Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

  • Announcements

    sgl_imaging_challenge_banner_lunar.thumb.jpg.ef4882eb5fb3610f8a68e5e6913de0e3.jpg

robin_astro

Advanced Members
  • Content count

    406
  • Joined

  • Last visited

Community Reputation

112 Excellent

1 Follower

About robin_astro

  • Rank
    Star Forming

Contact Methods

  • Website URL
    http://www.threehillsobservatory.co.uk

Profile Information

  • Gender
    Not Telling
  1. Hi John, If you are already experienced at using the Star Analyser and are using a high dispersion (low A/pixel) then it helps with resolution a bit. The improvement is pretty marginal with the SA100 (about a 20% improvement in resolution typically) but more useful with the SA200 where it helps to keep the focus constant across the spectrum with the higher diffraction angle. Other variable factors such as seeing, focus have a much greater effect though. One downside is that it makes the wavelength calibration non linear There is a comparison on Christian Buil's site here (tip 3) http://www.astrosurf.com/buil/staranalyser3/userguide.htm Cheers Robin
  2. An early evening target for more northerly observers? Probably the brightest supernova in the sky currently but few images or observations up to now
  3. sn2017gxq was discovered in ngc 4964 by the Gaia satellite on 17th September but not announced until 28th. I classified it as type 1a using spectroscopy on 30th. Details and the spectrum are on the Transient Name Server website https://wis-tns.weizmann.ac.il/object/2017gxq It is currently close to maximum at about mag 14 so is probably the brightest supernova in the sky at the moment but there are no observations or images currently in David Bishop's famous "bright supernovae" website. http://www.rochesterastronomy.org/snimages/ Robin
  4. Yes it is optically the same as eypiece projection. It will not work using a 500 l/mm grating without the eyepiece and camera lenses though. The key is that the eyepiece makes the beam parallel through the grating which is essential if you are using a grating with more than ~200l/mm. Here is a similar arrangement by Christian Buil using camera lenses. The LORIS spectrograph http://www.astrosurf.com/buil/us/loris/loris.htm Robin
  5. For a simple slitless setup, I always fancied trying something like the RS spectrograph, which is similar in concept to the junk box spectrograph. http://www.astroshop.eu/rigel-systems-spektrograf-rs-spectroscope/p,51408 At this price it is silly but it could be done quite cheaply if you already have one of those big wide field eyepieces and a DSLR. It is a fully collimated design and the camera is angled so you can use a high dispersion grating (I think this one uses 500 l/mm) and get good resolution. The only possible drawback with some DSLR eg cannon is that the front ring rotates as you focus. Robin
  6. 11 billion years ago.....

