkI also need to check my filters for dust)
-Denoising withMultiscaleLinearTransformon L and RGB, with a mask to protect the nebula from being blurred
- Stretched L and RGB images-Applied theDeconvolutionprocesswith a PSF(point spread function)generated from the image (uses the shape of the stars to inform the algorithm)
-Combined LRGB into one image-Reduced star sizes withMorphologicalTransformwith a star mask (needed because of the un-focused G and B channels)
-CurvesTransformationto boost saturation, brightness, and I also used it with a star mask to reduce the green halos that were a result of the un-focused green channel image.
Date: 9 December 2018
Location: Bortle 5 zone
Object: M1 Crab Nebula
Camera: ZWO ASI1600MM Pro
Telescope: Meade 127mm f/9 apo (club's)
Accessories: Astronomik LRGB Type 2c 1.25" filters
Mount: Celestron CGX-L (club's)
Guide scope: Celestron 102mm (club's)
Guide camera: QHY5
Subframes: L: 10x300sR: 12x180sG: 14x180sB: 15x180s
Total time: 2h53mGain/ISO: 139
Stacking program: PixInsight 1.8.5
Stacking method (lights): Average, winsorized sigma clipping
Post-Processing program: PixInsight 1.8.5Darks: 30Biases: 50
Flats: 0Temperature: -40C (sensor), 29F (ambient)
I'm continually amazed with what I can get out of an image that has sub-standard input data!This image has less than3 hours of total integration time,and yet I managed to get some reasonable SNR (signal-to-noise ratio) and detail on the nebula.Now just think if I had enough clear nights (and enough patience) to have much longer total integration times!
Having awesome software helps too...here's the comparison of the pre-and post-deconvolution images!
it is always fun to see what image processing can do for your astro images.At submissiontime, the comet image is still a work in progress –the calibration files are giving me some trouble. Hopefully I will have some time soon to finish that image. You can keep an eye out for it on my blog at AstronoMollyLog.blogspot.com, or on my AstroBin (www.astrobin.com/users/mollycule). You can also find me on Facebook and Instagram as AstronoMolly Images and astronomolly_images, respectively.170 nights down, many many more to come!