Kagoshima
member
Reged: 08/26/07
Posts: 24
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Hi All,
I have a little Vixen 110mm Cat on a Skypod and was wondering if anyone else also has one of these setups and is doing any photometry? I would like to eventually try this and to tell the truth I have no idea how it is actually done with a camera. Do you have to have a special camera? I have seen a used SX-7MC with STAR for $300 used. Is that adequate? I have read that the camera has to have a certain filter frequency? Also, how do you know what exposure time to use? Do you need a computer next to the telescope to download the data immediately? I am not at all electronically inclined but am interested in learning about this area of astronomy. I love observing but find I am interested in wanting to discover something and contribute to astronomy scientifically. I have read some of the AAVSO handbook about photometry but still find it confusing. 
Thanks for any help,
Chris
PS: Does anyone do any photometry in Toronto?
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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“Do you have to have a special camera?”
Yes and no. To do precision photometry a monochrome camera is preferred as it give he sharpest image, can be used with a photometric filter, and has a linear response. However, you can do basic photometry with just about any camera. Which camera you use will affect the details of how you go about it and how accurate your results are.
“I have seen a used SX-7MC with STAR for $300 used. Is that adequate?”
Hmmm, I couldn’t find any information this camera. What type of camera is this?
“I have read that the camera has to have a certain filter frequency?”
To do precision photometry you need to use one of the photometric filters such as a V (visible) filter. This is a way of getting your camera’s response to match that of the photometer used to measure the brightness of the reference stars.
“Also, how do you know what exposure time to use?”
It depends on the star. To do precision work you want the star’s image to be bright enough to easily measure, but not o bright it saturates the camera so the exposure you use depends on the star you’re measuring. It’s a bit of guesswork at the beginning, but you quickly figure out what exposures to use.
“Do you need a computer next to the telescope to download the data immediately?”
It depends on the type of camera you’re using and what you’re trying to accomplish. Precision work is generally done with a monochrome CCD which will require a computer to capture and process the images. I always spend my evening capturing source images in real-time and getting rough estimates of magnitudes manually off of the displayed image, I’ll get the ‘real’ numbers later.
Using a camera to observe variable stars is very effective and a lot of fun. How you go about it depends on the type of camera you have and what you’re trying to accomplish. You can use just about any camera capable of capturing images of the variable star and nearby reference stars. The pictures will provide a wonderful record of your observations and you should be able to see how the star varies in brightness. The easiest way to estimate the star brightness is just to use the visual method described on the AAVSO web site, observing manuals, and various observing guides. Things get a little more interesting if you want to do precision work our to use software to actually take measurements off of your images and for that you really need a monochrome camera fitted with a photometric filter. However, for starters if you already have a camera there’s absolutely no reason not to take some test images and to give it a try. That’s exactly how I started and it was a lot of fun. It’s only when I wanted to take ‘real’ photometric data that I could report to the AAVSO and have it compared t data from other observers did I get serious and learned how to take data using the methods described in the AAVSO handbooks.
Have fun! After all, that's kinda the point. 
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-John
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Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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RLTYS
Carpal Tunnel
Reged: 12/18/04
Posts: 2327
Loc: New York (Long Island)
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Kagoshima
Welcome to the Cloudy Nights Variable Star Forum. I'm sure someone on this forum should be able to help you.
Good luck and clear skies.
Rich (RLTYS)
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10" F4.8 Refl.
4" F5 Refr. (Genesis)
3" F4 Celestron FirstScope
50mm F12 Refr. (Tasco #6TE-5)
12x63 and 10x50 Binoculars.
"I want to do more then just look."
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3523
Loc: Doylestown, PA
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Chris,
John gave very good answers to your questions and I'll second his suggestion of starting out doing variable estimates visually. This will allow you to contribute to astronomy scientifically with your current telescope and accessories. Then you can do more research on imaging photometry and what is required. There is a fairly big learning curve for any imaging and photometry adds another step to this learning. Please don't get discouraged but persist and you will accomplish. I've done a bit of CCD photometry but do enjoy estimating variables visually.
I couldn't find anything about a "SX-7MC with STAR" camera. I though it may be an older Starlight Express camera but can't confirm this (do you have a link to that camera?). John is using the DSI cameras with good success and there are many cameras that will do nicely for photometry. There are many older SBIG parallel port cameras available used that work well. Post here about any camera you have questions about and hopefully we can help.
