robin_astro

Very true. They have no concept of time, a bit like me these days 😀

....and any case, since the (apparent) magnitude is a measurement of the photon flux at the earth, the brightness did peak at ~mag 12 in August 2013 😉

In who's reference frame ? not the photons 😉

They have spotted in that image what is left of SN 2013ej , a supernova which reached a bright mag 12 back in August 2013 https://www.wis-tns.org/astronotes/astronote/2022-147 Robin

Now there's an interesting challenge, to observe an occultation of a star by the ISS. I wonder how narrow the path would be ? I did catch a Jupiter transit quite a while back though when the data to be able to predict this sort of thing were not so widely available (and the ISS was quite a bit smaller) http://www.threehillsobservatory.co.uk/astro/astro2_image_67.htm Cheers Robin

Amen to that, though there is still some way to go in Physics. When I started my degree in 1969 there were just three women in my year (~5%). That has improved to ~25% now I understand Cheers Robin

If you want to dig further down this rabbit hole then you could visit NED Wright's cosmology FAQ https://www.astro.ucla.edu/~wright/cosmology_faq.html Cheers Robin

Extending this to more familiar wavelengths, at redshift 6 H alpha is at 7x656.3nm or 4.6um (JWST can see down to 28.3 um so theoretically it could see H alpha at 42 redshift or 60 million years after the big bang !) Robin

It doesn't in most "real" refractors (well it might do if you rack the focuser fully out.) If it did prime focus astrophotography would not be possible. Cheers Robin

The "recession velocity" depends on the observer (relativity) and the cosmological model so you can't really talk about a recession velocity. Using the currently accepted parameters for the universe, objects from 1 billion years after the big bang will show a cosmological redshift ~6. You can use Ned Wrights cosmology calculator to play with these figures. https://www.astro.ucla.edu/~wright/CosmoCalc.html The Hydrogen Lyman alpha line, at 121.6 nm in the UV at rest is commonly used to measure objects at high redshifts. This will therefore be at 7x121.6 nm or 0.85um so at the short wavelength end of the JWST spectrum (it is even in the passband of amateur CMOS/CCD detectors.) Here for example are some of my spectra showing Lyman alpha of objects at 4.5 redshift, shifted in to the red regiom of the visible spectrum (~1.3 billion years after the big bang) https://britastro.org/observations/observation.php?id=20210411_134753_85f4b3ebf4faaefe Cheers Robin

They are the same thing. You remove the eyepiece and focus the moon on the card. For objects at infinity (like the moon) the plane where the image is in focus is the principle focus. (rack the focuser all the way in so the focal plane is outside the tube) Cheers Robin

If that is the case then PEP photometrists are pretty rare animals these days. The AAVSO PEP section would probably be your best bet for advice https://www.aavso.org/aavso-photoelectric-photometry-pep-section Cheers Robin

What sort of photometry are you doing that requires just keeping the star within a 1-2 arcmin box ? (Photo electric photometry (PEP) perhaps, though I know nothing about it). Most photometry these days is done by measuring from images where the star image has to kept be moderately tight and round eg within say a few arcsec at most during the exposure. (Perhaps best moved to the variable stars subforum ?) Cheers Robin

Contrast that with H Beta in supergiant star Rigel for example. Although similar temperature to Vega and more massive, it is much larger so the surface gravity (which reduces as the square of the radius) is much lower so the lines (which are produced in the photosphere at the surface of the star) are less pressure broadened)

The FWHM looks about right. The resolution of the ALPY600 is ~12A and the lines in giant stars like Deneb are narrow enough to estimate the resolution but the Balmer lines of main sequence stars like Altair/Vega are intrinsically wider than this so don't represent the true resolution of the spectrograph. Here is H beta in Vega for example

I don't know the relative sensitivity/noise levels of these two cameras but pixel size-wise either way round should be ok with the the ALPY and the standard 23um slit. (To avoid undersampling of the spectrum you need to have at least 2 pixels per slit width). If you were to move to a narrower eg 10 or 15um slit though the Lodestar X2 used as a main camera would give an undersampled spectrum so should be avoided. Cheers Robin

Yes you can clean it no problem, the same way you would clean a lens or eyepiece etc

Another affect you can see with mirror slit guiders like the ALPY if the star is over exposed in the guide camera is a faint out of focus ghost image of the star next to the main image due to internal reflections in the mirror (Unlike usual astronomical mirrors is is silvered on the back, deliberately to prevent this reflected light entering the spectrograph) This is not a problem as it is very faint when the star is correctly exposed and does not prevent guiding on the overspill of the main star image on the slit.

