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  1. Andy Resnick

    Random Photos

    Snow on pine needles:
  2. Andy Resnick

    Random Photos

    Over winter break we took a trip to the Cleveland Museum of Art- in addition to Bellows' "Stag at Sharkey's", this is one of my favorite pieces: For the longest time I assumed this was some hyperviolent deity (it's adorned with skulls and stomping on people!), but it's not. Quite the...
  3. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    More or less- that happens all by itself. Remember, I am correcting for a few arcminutes of pointing error within a 2-degree FoV. The target doesn't need to cover the exact same pixels every frame, that's what the frame alignment step in stacking (as opposed to t-projection) accomplishes.
  4. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    Huh- I'll be a monkey's uncle, you are right! I found this reference, very careful measurements: https://www.sciencedirect.com/science/article/pii/S0042698900000213?via%3Dihub (see especially Figs 4-6 and 9) Clearly showing the L-cone has sensitivity all the way down to 400nm. Violet is...
  5. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    That's fine- I am color-blind, after all. How about this- given the sensitivity curves of human cone cells, explain how red hues are is perceived when the retina is excited by 405 nm light: https://askabiologist.asu.edu/sites/default/files/cones_graph.gif Yeah, yeah.... whatever. Bees see...
  6. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    No, I don't re-center the imaging target prior to mount model refinement, I simply perform the mount model refinement and then go back to the imaging target. Yeah, I know... that's the main reason it's taken me so long to figure out what is going on :)
  7. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    If the hue is generated by a continuous section of the EM spectrum, it's not a pure spectral color- by definition.
  8. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    I wouldn't- that wavelength simply appears as an intense blue to me. This diagram is pretty clear- violet does not correspond to an electromagnetic frequency. https://www.ledtronics.com/html/1931ChromaticityDiagram.htm There's often a lot of noisy "discussion" about color due to lack of...
  9. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Ok, let me break it down a little more. First- fiducial markers are, in the context of imaging, fixed reference points used to calibrate an imaging system: you can calibrate aberrations, magnifications, steering mirrors, wavelengths, any aspect of an imaging system by using appropriate...
  10. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    I'm assuming this is a joke?
  11. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Model targets ('fiducials') are always placed in the center of the FoV, and it does not impact the z-projection. 'refining the model', according to Losmandy, means re-aligning to prior fiducials and aligning to new fiducials as they rise and become visible as the night goes on. For example, I...
  12. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    I mean that no points on the violet-purple line on the CIE chromaticity diagram correspond to electromagnetic frequencies. Alternatively, the electromagnetic spectrum does not contain purples/violets (or pinks and browns, for that matter...)
  13. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Last night I had 7 moon-free hours of viewing and now I have a better (but still incomplete) understanding of what my mount driver is doing, PEC correction, etc. It's a long-ish post but the content may be useful to the community. I tried to experimentally validate parts of @collinsmark's...
  14. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    @Drakkith , @Tom.G , @collinsmark : Thanks for the education! This weekend is supposed to be clear, so I'll have some time to try all this out.
  15. Andy Resnick

    Light's properties (Why isn't the sky violet?)

    Part of the reason is that violet is not a spectral hue. Wavelengths shorter-than-blue (ultraviolet spectra) are indeed scattered more than blue, but your photoreceptors don't respond to UV. "snow blindness" occurs from the large amount of UV light scattered from ice/snow.
  16. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    I got this one: https://www.amazon.com/dp/B08XKMBTXH/?tag=pfamazon01-20 14 TB should last me a while :)
  17. Andy Resnick