A should be able to find an older Monochrome DSI camera fairly inexpensive they do work.
If you have any additional questions, please ask.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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Walt; and interesting variation on the imaging technique is to use the visual method, but with the image on my computer instead of at the scope. I'll usually sit at my computer and look at the image with a magnifying glass as the images stack on my screen (my eyes are awful and getting worse) and I've found I can usually peg the magntitude pretty well using the AAVSO comparison charts 'live'. I can't wait until spring so I can actually get outside at the scope and give this a try.
Fun stuff.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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Kagoshima
member
Reged: 08/26/07
Posts: 24
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Hi John, Walt and Rich,
Thanks for the information. I will try the visual way first. Maybe it is best to draw the starfield if I can. I too up in Toronto am waiting for spring. The skies are clear in winter but usually unbearably cold except for a few quick jaunts. I hope to see Comet Lulin before a storm reaches us Thursday evening.
Thanks again,
Chris
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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Chris, drawing the field is a great idea. Way back when I used to observe asteroids I would do that and I found it's a great way to really look at a field and record what you see. The neat thing about variables is they don't move, so you should quickly become familiar with the surrounding field and be able to note the variable's changing brightness.
Have fun!
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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btieman
professor emeritus
Reged: 07/24/08
Posts: 567
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Chris,
I'll add my vote to a DSI camera for starters. A new DSI II is ~$300. There were a lot of these systems sold so you can often find good deals on them used.
My advice would be if you do get a CCD learn how to use the camera first. Don't try for variables right away--your images will be a bunch of dots and may be difficult to identify the field you're looking at--the camera will likely catch many more stars than you're used to, the field of view will take some getting used to as will orientation. Take some images of extended objects you easily recognize. A common problem is people try and image M31 but claim they can't. Usually turns out they have very nice images of M31's *CORE* and nothing else due to field of view on a small CCD sensor. Once you get a little familiar with working the camera, finding and imaging variables will be easier.
Brian
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CPC 1100
Orion 120st
Milburn Wedge
QSI 520i
Meade DSI Pro
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3199
Loc: 55.215N 6.554W
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Quote:
interesting variation on the imaging technique is to use the visual method, but with the image on my computer instead of at the scope. I'll usually sit at my computer and look at the image with a magnifying glass as the images stack on my screen (my eyes are awful and getting worse) and I've found I can usually peg the magntitude pretty well using the AAVSO comparison charts 'live'.
Fun stuff.
Sure, but don't forget that the camera spectral response is different to the eye's, so the magnitudes you get that way aren't strictly comparable with visual estimates. You should be able to get consistency to the order of 0.1 mags working this way so it is a good way of for example timing the minimum of an eclipsing binary.
Using the photometric tools in e.g. AIP or Astro Art you can (with care and suitable exposures) get results consistent to around 0.02 mags and obtain error bars.
I've just started doing a sequence on Epsilon Aurigae prior to the eclipse which is due later in the year - using an ordinary DSLR with a 50mm lens on a static tripod; 1 sec exposures are enough for an object that bright! With 2 min unguided exposures (on an Astrotrac mount) and a 300mm f/4 lens a DSLR can do reasonably accurate differential photometry down to about mag. 13.
A set of good "flats" is essential.
I've no idea where you'd get UBVRI filter sets from, in any case the passband would need to be tailored to the exact spectral response of the CCD chip to get accurate results if the aim is absolute as opposed to differential photometry.
Still, with most variables, one is concerned with detecting variations; the fact that a rise of 0.5 mags occurred is of much more interest than the exact magnitude before (or after) the event.
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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It's interesting to hear about the different experiences observers have had. Just for yucks I started using both my DSI and DSI Pro (after seeing the test image that Brian posted, thanks Brian!) and found I could get fairly good results with either camera, but I quickly migrated over to my DSI Pro I & II. The data I was getting was very good, but for some stars there was a bit of an offset that I understand now was due to the color of the reference and variable stars. As soon as I started using a real V photometric filter (thanks Walt!) my data cleaned up beautifully and fell right into place. I'm in the very early stages of collecting data on at least one fast eclipsing variable (UX Uma) and that should give me a great opportunity to get an accurate measure of the variance in my data, but it looks like I'm getting numbers that are on the order of 0.01 magnitude, but I'm sure the real variance will sneak up just a bit once I get some good data to do an analysis with. With UX Uma I'm set up to average a small set of about 10x20sec exposures every 10 minutes for up to about 4 hours (from the time UX Uma rises over the trees until it reaches the meridian).