I assume this is with the star away from the slit ? With the star on the slit the star will get fainter (and can even disappear, needing an increase in exposure to see the overspill ) as the star comes to focus and more of the light goes through the slit. When in focus and centred on the slit the star image will be split by the slit and look like a "burger"

Hi Graham, Regarding (2) The procedure is to first adjust the guider system to produce a sharp image of the slit. The star should then be brought to focus in the guider. The ALPY 600 guider image shows a lot of coma off axis, particularly with fast telescopes but the star image should be round and tight around the central region where the slit is. Also, although the focus of the guide camera can be adjusted, for best image quality the camera should also be at the correct focal plane (It is optimised for 17.5mm C mount back focus so best used with a 5mm spacer with 12.5mm back focus cameras for example). Check that the core module is fully inserted into the guider module. (It should butt up positively against a shoulder) and tighten the 6 screws evenly and progressively. Is the core module rotated correctly relative to the guider module? You need to consider both the orientation of the core module and the guide camera. The image of the slit can look square in the guide camera but one misalignment can be compensating for the other. (The clue if this the the problem is that the guide camera is not square to spectrograph when the slit is square in the image) The instruction manual describes the right orientation. If you still have a problem feel free to contact me off list. Cheers Robin

As an interesting twist on this I have recently been getting spam invitations to joint the "editorial panel" of one of these, presumably in some attempt to give them an increased air of credibility, though they must be getting desperate if they think approaching me would give them that 😀

Reminded me of a project "Light Beam Communicator" in "Electronic Novelties for the Constructor" by EM Bradley (which predates the invention of the laser), one of the first books on electronics I had as a lad in the early 1960's

Have done this twice on their cheapest cameras. One was a bargain (a discounted run out model which slipped past customs with no fees resulting in a good saving over buying locally) The other was also a saving on paper but worked out almost exactly the same as the local price once the customs and courier took their cut so probably wont do it again, certainly not on a big ticket camera. Cheers Robin

Sounds about right. They are still calibrating the instruments https://blogs.nasa.gov/webb/2022/05/12/seventeen-modes-to-discovery-webbs-final-commissioning-activities/ so except for some test calibration images eg https://blogs.nasa.gov/webb/2022/05/09/miris-sharper-view-hints-at-new-possibilities-for-science/ they haven't actually taken any science images yet but I doubt there will be much delay once they have some interesting images to show.

The UV end sensitivity limit can depend largely on the rest of the optics, particularly the antireflective coatings that are designed for visible wavelengths. If you pick the right camera sensor and optics though it is possible to measure down to at least 320nm with the ultimate cutoff at ~300nm due to the ozone layer. See this spectrum by Christian Buil and a UVEX spectrograph for example http://www.astrosurf.com/buil/UVEX_project_us/images/image-collee-810.jpg At the IR end an unfiltered camera can potentially reach to ~1200nm (with holes due to absorption of oxygen and water vapour) but with colour cameras the spectrum tends to look white beyond ~750nm as there all three colour filters become transparent, like this "rainbow" of a cool star at the top taken by Christian Buil and an unfiltered DSLR for example http://www.astrosurf.com/buil/staranalyser/wr5_2.jpg When photographing the full spectrum of a rainbow though I suspect the limiting factor will be the low contrast between the rainbow relative to the sky background as the sensitivity of the camera drops off in the UV and IR. I too would be interested to see an example ! (Rainbows taken with UV and IR pass filters could be interesting) Cheers Robin