Skip to main content

Star Analyzer

uis01
uis01's picture
Offline
Joined: 2010-07-25

I thought it might be helpful to start a separate topic on the Star Analyzer so that people looking for that don't have to page through the other topics in this forum.

My question was mainly for Jeff, but if anyone else can answer I am happy to hear from you too.

The Star Analyzer SA-100 says that it can be used as a normal 1.25-inch filter.  Has anyone tried mounting it in a filter wheel?  I'm curious because I think that it would improve the ease of use if it was just another slot in the filter wheel that people could have as an option for imaging.  

-John

Star Analyser
HPO
HPO's picture
Offline
Joined: 2010-07-23

 

Hi John,

Yes, people have mounted the Star Analyser in filter wheels. This forum was started by someone doing that. The problem was orientation and spacing. As you know the spectrum should be as horizontal as possible (requiring a rotation of hte Star Analyser relative to the CCD) and there are optimum spacings for the Star Analyser and CCD, one to include the zero and first order spectra and one for just the first order.

If the orientation and spacing can be set properly, it doesn't matter where the Star Analyser resides. The big problem is what do you do with the other filter wheel positions? If you are doing astrophotograph and/or photometry, there may be a focusing problem. 

Using a filter wheel is certain doable if done properly, however, it may be better to think a speparte system. Even a 60 mm telescope piggyback with a Star Analyser could do a lot of work. As you know it is only when you go to higher resolution that the bigger telescopes are need even on fairly bright stars.

Jeff

Star Analyser in filter wheel
Robin Leadbeater
Robin Leadbeater's picture
Offline
Joined: 2012-10-08

Yes,   As it happens it is the subject of a current thread on the star analyser group.

http://tech.groups.yahoo.com/group/staranalyser/message/2684

This is my standard Star Analyser setup but with the filter wheel spaced at the optimum distance from the camera. (For faint objects you also need to arrange for the camera+wheel to be rotateable to avoid interference from background stars/spectra. ) 

 

Cheers

Robin

Thanks
uis01
uis01's picture
Offline
Joined: 2010-07-25

This makes sense.  The image of the star should be focused at the star analyser for it to work.  Therefore the CCD needs to be 3-5 cm behind the analyser so that the spectrum spreads out enough to get good resolution per pixel.  And in order to re-focuse to do imaging you need at least the 3-5 cm travel to switch back and forth.  Thinking that through, I agree that can be a tall order.

I was hopeful that the analyser software could deal with a spectrum that did not fall exactly along a CCD row.  It is disappointing and adds a dimension of difficuly for filter wheel mounting, if the software doesn't handle a mis-aligned grating axis well.

The piggy back idea sounds like a good one but it requires a second CCD camera.

I was asking because I was thinking about getting my hands on one of these myself so I was more directly familiar with it.  In my mind I was thinking that this might have some of the advantages of an objective prism, without the fuss of mounting a big piece of glass on the front aperture of your scope, but I'm realizing I had some bad assumptions in my mental comparison there.

Robin, Thanks!  That is
uis01
uis01's picture
Offline
Joined: 2010-07-25

Robin,

Thanks!  That is helpful.  As I said to Jeff, I was thinking of getting one of these myself to play with and your experience is really useful for helping me uncover the bad assumptions I may be making.

 

-John

Star Analyser
HPO
HPO's picture
Offline
Joined: 2010-07-23

 

Hi John,

The RSpec software has the ability to rotate the spectral image so it can be adjusted in software. It is still best to get it as horizonal as possible without software rotation. Somethings the software does strange things with the rotated spectrum.

As for a separate CCD camera for a piggyback unit, the Orion Star Shoot G3 mono has recently been on sale for $399 with free shipping and no tax. I use one of these with my Lhires III. It has the same chip as the DSI Pro II, but also has TEC cooling. You could also use a DSLR with the piggyback scope. Even web cams have been used. The G3 is what I would recommend. See

http://www.telescope.com/Astrophotography/Astrophotography-Cameras/Orion...

Jeff

Nova Cep 2013 with the Star Analyser
Robin Leadbeater
Robin Leadbeater's picture
Offline
Joined: 2012-10-08

This is what nova Cep 2013 looks like  through the Star Analyser.  (Taken on 6th Feb, v mag ~12)

The equipment setup used  is described here

http://www.threehillsobservatory.co.uk/astro/spectra_42.htm

Dispersion is 17.2A/pixel and the spectrum was focussed at  the H alpha line.  It was taken as a test after a rebuild and the collimation needs tweaking.

Robin

deleted duplicate post
Robin Leadbeater
Robin Leadbeater's picture
Offline
Joined: 2012-10-08

deleted duplicate post -

note to webmaster  - please add delete post option.

star analyser in filter wheel
Robin Leadbeater
Robin Leadbeater's picture
Offline
Joined: 2012-10-08

uis01 wrote:

This makes sense.  The image of the star should be focused at the star analyser for it to work.  Therefore the CCD needs to be 3-5 cm behind the analyser so that the spectrum spreads out enough to get good resolution per pixel.  And in order to re-focuse to do imaging you need at least the 3-5 cm travel to switch back and forth.  Thinking that through, I agree that can be a tall order.

I was hopeful that the analyser software could deal with a spectrum that did not fall exactly along a CCD row.  It is disappointing and adds a dimension of difficuly for filter wheel mounting, if the software doesn't handle a mis-aligned grating axis well.

 

Rather late in replying to this I know but I am still getting hits on my website from  the thread so it is probably worthwhile correcting and clarifying  this to avoid any confusion, particularly since the new Star Analyser 200 model designed for use in filter wheels has just come out.

The focal plane remains at the CCD. You do not refocus the star image at the Star Analyser plane. The unfocussed beam is dispersed at the Star Analyser and the various wavelengths all come to focus on the CCD (approximately, there are some aberrations, the limitation of the simple converging beam configuration)  There will be some small refocussing needed due to different optical thickness of the Star Analyser relative to other filters and the fact that the focal plane of the spectrum is slightly tilted but nothing  that a normal focuser would not be able to cope with.

Most (All?) spectrum processing software can make geometric corrections to the spectrum image. to correct for tilt, slant, smile etc for example. It is advisable though to rotate the grating to make the spectrum as horizontal (or vertical) as possible to avoid the risk of artifacts, particularly if a small aperture short focal length telescope is used where  the spectrum may be very narrow and cover only a couple of  rows.  This is achieved when using a filter wheel by unscrewing the Star Analyser slightly and fixing in position eg with plumber's PTFE tape on the thread or a spot of hot melt adhesive. The SA200 also has a mounting kit for wheels which take larger filters and this allows the grating to be fixed in the correct orientation.

Note It can be useful to be able to rotate the spectrum relative to the field to avoid contamination of the spectrum from background stars  when measuring faint objects in crowded fields but by preference the camera and grating should be rotated together so the spectrum remains horizontal (or vertical) in the field.  

Rotating to avoid background stars can be inconvenient and difficult to achieve in robotic setups of course so an alternative in this situation is to use 2 gratings in the wheel at 90 or 180 deg to each other to maximise the opportunity to miss background stars or at least identify where contamination may be occuring.

Cheers

Robin

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484