This stuff is sooo neat.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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Steven Aggas
sage
Reged: 04/15/08
Posts: 474
Loc: Arizona
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Would a SBIG Deep Space Spectrograph (DSS-7) coupled to a ST-7XME camera and using the Zeroeth Order image work for variable star brightness (shown here in Figure 6)?
Is the Zeroeth Order image of sufficient quality? If so, then I could also get the variable's spectrum following its magnitude estimate, sort of a two-fer.
Thanks, Steven
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Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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That would be a neat trick. However, the same rules apply to the 0th-order image as a straight CCD image; to get accurate photometry it will need a photometric filter in the light path. The challenge of getting a spectra will be sensitivity; once you've spread that light out to get the spectrum the intensity will drop way off. You can pick up a bit by removing the photometric filter when taking the spectra.
This sounds like a really neat idea. A relatively inexpensive way to find out how well it will work would be to use a transmission grating (like the one from Rainbow Optics) right in front of the CCD, this will give a sustem with a relatively low disersion that should give relatively bright spectra and the 0th order image in the same frame. I tinkered with using a cheapy holographic film grating to take spectra of bright stars and I was impressed with how well it worked. I'd think a 'real' grating would work much, much better.
This is an example of a 50x0.1sec image of Sirius taken with my original DSI on my LXD75 SN6.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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Steven Aggas
sage
Reged: 04/15/08
Posts: 474
Loc: Arizona
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So the system I proposed may work if it had a filter wheel to bring the V filter (or others) in for photometry and out for the spectrum with focus adjustments as needed, as long as the 0th is of sufficient quality?
It sounds like we need someone who has a grating to produce the Zeroeth Order image and verify that it is usable for estimating variable star brightness. Would anyone be willing to test that?
Steven
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Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3199
Loc: 55.215N 6.554W
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Quote:
to get accurate photometry it will need a photometric filter in the light path.
Possibly more to the point, to get even differential unfiltered ("C band") photometry, the exposure must be short enough not to saturate any of the pixels in the star image photometry aperture ... that's going to be hard to reconcile with getting enough light into the spectrum to record it.
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walt r
Post Laureate
Reged: 02/13/07
Posts: 3523
Loc: Doylestown, PA
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and another point is you do want to have the known comparison stars in the same FOV to do differential photometry.
With some careful planning it sounds like this idea could work.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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A really simple way to try this would be to have a filter wheel/slide that has a V filter for the photometric image (taken and analyzed in the normal manner) and a grating for the spectra. The trick will be getting the grating close enough to the CCD. In the example I showed the grating was placed about 5mm above the CCD. I'd suspect Nytecam would know how to do this. I've been looking for an excuse to pick up a Rainbow Optics grating and this might be it.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
Would a SBIG Deep Space Spectrograph (DSS-7) coupled to a ST-7XME camera and using the Zeroeth Order image work for variable star brightness (shown here in Figure 6)?
Is the Zeroeth Order image of sufficient quality? If so, then I could also get the variable's spectrum following its magnitude estimate, sort of a two-fer.
Thanks, Steven
Hi Steven
Unfortunately the zero order image is effectively rather oddly fitered as it is the light that is not dispersed by the grating. Depending on the exact grating design it typically is biassed in the blue and red with the mid range wavelengths missing. This is going to be very tricky to transform to anything which can be compared with standard photometry.
Additionally you need a comparison star in the same field do differential photometry
Best
Robin
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www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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Something like this could prove interesting...
http://www.astrovid.com/prod_details.php?pid=3490
...or just to tinker with for proof-of-concept...