    Hi Steve, The total exposure was 8x600s but the key features in the spectrum are visible in a single 600s exposure. Yes it is a slit spectrograph which isolates a narrow (23um) strip of sky including the target, described here http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm You still have to isolate the wanted star from any others which might be trapped in the slit and remove the spectrum of the sky background which includes light pollution and natural air glow. I use Christian Buil's ISIS software to do all the data processing http://www.astrosurf.com/buil/isis/isis_en.htm You can see the raw spectrum image with and without the sky background subtracted below. (The slit can be seen on the left and the QSO is the bottom of the three spectra) This was just a bit of fun really. The main use of this setup is to confirm and classify supernovae down to ~mag 17. This is my latest classification https://wis-tns.weizmann.ac.il/object/2017gjn Cheers Robin
  7. Yep that's amateur spectroscopy pioneer Maurice Gavin. He's on here from time to time. He made a lot of innovative unusual designs and observations and is one of the people who helped me when I picked up spectroscopy. Yes you can use a Barlow to collimate the beam from the telescope ahead of a prism (in this case an Amici prism which keeps everything in line) or grating and then use a camera lens to focus the spectrum. Similar to the junk box spectrograph but using a concave barlow lens as a collimator instead of an eyepiece. http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm The Barlow does not produce a secondary image ahead of the grating though so you cannot add a slit with this design. Cheers Robin
  8. If fact I see the paper I just referenced here does this https://arxiv.org/pdf/1707.05873.pdf (fig 2) so I am in good company Cheers Robin
  9. You cannot form a decent spectrum at all with a prism in the converging beam, the aberrations are just too severe. You need a parallel beam and the camera at an angle. You can also angle the camera with a grating to minimise the effect, or use a shallow angle wedge prism to deflect the beam back on axis but once you go beyond ~200l/mm various other aberrations rear their head and you really need collimator to give a parallel beam and an objective to refocus on the detector to get a decent spectrum.
  10. Ah, this is astronomy though. No SI unit police there. Plenty of Angstroms still around even in the latest literature, and the flux Y axis is often given in erg/cm2/sec/Angstrom. Mind you that's nothing compared with some of the units we had in the paper industry Robin
  11. Hi Steve, Lose the Barlow. It is only making the star image bigger and losing you resolution The Star Analyser etc are only 100-200l/mm for a reason. At 500l/mm the angle and curvature of the spectrum is so large where it hits the camera sensor that there is a big change in focus from one end of the spectrum to the other. (There are other aberrations in the converging beam arrangement that make lower resolution gratings at larger distances work better too. ) If you want to try building a "Star Analyser on the cheap" you could go for the Paton Hawksley school 100 l/mm grating that I developed the Star Analyser from. http://www.patonhawksley.co.uk/transmissiongratings.html (TE218) http://www.threehillsobservatory.co.uk/astro/spectroscopy_jeulin.htm It is not quite as efficient as the SA100 and can vary in quality as the QC is not as strict but will perform much better than the grating you have. (Note it is on glass so cannot be cut like the cheap plastic film gratings so you will need to make a mount for it. This is what I used for a web/video cam. http://www.threehillsobservatory.co.uk/astro/spectroscopy_3.htm Aim for a larger spacing than this though, giving around 10A/pixel as a starting point. Use the PH calculator as a guide http://www.patonhawksley.co.uk/calculator/ Robin
  12. Thanks for the "heads up" on this. I pointed my low resolution spectrograph at it last night. The spectrum confirms the z=2.4 redshift https://stargazerslounge.com/topic/300597-11-billion-years-ago/ Robin
  13. ...... a few photons set out from the recently discovered quadruple gravitationally lensed quasar J014709+463037 and ended their journey kicking out electrons in a chip of silicon attached to my C11/ALPY200 spectrograph. Here is the spectrum with an overlay of the confirming spectrum from the Keck telescope published here https://arxiv.org/abs/1707.05873 The broad UV emission lines from Lyman alpha, Si IV and C IV have been shifted at z =2.4 into the visible. Robin
  14. Hi Billy, I do not do photometry regularly but I know some of the basics. How did you chose your reference stars, where did you find their brightnesses and how are the values described (eg which filter) ? Normally these are chosen from charts produced specifically for the star you are trying to measure eg BAA AAVSO etc where the reference stars have been checked for variability, the brightness measured to high accuracy and chosen to be similar in brightness and colour to the target, particularly important for unfiltered measurements, which can be very different from V mag values depending on the colour of the star. Note 0.01mag absolute accuracy in magnitude (~1%) is tough to achieve. Are you sure the 0.01 figure quoted for DSLR was not precision or uncertainty ie how repeatable the measurement is, rather than the absolute accuracy ? Cheers Robin
  15. Hi Vlaiv, If you have not come across them already (thanks for the hat tip Andrew :-) ), here are my two configurations using the Star Analyser which can incorporate a slit : SEPSA. This keeps the converging beam but adds a focal plane to place the slit at http://www.threehillsobservatory.co.uk/astro/spectroscopy_18.htm Fully collimated "junk box"design (without slit though one could be added http://www.threehillsobservatory.co.uk/astro/spectroscopy_19.htm You might also be interested in this grism design (note the mirror slit guider which (almost) all commercial slit spectrographs use thse days) http://www.burwitz-astro.info/spectrographs/tragos/ and my modification of the similar Shelyak ALPY spectrograph using a 200 l/mm grism http://www.threehillsobservatory.co.uk/astro/spectroscopy_20.htm As Andrew said though, adding a slit is the easy part, finding the target, focusing it and guiding it on the slit are the hard parts. With bright targets though you can get away with defocusing the target into a large blob so it is impossible to miss the slit. (See my Vega spectrum using the SEPSA for example) Cheers Robin
×