Light polluted city - which spectroscope?

Design, construction, tuning of spectroscopes
Information and discussion about softwares (telescope remote, autoguiding, acquisition, spectral processing ...)

Light polluted city - which spectroscope?

Postby Ken Hudson » Tue Apr 10, 2018 2:16 pm

Hi All,

I'm curious, which is "better" or more useful in a light polluted city: an Alpy or a Lhires?

Thanks,

Ken
Ken Hudson
 
Posts: 6
Joined: Thu Oct 06, 2011 4:02 pm

Re: Light polluted city - which spectroscope?

Postby Olivier GARDE » Tue Apr 10, 2018 3:58 pm

Hi ken,

From my point of view, you can use both, because in any case during the process, you subtract background sky and therefore you will remove light pollution ...

Valerie use a LHIRES III in the center of Paris and I know some people using an Alpy close to a big city.

Olivier
LHIRES III #5, LISA, e-Shel, C14, RC400 Astrosib, AP1600
http://o.garde.free.fr/astro/Spectro1/Bienvenue.html
Olivier GARDE
 
Posts: 878
Joined: Thu Sep 29, 2011 6:35 am
Location: Rhône Alpes FRANCE

Re: Light polluted city - which spectroscope?

Postby etienne bertrand » Tue Apr 10, 2018 8:38 pm

Alpy low resolution and Lhires High resolution 2400 ? I think you can use the both.
I use the both in a little city, i have sodium floor lamp many mètres next to it hidden by a wall of my telescope and i can use Alpy for catch QSO m~14 whith good SNR but it's possible to catch QSO m~15-16 (Exposure with Atik414EX 1800s or 2400s bin1x1 on C8 f/6.3)
In the night around 1 am the fuck*** floor lamp close the light and it's very very better !!!
With the lhires i work around Ha and i haven't pollution raies with 2400tt, but the image is a little bright. When the floor lamp close at 1h, it's perfect ! the images are very black !

I think that Christian use Alpy in south of Toulouse in sodium city with Atik460EX bin2x2 and 600s and exellents results.
etienne bertrand
 
Posts: 689
Joined: Mon Aug 31, 2015 11:26 am

Re: Light polluted city - which spectroscope?

Postby Robin Leadbeater » Tue Apr 10, 2018 11:06 pm

Hi Ken,

It is more to do with the resolution you are working at and how faint you want to go rather than the spectrograph. Light pollution is less of a problem at high resolution because your targets are necessarily brighter so the effect of light pollution is less. (ie the difference between skies with low and high light pollution when using the LHIRES at high resolution will be much less than when using low resolution on faint targets with an ALPY). Compared with deep sky imaging, slit spectrographs in general are very forgiving in terms of light pollution though as the slit keeps out most of the sky background, and what is there can be subtracted. (The sky background will still add some noise and at low resolution ultimately will limit how faint you can go so for example if you want to measure mag 17.5 supernovae as I have been doing recently with my modified ALPY, you will still need dark skies)

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1117
Joined: Mon Sep 26, 2011 4:41 pm

Re: Light polluted city - which spectroscope?

Postby Robin Leadbeater » Tue Apr 10, 2018 11:21 pm

Valerie's observatory in the centre of Paris
http://valerie.desnoux.free.fr/paris%20 ... bs_en.html
Christian's observatory near toulouse
http://www.astrosurf.com/buil/pollution/measure.htm
http://www.astrosurf.com/buil/nova_sgr2015_2/obs.htm
mag 17.5 supernovae at my rural location
http://www.threehillsobservatory.co.uk/ ... tra_46.htm
epsilon aurigae at high resolution in very bright twilight
http://www.threehillsobservatory.co.uk/ ... tra_40.htm
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1117
Joined: Mon Sep 26, 2011 4:41 pm

Re: Light polluted city - which spectroscope?

Postby Peter Somogyi » Wed Apr 11, 2018 5:55 am

Something previous comments did not mention, the focal ratio.
I've used both Alpy and LHiresIII (+ 150/mm grating) at low resolution - latter recently at sunrise, also an important factor - many times necessary to shoot in the sunlight till meaningful. With the LHires, background is much less of a problem (anyway Alpy is my default low res choice - in many aspects it's the better -, just the LHires grating is switchable relative quickly).
Alpy has in my system f/4, and LHires f/10 scope focal ratio.
LHiresIII is taking up much less background light, and the main reason I think it's because of f/10.
Of course, at highest resolution (2400/mm grating) AND f/10 you will never have any pollution problem (except when street lamps reflecting directly into the scope tube).

- Peter
Peter Somogyi
 
Posts: 350
Joined: Sun Jul 13, 2014 8:56 am

Re: Light polluted city - which spectroscope?

Postby Robin Leadbeater » Wed Apr 11, 2018 5:51 pm

Hi Peter,
Peter Somogyi wrote:Alpy has in my system f/4, and LHires f/10 scope focal ratio.
LHiresIII is taking up much less background light, and the main reason I think it's because of f/10.


Yes I wondered about this effect, but are the background counts per resolution interval still lower for the LHIRES if you compensate for the increased size of the star image ? (The slit should be 2.5x wider and the binning zone 2.5x higher at f10 compared with f4)

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1117
Joined: Mon Sep 26, 2011 4:41 pm

Re: Light polluted city - which spectroscope?

Postby Peter Somogyi » Wed Apr 11, 2018 7:43 pm

Hi Robin,

You are true, compensating the slit width for seeing disc size is complicating my statement (in fact, I've never used larger than 35 micron slit, and practice dictating not to loose any more resolution...). You are true in this aspect, Alpy is a bit overtargeting the slit size, causing generally higher background just because the fixed slit size.
However - if I haven't missed anything -, increasing slit width (to match seeing disc) effect is linear for background, but changing f-ratio (in other words: magnification) effect should be in _square_ relation for the background to match the same seeing disc size / slit width ratio.

Cheers,
Peter
Peter Somogyi
 
Posts: 350
Joined: Sun Jul 13, 2014 8:56 am

Re: Light polluted city - which spectroscope?

Postby Robin Leadbeater » Wed Apr 11, 2018 9:57 pm

Hi Peter,

I think the sky background included in the measurement also squares with focal ratio because you must also bin 2.5x the number of rows to include all the light from the star.(Or for a fibre spectrograph the diameter of the fibre has to be 2.5x larger) In the case of going from f4 to f10, the background per pixel reduces 6.25x but area of the star image is 6.25x larger.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
 
Posts: 1117
Joined: Mon Sep 26, 2011 4:41 pm

Re: Light polluted city - which spectroscope?

Postby Peter Somogyi » Thu Apr 12, 2018 5:48 am

Hi Robin,

You are true, I've forgot that the star image also scales, so there should be no difference about f/4 vs f/10, in the case we were using the right slit width.

Now I'd rather think, the relative narrower slit (max 35 micron) we are often using with the LHires provides a higher star signal ratio, whilst with the Alpy we never change slit, and that's typically left larger - and getting less real signal (increasing slit width scales the signal far less than linear - as the average star shape is not a cube in 3D).

Cheers,
Peter
Peter Somogyi
 
Posts: 350
Joined: Sun Jul 13, 2014 8:56 am

Next

Return to Spectroscopes and Softwares

Who is online

Users browsing this forum: No registered users and 5 guests

cron

x