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1490
The later is what I used to take my little test spectra. The 'real' grating would give much better results, but I'll have to wait a bit on that one.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
A really simple way to try this would be to have a filter wheel/slide that has a V filter for the photometric image (taken and analyzed in the normal manner) and a grating for the spectra. The trick will be getting the grating close enough to the CCD. In the example I showed the grating was placed about 5mm above the CCD. I'd suspect Nytecam would know how to do this. I've been looking for an excuse to pick up a Rainbow Optics grating and this might be it.
You mean like this 
http://www.threehillsobservatory.co.uk/astro/Classifying_red_stars_using_a_Star_Analyser_VdS_poster.pdf
I have V and R filters and a Star Analyser grating mounted in the filter wheel. (The 100l/mm grating Star Analyser is mounted twice as far away as the Rainbow Optics grating to give the same length spectrum)
(The long pass filter shown here was because I was taking IR spectra, this is not normally in the optical train)
Robin
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www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
Edited by robin_astro (02/26/09 01:05 PM)
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
Something like this could prove interesting...
http://www.astrovid.com/prod_details.php?pid=3490
Here's the Star Analyser manufacturer's page which has more info.
www.patonhawksley.co.uk/staranalyser.html
Robin
www.threehillsobservatory.co.uk
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
Edited by robin_astro (02/26/09 01:11 PM)
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1288
Loc: Duncan, BC
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Quote:
...or just to tinker with for proof-of-concept...
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1490
Ok, so, I had to go look at these things. That stuff from edmunds works out to just over a dollar each for the 2 inch cards in a 15 pack. Lots of room for cheap experiments there. They claim 1000 lines/mm, whereas the 'star analyzer' says 100 lines/mm.
Other than the convenience of a ready manufactured screw in gadget, what advantage would the star analyzer type gadget have over something homebrewed out of the holographic film stuff ?
I'm starting to become very intrigued by the possibilities of some of this stuff, and the ability to experiment 'on the cheap'. There is a world of difference between tinkering with 20 bucks worth of stuff I ordered online, and that $1600 sbig spectrograph.
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Steven Aggas
sage
Reged: 04/15/08
Posts: 474
Loc: Arizona
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Quote:
Quote:
Would a SBIG Deep Space Spectrograph (DSS-7) coupled to a ST-7XME camera and using the Zeroth Order image work for variable star brightness (shown here in Figure 6)?
Is the Zeroth Order image of sufficient quality? If so, then I could also get the variable's spectrum following its magnitude estimate, sort of a two-fer.
Thanks, Steven
Hi Steven
Unfortunately the zero order image is effectively rather oddly fitered as it is the light that is not dispersed by the grating. Depending on the exact grating design it typically is biassed in the blue and red with the mid range wavelengths missing. This is going to be very tricky to transform to anything which can be compared with standard photometry.
Additionally you need a comparison star in the same field do differential photometry
Best
Robin
Hmmmmm,
What I think I'm hearing is: The image in Figure 6 of the site listed above is/possibly not large enough to contain comparison stars whith which to do DP? If it's not, what field size is typically used?
Additionally,
You call the Zeroth Image an oddly filtered, biased image.... I can see how it can happen. I've sent an email to SBIG regarding their zeroth order and possible filtering relating specifically to DP's use.
Thanks,
Steven
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Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
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Steven Aggas
sage
Reged: 04/15/08
Posts: 474
Loc: Arizona
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I just got SBIG's response.
"...the spectrum of the zeroeth order is greatly modified from the original by the action of the grating across all orders. You should not expect accurate photometry on it. That being said, you can multiply the measured spectrum by the V-filter curve and obtain the V-filter response that way, which would work if you have enough signal."
--------------------
Mr. Wizard
Walking on a Dob: http://darkskyobserving.com/MtGraham.html
Elements in Harmony I, an 8"f6 German Equatorial - Stellafane Winner,
II a 20"f4.2 Newt-Dob - Astrofest Winner,
III a 6"f3.5 Finder/Newt-Dob, and
IV a 36"f4.5 Newt-Dob - "If it's up there, it's in here."
Apache-Sitgreaves Center for Astrophysics
www.DarkSkyObserving.com
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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Replica gratings on film aren't very good, kinda the department store telescope of gratings, only not quite that good. They're great for proof-of-concept, but the spectra are fuzzy. The star analyzer should give much sharper spectra. I'd also want to run the math on the two gratings (lines/mm) as I think the star analyzer may have fewer lines, but that could be a good thing. If it gives sharp spectra the dispersion would be lower which translates into a brighter spectra and a better chance of getting the 0th and 1st order spectra on the same image. It could also let you have a greater spacing between the grating and the CCD array.
