robin_astro

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

8 minutes ago, robin_astro said:

 (Photo electric photometry (PEP) perhaps, though I know nothing about it)

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

9 minutes ago, robin_astro said:

Here is H beta in Vega for example

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)

5 hours ago, Graham Beamson said:

Here are my Altair, Deneb and Vega spectra

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

4 hours ago, SteveBz said:

Everyone says that guiding is less sensitive to sampling than image quality, so you might improve the resolution of your spectra if you switched round X2 for guiding and 178 for spectra.  But I'm not even off the ramps yet!!! What does @robin_astro think?

 

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

4 hours ago, Graham Beamson said:

By the way, I've got a lot of dust on the reflective slit that won't come off with a blow duster. As the coating is on the back is it safe to gently wash the front surface with a lens cleaning cloth and lens cleaning solvent ?

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.

3 hours ago, Graham Beamson said:

I can see a star get bigger as I go away from focus, but going the other way the star gets fainter and disappears. Also, at the best focus I can get the shape of the star is very strange - there is lots of flaring so that it looks a bit like a comet.

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

13 hours ago, andrew s said:

Having given a talk at a joint BAA/AAVSO meeting I often get invited to publish in such journals.

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 😀

On 16/12/2020 at 22:13, vlaiv said:

Reminded me of Ronja

http://ronja.twibright.com/tetrapolis/spec.php

It uses lens for transmission part:

http://images.twibright.com/tns/lvl3/1409.jpg

I guess small mak would have much better performance than these Chinese magnifying glasses . Still interesting project for anyone interested.

 

 

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.

11 hours ago, pipnina said:

 

For UV I have a harder time guessing. I think our CMOS sensors are typically only sensitive about 50-100NM below the visible spectrum on that end because there is a harsher dropoff.

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

20 minutes ago, StarryEyed said:

The British Astronomical Association have merged their variable data with the American Association Variable Star Observers

The variable star brightness data submitted to the BAA is still held independently in their database but that data is then also periodically uploaded to the AAVSO database. 

Robin

T200 is the telescope (0.2m aperture).  Rotation velocities are quite high so you don't need particularly high resolution. An f4.5 Newtonian is a good choice with the ALPY.  A much better match for an ALPY than the slow etx90. 114mm aperture is rather small  for  objects as faint as this though and exposures are long even with bigger apertures (2 hours in 20min exposures say) so you will need a good mount with good guiding

For exactly edge on galaxies the conversion to a rotation curve  is not  straightforward though as at any given location, the spectrum is potentially a combination of regions at different radii, each seen with a different component velocity in our direction. 

Hi Steve,

There are several examples of this measurement by amateurs on the ARAS forum. The observers might still have the spectra. Otherwise perhaps contact the authors of papers where rotation curves measured using optical spectroscopy have been published ? (I think most of them use radio astronomy so you can measure further out than you can do at visible wavelengths)

https://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2618

https://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2232

http://www.astrosurf.com/buil/UVEX4/_demo_m82.jpg

https://www.spectro-aras.com/forum/viewtopic.php?f=6&t=2420

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=1682

http://www.astrosurf.com/buil/forum/ngc7331_poster.png

Cheers

Robin

S&T  picked up on our black hole being "on its side" so I did hear right. Apparently this is not unusual with supermassive black holes in the centre of galaxies, with low accretion rates like ours having a wide range of orientations, assumed to be the result of their formation from merged smaller black holes. (Any with jets in the galactic plane must be interesting places to live !)

https://skyandtelescope.org/astronomy-news/astronomers-unveil-image-of-the-milky-ways-central-black-hole/?utm_source=cc&utm_medium=newsletter

Cheers 

Robin

19 hours ago, saac said:

So the image may be such that we are not looking directly at it in the same plane of rotation as the excretion disc. 

I was going by  what was  said though rather than the appearance in the image. There was  also a comment on the jet (if there is one) being difficult to detect as, unlike the one in M87 this would be in the plane of our galaxy.  Anyone got a link to the original press conference? I cant find it at the original address

Robin

So.. from the press conference I picked up that we see the spinning black hole ~face on (even though we live ~ in the plane of the milky way) and the spin is in the opposite direction to the orbiting  accreting material.  This seems quite surprising to me. I perhaps naively expected everything to be in the same plane

Yes definitely worth a look with the Star Analyser if you have a deep sky capable setup. The broad Silicon absorption line confirming it as a type 1a is very clear

https://stargazerslounge.com/topic/394071-supernova-in-ngc-4647/?do=findComment&comment=4238399

Cheers

Robin

If you pop a diffraction grating in front of the camera you can even measure the red shift as here in Patrick Moore's back garden back in 2005 using a prototype of the Star Analyser (bottom of the page)

http://www.threehillsobservatory.co.uk/astro/spectra_12.htm

Cheers

Robin

EDIT: You would have to hack the Stellina though. Anyone doing that yet ?