1,656 reputation
615
bio website linkedin.com/in/neilsen
location Illinois
age 44
visits member for 3 years, 9 months
seen 6 hours ago

Astronomer and software developer with interest and expertise in sky surveys, observing strategy and efficiency, data science, open science, and image processing.


Mar
30
comment Andromeda/Milky Way collision: How, and how accurately, can a galaxy's lateral velocity be measured?
The Loeb et al. paper is available on astro-ph: arxiv.org/abs/astro-ph/0506609
Aug
29
comment How much energy does a super nova generate?
No, neutrinos don't interact much... this is why shielding is hopeless. SN produce so many, though, that the minimal interaction may be overwhelmed by sheer numbers. Yes, though, by guess is that other forms of radiation would be more immediate problems; neutrinos are just what I could find the most obviously applicable reference for, as the planet can just be ignored.
Aug
25
comment How much energy does a super nova generate?
Even if the planet could shield you from the light, supernova release an awful lot of energy in neutrinos, and hiding behind a planet would be completely useless. Phil Plait's book Death from the Skies estimates that they would give you cancer out to about 30 light years, with large uncertainties.
Aug
9
comment How to create unusual sundial?
The empirical method is likely to be the most practical (and probably the most fun). If you want a good reference, I suggest Sundials: Their Theory and Construction by Albert Waugh.
Aug
7
comment Why don't we have a better telescope than the Hubble Space Telescope?
It is also important to realize that we have been regularly updating the detectors on the HST, so the HST we have now is vastly superior to the one we had in the 90s.
Jul
27
comment How to identify the objects in an astrophoto, and what portion of the sky it covers?
If your image is undersampled (that is, the full width at half maximum (FWHM) of a star is less than 2.3 pixels or so), you may need to blur your image for some of the object detection routines used in these tools to work properly. Most can handle undersampled images at some level, though.
Jul
27
comment How to identify the objects in an astrophoto, and what portion of the sky it covers?
Most of the things I mentioned are probably better tested at higher resolution that you requested, but I don't expect that it will matter very much: they should be able to handle the lower resolution images just fine. Of more concern is less complete starting metadata, particularly your initial guess about where you are pointing. I doubt if this is a show-stopper, but it will make using these tools on amateur data harder than data from a properly working professional instrument.
Jul
24
comment Maximum depth for bacteria on rogue planets
There is a recent paper (Abbot & Switzer 2011) which presents an estimate that the liquid water oceans can exist below glaciers of order 1 to 100 km thick for such a planet.
Jul
24
comment Where does a star's angular momentum go as its spin slows down?
I believe it is carried away through magnetically driven stellar wind. See Lau et al. 2001, Tout & Pringle 1992, Ud-Doula et al. 2009.
Jul
24
comment Is there evidence of dark matter in our galaxy?
Measuring the dark matter content of our own galaxy is much harder than measuring those we can see from the outside. It is not impossible, though. One relatively recent example is Xue et al 2008 (2008ApJ...684.1143X), who measured the distributions of the line of sight velocities of stars at different galactic radii, and compared these to different galaxy models.
Jul
21
comment Why is the universe so organized?
The short answer to why our solar system lies mostly in a plane, and why some (but far from all) galaxies have a disk shaped component, is "because angular momentum is conserved." I realise this needs more explanation, but I think it deserves its own question. One cannot hope to describe all instances of regularity in one question! Indeed, one might argue that is what the entire field of astrophysics is about...
Jul
17
comment Why is the universe so organized?
Of course, in our solar system, elliptical orbits with very low eccentricities (that is, nearly circular orbits) are more common than very elliptical ones, to the extent that the big stuff (planets) all have roughly circular orbits. Why this is the case would make a good, separate, question.
Jul
17
comment “Blue Bumper” Stars
Can you provide a reference for the MACHO/EROS papers you saw them mentioned in?
Jul
2
comment What are the easiest ways to get data out of a FITS file without a library?
The hexdump recipe I gave works just fine for headers of extended HDUs (I assume that's what you mean by subheaders). You just need to scroll past the data. Note that FITS requirements on the arrangement of the data mean that the header data cards of subsequent headers will line up correctly.
Jun
29
comment Standards for scientific metadata in common-use image formats?
The exceptions I refer to are compression and heaps. I personally find the growing use of the compression features of some libraries before they are properly documented foolishly short-sighted: I want a proper published standard.
Jun
28
comment What are the easiest ways to get data out of a FITS file without a library?
Of course on my own system I use appropriate tools (ds9, fv, TOPCAT, pyfits), but I also want to be able to write a bash script I can hand to someone and reasonably expect to work without any additional effort. Maybe I should just expect that everyone will have a python interpreter, at least.
Jun
27
comment What are the easiest ways to get data out of a FITS file without a library?
The FITS standard specifies big endian. Some formats that used to be used by IRAF were very close to FITS, but were machine dependent. Some raw files that were produced by the SDSS data acquisition system were little endian, and used a "SIMPLE=FALSE" keyword assignment to indicate that they were not purporting to be real FITS.
Jun
27
comment What are the easiest ways to get data out of a FITS file without a library?
Other file types (ASCII tables and images) should be slightly easier. FITS heaps are pretty much impossible without lots of code, and fortunately obscure anyway.
Jun
27
comment What are the easiest ways to get data out of a FITS file without a library?
The sample file I used can be found here.
Jun
18
comment Why don't we have a better telescope than the Hubble Space Telescope?
The size issue is the mirror. JWST's mirror consists of multiple segments, but this is technically challenging.