--------------------
-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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gavinm
scholastic sledgehammer
Reged: 08/26/05
Posts: 846
Loc: Auckland New Zealand
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Correct, more lines/mm means your 1st order spectrum is displaced further from you zero. You either need a larger CCD (or going diagonal) or move grating closer to your CCD, which may not be possible.
The number of lines/mm is roughly inversely proportional to displacement ie 1000 lines/mm vs 100 means 10 times larger displacement.
--------------------
Gavin
Mt Albert Grammar School Observatory
Auckland, New Zealand
http://www.mags.school.nz/astronomy/index.html
12" LX200R F6.8 AP
SBIG ST7-XME + CFW10
Moonlite SCT focuser w/ temp
Skywatcher Equinox ED80 Pro (ADM dovetail)
+ other stuff
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
I just got SBIG's response.
"...the spectrum of the zeroeth order is greatly modified from the original by the action of the grating across all orders. You should not expect accurate photometry on it. That being said, you can multiply the measured spectrum by the V-filter curve and obtain the V-filter response that way, which would work if you have enough signal."
Yep that would work but then you need to work out the response of the whole instrument system too (CCD plus grating) You can do this by measuring a star with a known spectrum (the spectroscopic equivalent to differential photmetry but unless you are lucky and have a known star in the same field, you will have to move the scope and take another image. This is the spectroscopic equivalent of all sky photometry and needs photometric sky conditions ie unchanging over time and sky position - the sort of things you usually need to go to the top of mountains to achieve. Even most pro spectroscopists try to avoid this sort of thing. Doing differential photometry the usual way and taking the spectrum separately is far easier in the long run.
If you are interested in the steps needed to convert a spectrum image to useful data you could take a look at he second half of my presentation to the BAA on spectroscopy of variable stars here.
http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm
This study of a very simple technique to measure bright star spectra might be of interest too
http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm
These are both based on spectra taken using the Star Analyser grating but the same techniques apply to all low resolution spectra.
Best Regards
Robin
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
Replica gratings on film aren't very good, kinda the department store telescope of gratings, only not quite that good. They're great for proof-of-concept, but the spectra are fuzzy. The star analyzer should give much sharper spectra. I'd also want to run the math on the two gratings (lines/mm) as I think the star analyzer may have fewer lines, but that could be a good thing. If it gives sharp spectra the dispersion would be lower which translates into a brighter spectra and a better chance of getting the 0th and 1st order spectra on the same image. It could also let you have a greater spacing between the grating and the CCD array.
All this is true but one additional word of caution. Simply placing a grating in front of a CCD will never give a perfectly sharp spectrum because of the various optical aberations produced by the light beam not being parallel through the grating. This is why perhaps paradoxically there is no advantage in using a grating with a large number of lines. You just increase the amount of aberation while making the spectrum longer and fainter. 100-200 l/mm is about optimum. The French amateur spectroscopist Christian Buil has some calulations on this simple setup here which you might find interesting.
http://www.astrosurf.com/buil/us/spe1/spectro1.htm
and you can find lots of examples of Star Analyer spectra on my website here
http://www.threehillsobservatory.co.uk/astro/spectroscopy_results.htm
Cheers
Robin
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
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Coromandel
sage
Reged: 02/27/09
Posts: 331
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This is GREAT stuff!
Hi guys, first post. A friend and I have been visual observers for too long and now we want to do something productive. I'm thinking spectroscopy and since John and Robin appear to be experienced experts on the subject perhaps you and others can give me a shopping list of things we will need or should buy.
Between us we are prepared to spend some cash, so suggestions are welcome! Everything from the ideal telescope for the job to all the other bits and pieces.
Thanks gys, this is an awesome site!
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
I'm thinking spectroscopy and since John and Robin appear to be experienced experts on the subject perhaps you and others can give me a shopping list of things we will need or should buy.
Between us we are prepared to spend some cash, so suggestions are welcome! Everything from the ideal telescope for the job to all the other bits and pieces.