    Thinking about quitting teaching

    I empathize, I've struggled with burnout the past few years and am slowly digging myself back out. Are you teaching the same course over and over and over? Do you have the ability/option to vary the class or course you are teaching? I've found that helps a lot- teaching Intro physics I and II...
  18. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Many thanks for the suggestions- for example, I do have a backlash compensation setting, I'll give it a try and see what happens. The paragraphs above are well-written but I can't follow along. In my mind, it's a conceptually simple problem (for a German equatorial mount [GEM]): Imagine you...
  19. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    The GM-8's worm rotates once every 8 minutes, so I only have to stay hunched-over-motionless-staring_at_a_bright_dot for 8 minutes, not pleasant but not terrible. :P Go outside and enjoy nature! All the critters come out to play :)
  20. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    FWIW, I've played around with putting a transmission grating at the aperture stop, that gives nice spectra at the image plane: Here is Jupiter and Spica, an image I took a few years ago-
  21. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Long post follows- sorry! I have a really hard time visualizing what is happening when my mount is not perfectly polar aligned (know any good URLs that illustrate this?). In my mind, the idealized heavenly sphere (apparently) rotates only in RA and polar alignment error introduces slow RA...
  22. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    No. I'm lazy- my conditions for a PEC run are (1) imaging @ 800mm, and (2) weather warm enough so I'm not freezing my giblets :) I typically PEC train in Fall and Spring. Yes- I meant that PEC only compensates for the RA worm drive. Polar alignment (meaning the manual pointing of the mount)...
  23. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Thanks! I don't have a pier. My understanding is that PEC (rather, the error itself) arises from mechanical (machining) errors in the worm gear, and so PEC acts on the errors inherent to the mount, as opposed to polar alignment error (which does vary from night to night). I'm not sure about...
  24. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    I empathize! Even with my simple setup, it seems there are several dozen things that can go wrong on any given night.....
  25. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Well, it's been about 3 straight weeks of 100% overcast skies, day and night.... there were only a few hours here and there Here's what I have for Neptune, in October it was at the bottom, moving upwards until early December when Neptune switched back to prograde motion. I have been able to...
  26. Andy Resnick

    Plagiarism and ChatGPT

    A few department colleagues and I starting talking about this exact issue last semester; we don't have a definitive response yet but are thinking along the lines of treating AI-generated text by students similarly to how the student use of graphing calculators and symbolic math software has been...
  27. Andy Resnick

    I think I ended up in the wrong Laboratory? (Theoretical Cosmology)

    Just because QFT has found applications in many subfields, it does not follow that all research in those subfields requires expertise in QFT. In addition, while some research activities do require QFT, students can often get involved in those efforts without having prior expertise in QFT: for...
  28. Andy Resnick

    I think I ended up in the wrong Laboratory? (Theoretical Cosmology)

    If this is truly how you feel, then have an honest conversation with your advisor ASAP and together, figure out a better match for your interests. I often have students who, once they get some hands-on experience with my research, realize that they actually prefer other approaches...
  29. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    I only used data points generated when I imaged on consecutive nights (images 24 hours apart) to reduce the ambiguity about when the angular velocity was measured. Yes, I could synthesize more data points since I know the dates I acquired the images, but I have limited time to goof around.
  30. Andy Resnick

    Random Photos

    Went for a night hike a few days ago, shooting 3s exposures @ 105/1.4, handheld. Because it was so dark (heavy overcast, rural area), I could not focus with intention. That, and the amount of camera shake from the long exposure time result in images that (I think) accurately convey the...
  31. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    The weather has been uncooperative lately, so I'm posting this (hopefully) intermediate image: That's a composite image of Neptune, taken from 10/22 (bottom) through 12/4. As you can see, Neptune was moving apparently retrograde and was nearly switching direction by 12/4, here at 200%...
  32. Andy Resnick

    A How to compute phase gradient from Snell's law?

    That's a pretty cool way to use oblique illumination! I need to read these carfully, but I suspect using Snell's law will be an excercise in frustration.
  33. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Thanks! Regarding magnification, 800mm provides about 16X magnification, using the convention that a 50mm lens is matched to the field of view of a naked eye.
  34. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Thanks! Certainly! (thanks for asking first :) )
  35. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    It's been cloudy for a week and there's another week to go, so this is as far as I go for the year with M31 (Andromeda galaxy): M31 just barely fits in the frame @800mm (35mm image format). The image is still a little too noisy, I need another pass or two to get sufficient integration time...
  36. Andy Resnick

    I Plane wave decomposition method in scalar optics

    Scalar diffraction is usually a very good approximation, so that's why it's often used. When vectorial diffraction is required, for example imaging with a high-numerical aperture lens, a variety of polarization effects can be obtained, for example depolarization. Similarly, when calculating...
  37. Andy Resnick

    I How to interpret this new Keck spectrum picture of Jupiter?

