robin_astro

10 minutes ago, robin_astro said:

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

In which case substitute neutrino for photon in the above 🙂

3 hours ago, andrew s said:

photons don't have a reference frame. If they did they would be a rest in it? 😊

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

26 minutes ago, robin_astro said:

In  who's reference frame ? not the photons 😉

 

 

....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 😉 

3 hours ago, StuartT said:

...although probably a bit earlier back in time than that, in all fairness.. 😉

In  who's reference frame ? not the photons 😉

On 19/07/2022 at 08:00, IB20 said:

Some nice images of Messier 74 yesterday from JWST

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

15 hours ago, Mandy D said:

Thankfully things have changed.

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

21 minutes ago, robin_astro said:

The "recession velocity"  depends on the observer (relativity) and the cosmological model so you can't really talk about a recession velocity.

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

11 minutes ago, robin_astro said:

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

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

14 hours ago, ollypenrice said:

It seems to me that the diagram in James' original post is confusing. Why does the point of focus lie so far inside the tube?

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

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.