Ok, shall I go first? (Takes a deep breath)
Firstly this is my own take on this based to some extent on the route I took.
Apologies also for any European perspective but it is where I live 
If you are coming straight from visual observing then I would recommend getting
some experience in using CCD cameras for imaging first before diving into
spectroscopy. The experience gained there of taking images and processing them
will all be needed in spectroscopy. The next step would then be to get some
experience in spectroscopy using the simple (Star Analyser or Rainbow Optics type)
grating in front of the camera approach. You can learn most of the skills of
spectroscopy using these, and have a lot of fun for a modest outlay of cash. If
you then get the bug, you can then build or buy a "proper" spectrograph.
What follows applies to a bespoke system designed to take scientifically useful
spectra. If you are just doing it for your own general interest then the specs can
be greatly relaxed and almost any scope camera system can be pressed into service.
This is what most spectroscopists did to start with before they became hooked.
Observatory
If you are going to go into spectroscopy seriously you really need a permanent
installation. Once you get beyond simple "grating in front of CCD" setups,
Spectroscopy is tricky enough without having to reassemble everything every night
Telescope
If you are looking for an imaging system which is compatible with spectroscopy you
should go for large apertures and definitely not achromatic refractors. Unless you
are super rich this means going for a reflector. An SCT is probably the most
flexible option. (note that high optical performance off axis is not needed for
spectroscopy as you are generally working on axis. A nice tight star image on axis
is what you are after). Celestron C11, C14 are popular among the top
spectroscopists in Europe (I have an 8 inch vixen VC200L - I aspire to something
bigger )
Mount
You need a good equatorial goto mount which tracks well. (How deep are your
pockets?) I have a GPDX with a SkySensor 2000 which is fine but is overloaded - I
would love a Paramount ME
Camera
The camera should be suitable for scientific imaging. This mean monochrome and for
spectroscopy as wide as you can afford. Pixel size is not super critical - in the
range say 7-12um. It should be low noise (which for serious spectroscopy work
means cooled, preferably with temperature control so you waste less time
repeatedly taking darks. mine is not - I wish it was) It should linear over at
least 50% of its range. Some people say they need to be non anti blooming though
in general, modern anti blooming cameras are fine up to typically 2/3 of their
maximum range. A CCD with a good spectral response is useful, particularly at H
alpha and extended response at the blue end is nice. If you go for an SBig SGS
spectrograph then you need an SBig camera. Shelyak and Baader spectrographs use a
Separate guide camera so are more flexible in camera choice
Spectrograph
Many amateurs build their own spectrograph, this is mainly because they want to and
to reduce costs. Until recently (with the introduction of the LHIRES III) this was also the only route to the highest
resolution spectroscopy. If you got this route do not underestimate the challenge.
To build a practical spectrograph capable of taking scientific quality data of
faint stars is not trivial. Many amateur built spectrographs take a few spectra of
bright stars and are then abandoned.
Suppliers of simple filter size gratings of suitable quality are
Paton Hawksley (UK)
Rainbow Optics (US)
Baader (Germany) though I believe they are out of production at the moment
I have a Paton Hawksley Star Analyser but I am of course biased as I developed it .
There are 3 companies who supply "proper" spectrographs suitable for amateur use
SBig (DSS, SGS)
Shelyak (LHIRES III, eShel)
Baader (DADOS)
I have a LHIRES III - I would love an eShel and a large enough scope to do it 
justice
Software
The best software for spectroscopy is also free - IRIS and Visual Spec. If you are
going to publish then you need to use the professional's software IRAF or MIDAS
Oh BTW here is a picture of my own current setup for high resolution spectroscopy
http://www.threehillsobservatory.co.uk/astro/observatory/observatory_1.htm
Cheers
Robin
www.threehillsobservatory.co.uk
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
Edited by robin_astro (02/28/09 07:43 AM)
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jgraham
Postmaster
Reged: 12/02/04
Posts: 7029
Loc: Dayton, Ohio
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Most excellent! I like Robin's thoughtful comments as this isn't an easy question to answer in one post. Spectroscopy is a fascinating branch of science and like a lot of scientific endeavors you can start simple to test the waters and follow a path that's as simple or sophisticated as you want depending on your interests and objectives. I've done a lot of spectroscopy professionally (mostly in the UV from 160-350nm and near-IR from about 900-3500nm ), but that doesn't have a lot of relevance as the goals, objectives, and resources in both time and money and quite a bit different than what I’ve done at home. Most of what I’ve done at home so far has been along the line of exploring what can be done with simple equipment and put on the back-burner until I have the time and money to take it seriously (my youngest is about to start college so I’ve got a couple of years to go yet). However, I have poked around the web looking for ideas and there are quite a few very interesting sites out there including Robin’s. Another I’d highly recommend is Nytecam’s…
http://www.astroman.fsnet.co.uk/spectro.htm
…as he shows many excellent samples that span the range from very simple to very sophisticated.