    If I had to guess, I would say that the spectrometer design is good enough to measure atmospheric components in the part-per-million (ppm) range.
  38. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    In a strange, inverted, turn of events, I had to wait until there was a break in the clear nights to catch up :) This time of year I can image 4 planets each night- First, Neptune (and Triton) (50% downscaled): Jupiter and 4 Galilean moons (composite, 100%): Mars (100%)- nothing amazing...
  39. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    The moon, Jupiter, and 22° halo: Shot @ 15/4, 30s ISO 64. Image cropped but otherwise straight-from-camera jpg.
  40. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    I have considered a few models a while ago (Orion makes some nice ones, for example) and rejected the idea for a few reasons, for example: 1) Increased complexity. For example, I don't want to set up a computer every time I go image. 2) No obvious way to mount a tracking scope on my setup...
  41. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Problem solved! Before opening up the mount- a potentially risky step- I looked around on other discussion boards (e.g. cloudynights.com) for discussions about similar tracking issues and based on what I found, I realized I still hadn't done all of the basic check-out steps. Specifically, I...
  42. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Thanks for the suggestion- the lens is mounted to a dovetail that mates to the GM8 and by sliding the dovetail back and forth, I achieve DEC balance. Since I have to do this every night I image, adding or removing the tele is always accounted for.
  43. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Another excellent weekend, I had three chances to try out the various suggestions. Unfortunately, I have mixed results. To be fair, I've also noticed that over time, I have been setting more and more stringent requirements on the images I retain for stacking. The two main things I tried this...
  44. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Yes, the entire reticle rotates to permit proper alignment.
  45. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Wow- thanks! There's a lot here for me to think about. To answer your questions: 1) I am using a Losmandy GM-8 mount, a German equatorial mount. 2) Balancing is pretty easy... Ohh... wait a minute- when I switched to 800mm (adding the 2x teleconverter) I didn't adjust the counterweight...
  46. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Thanks for the ideas! Camera is triggered automatically, using the built-in intervalometer and 3-second shutter delay. I polar align using this reticle: https://www.primalucelab.com/media/wysiwyg/montature/Losmandy_cannocchiale_polare_GM8_G11_reticolo.jpg Which does account for the...
  47. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    I'm fairly certain it's not seeing. Fortunately, I didn't wipe the SD card yet, so I was able to get some raw images to show (these have been converted to jpegs for obvious reasons). This is a *typical* bad image- not the worst, but very average- scaled 200%. Right ascension is up-down...
  48. Andy Resnick

    Random Photos

    I read about that lens (minor correction: it's 85/1.4, not 85/1.2), it seems to be a great deal- alternatives cost several times as much. When I was shooting with a Sony Alpha, I used the 85/1.4 lens (Sony FE 85mm f/1.4 GM) and it was a joy to work with. Good lenses cost money, but the...
  49. Andy Resnick

    Our Beautiful Universe - Photos and Videos

    Had a moon-free clear skies weekend- 3 consecutive (relatively) dark nights when I can stay up past midnight! Mostly I imaged M31 (Andromeda galaxy) and am still processing those. But each night when M31 moved behind a house, I was able to image Neptune and Triton for a few minutes- this was...
  50. Andy Resnick

    B Hydrostatic pressure in a bottle of water on ISS

    Yes, and this has a variety of ramifications besides being able to drink out of a bottle of water. It also impacts (fluid) fuel management in tanks, supplying water to the roots of plants, boiling and other multiphase fluid separation, including oil-water systems, and is possibly the reason for...
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