Another excellent resource on the general topic of spectroscopy is…
http://www.cs.cmu.edu/~zhuxj/astro/html/spectrometer.html
…which is one of several web sites that describe how to make a spectroscope using a CD-ROM as a reflection grating. I actually built a couple of these including one tunable spectroscope that had two gratings; one made from a CD, the other a DVD, and by the way a DVD has about twice the dispersion of a CD. These work surprisingly well and the absorption lines in the solar spectrum are sharp and well resolved. My next step is to mount one of these on my Canon XTi and take a stab at wide field stellar spectroscopy. At the scope I’m hoping to save up to get one of the Rainbow Optics gratings for use with my DSI Pro.
Very fun stuff.
--------------------
-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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Coromandel
sage
Reged: 02/27/09
Posts: 331
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Thanks a lot guys! This is really helpful. So maybe a 14" SCT. Would an EQ6 Pro mount be up to the job do you think? We have one of those and it's worked out great for astrophotography so far with no mods at all. I'll have to check the specs to see if it can carry the load of a 14".
Sorry for not mentioning that we are experienced with CCDs and DSLRs too. We also have a permanent observatory to use.
Are there any collaborative spectroscopy projects we could join? Working in isolation doesn't sound like much fun. The whole point is to contribute a bit of useful science, otherwise we could just stick to taking nice pictures.
Thanks again for all the excellent info and links you guys!
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brianb11213
Post Laureate
Reged: 02/25/09
Posts: 3199
Loc: 55.215N 6.554W
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Quote:
So maybe a 14" SCT. Would an EQ6 Pro mount be up to the job do you think?
In my experience an EQ6 Pro will struggle with an 11" SCT....
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
Are there any collaborative spectroscopy projects we could join? Working in isolation doesn't sound like much fun.
There are a couple of groups I am connected with(in Germany and France) who coordinate Pro Am studies.
http://spektroskopie.fg-vds.de/index_e.htm
http://www.astrosurf.com/aras/
they are nationally based but welcome participants from other countries and run beginners sessions too, as amateur spectroscopists working at pro level are pretty rare.
I expect there may be similar activity in the US, for example SAS sometimes includes spectroscopy papers.
http://www.socastrosci.org
As an example, I am currently involved in this project on the periastron of colliding wind binary WR140
http://www.stsci.de/wr140/index_e.htm
which involved a team of amateurs, aided by profesionals manning a half metre pro telescope on Tenerife for several months, supported by amateurs using their own observatories to give daily coverage of this event. I think we all learnt a lot about how to produce spectra to the high precision professional level needed for this study.
I would think a Eq6Pro would carry a C11 OTA ok (at least that is what I am considering as a possible move) but not so sure about a 14 inch.
What do you have on the EQ6 currently?
Robin
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
Edited by robin_astro (03/01/09 07:47 AM)
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RLTYS
Carpal Tunnel
Reged: 12/18/04
Posts: 2327
Loc: New York (Long Island)
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Quote:
which is one of several web sites that describe how to make a spectroscope using a CD-ROM as a reflection grating. I actually built a couple of these including one tunable spectroscope that had two gratings; one made from a CD, the other a DVD, and by the way a DVD has about twice the dispersion of a CD. These work surprisingly well and the absorption lines in the solar spectrum are sharp and well resolved. My next step is to mount one of these on my Canon XTi and take a stab at wide field stellar spectroscopy. At the scope I'm hoping to save up to get one of the Rainbow Optics gratings for use with my DSI Pro.
Very fun stuff.
jgraham
I've also experimented with a CD as a apectroscope and it works surpriseingly well.
Rich (RLTYS)
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10" F4.8 Refl.
4" F5 Refr. (Genesis)
3" F4 Celestron FirstScope
50mm F12 Refr. (Tasco #6TE-5)
12x63 and 10x50 Binoculars.
"I want to do more then just look."
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Coromandel
sage
Reged: 02/27/09
Posts: 331
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Yes Brian it looks like you're correct. The EQ6 Pro is rated to 40 pounds max and the C14 tube is 45 pounds. The C11 is 27.5 pounds and would probably be fine because my friend loads his up pretty good with gear. I'd say easily 25-30 pounds and his pics are great.
It looks like we are in the market for a new mount. This darn exchange rate. A year ago we almost pulled the trigger on a PME but chickened out at the last second. Now it will cost us twice as much, maybe even more than that. 
Thanks agiain for the info and links Robin. It'll be much better to work in with a group than just playing around with spectroscopy on our own. Was it the same with you guys where you got over the visual and imaging and itched to do something scientifically useful? I think some of us reach a point where we either continue to progress or we lose interest.
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groz
Pooh-Bah
Reged: 03/14/07
Posts: 1288
Loc: Duncan, BC
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On the question of mounts.
I have an eq6-Pro, and I believe it'll carry the 11 inch sct just fine for this kind of stuff. Mine currently carries an 8 inch sct, and I'm using a 5 inch mak side by side mounted. The mak weighs as much as the sct, and i get perfect results using a guide camera in the mak.
On Chris setup, we are guiding using the ssag in a 9x50 finder. The results from that, suggest that a 12x80 finder would be more than adaquate for guiding the sct, even up to the focal lenghts of the 11 inch for this kind of work. For this kind of stuff, 5 minute exposures appear to be more than adaquate. If I lost the side by side bar, and the 5 inch mak, then upscaled my sct to the c-11 with the 12x80 finder in the guide role, I expect it'll be fine. I'm confident enough, i'm already watching for a good deal on the 11 inch scope.
FWIW, using the short tube 80 in my side by side setup, guiding the 8 inch sct + dslr combination, i'm getting results that far exceed my original expectations. At 0.66 arseconds per pixel, the 'on axis' stars have no measureable elongation using the ssag in the st80, 2 second guide exposures, and 5 minute image exposures. I am going to do some tests and measures in the near future, guiding the 8 inch sct with the 9x50 setup, which is 175mm focal length on the guide scope. I'll post results when I have them.
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robin_astro
super member
Reged: 12/18/05
Posts: 164
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Quote:
For this kind of stuff, 5 minute exposures appear to be more than adaquate.
That is true for low resolution spectra on brightish objects but on faint objects like galaxies, nebulae, supernovae, quasars and the like, or at high resolution, total exposure time can end up being much longer because you are spreading the light out so much. With a simple grating like the Star Analyser in front of the camera, I reckon on something like 200x the exposure needed to capture the object in an image, so 5min total exposure is needed for something you would need just a couple of seconds to capture in an image. If you are working without a slit though, the sky brightness tends to limit how faint you can go.
At the highest resolutions using a slit, light pollution is rarely a problem and total exposures can be over an hour. (I am currently taking over an 1hr (in 20 min exposures) to reach 100 signal/noise on a 7th magnitude star and that is only at a relatively moderate 0.6A/pixel. My highest resolution is less than 0.1A/pixel and a complete spectrum covering the full range would be about 50000 pixels long  . I am limited to about mag 6 or 7 with my 8 inch scope at this resolution.
With the LHIRES III spectrograph the guiding is done using the unused star light which spills over the edge of the polished slit and is reflected back. Other slit spectrographs use similar systems to keep the star positioned on the typically 20-35um wide slit.
You can read all about the design and development of the LHIRES III here.
http://www.astrosurf.com/thizy/lhires3/index-en.html
This is the website which covered the spectrograph before it was made commercially, when it was available at cost as a kit. There are even drawings there if you fancy building one yourself
Cheers
Robin
--------------------
www.threehillsobservatory.co.uk
Vixen VC200L GPDX Skysensor 2000
Modified web and video cams, Spectrograph
Edited by robin_astro (03/02/09 06:18 PM)
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