Making a Start in Stellar Spectroscopy

[Ganymede12]

I recently bought a Rainbow Optics two-piece spectrometer and have so far used it only once (due to clouds) and that was using the smaller of my two telescopes (70mm frac) in low lying cloud. I only really looked at Jupiter and Betelgeuse but I am already loving the spectrometer. 

So tonight I'm hopefully going to get a chance to try it out with the 8" dob and have an attempt at taking an image. My plan was to take the image in the same way I took an Image of Saturn. That is using a Phillips Webcam, SharpCap, Manual tracking and then processing with Pipp and Registax6. 

So is that all I need? Is it feasible to image a stars spectra in this way or is there a special piece of software I need first (note that for now I jus5 want an Image of the spectra's colours and don''t want to turn it into a graph just yet).

Thanks in advanced! 

[MattJenko]

Don't you just need to take a longer exposure rather than go for a avi, multiple short frame approach? You want to get a decent signal from the star's spectrum on the sensor, which will be quite dim compared to the star itself, so short exposures don't strike me as the way to go, you want to build up a strong signal by a longer 30 sec+ exposure. This is obviously going to be tricky on a Dob...

[Ganymede12]

So won't I be able to get any sort of image out of it?

[MattJenko]

One can only try and see. I don't think it won't work, its just that for spectrum images, you need a total exposure in the order of multiple seconds. If this can be obtained via many short webcam images, then cool, its just not the easiest way to get a spectral image. Don't take what I say as a reason not to give it a go!

[Ganymede12]

T be honest I was always going to try . Is there any piece of software that you'd recommend for either capturing the image or processing later? 

[MattJenko]

I think your list sounds perfect, although you could try AutoStakkert2 if you struggle with Registax.

[Ganymede12]

I've just come back in from my first attempt at stellar spectroscopy. Here are the images I got; let me know what you think! 

So the Betelgeuse ones are  snapshots.The Adebaran pictures are all the product of one Avi being put through PIPP then Registax6. 

Now all of these images are out of focus. I know this because when I pointed my telescope at the star it was blurry. 

So although no detail in the spectra is discernible I think that, as a proof of concept, it was fairly successful. It has at least proved that my set up is capable of actually detecting the spectra so now its just about getting the image to look right, which I imagine will require a tracking mount of some sort. 

The focus is the big ssue at the moment. The focus improved as I moved the focuser inwards but it didn't reach perfect focus when it was all the way in. Is there a way around this? 

[MattJenko]

Excellent!

To get that final few mm of focus on some Newtonians not designed for photography you have a few choices. You can try and move the primary mirror up the tube a bit, by having longer collimation bolts (not done this myself, just read about others doing it). You can also look to see if you can find a lower profile focuser, which will allow you to get those last few mm( also not something I have done), or use a barlow, which will push the focus out of the tube and further up so you can get focus. You probably don't need a powerful barlow, so a decent x1.5 or x1.6 one should do the trick, or if you have one already, give that a go. This I have done, for my 250px and my DSLR and a x2.25 one.

[Ganymede12]

I have a 2X Barlow but won't that reduce my FOV? I'll give it a go still. The other two options involve messing around with the telescope which I am fairly reluctant to do. 

[Ganymede12]

Tonight was also a good night so I had a second go at imaging Betelgeus' spectra and his time I used a 2X Barlow. 

I'm a bit happier with it because the focus is better. The cameras' small fov combined with the limitations of manual tracking means that the final image is missing the end  but it is sharper than my first attempt so thanks for recommending the Barlow It works! 

I don't know why it fans out at one end but when I was filming the spectrum was shimmering like the aurora. I've put it down to atmospheric turbulence.  So is there a way of eliminating the fanned end?                                   

[frostynixon]

Very interesting read! I guess the  barlow lens/ narrower field of view would help to isolate stars in a "crowded" area or a cluster. Otherwise you will just measure the spectra of multiple stars. It would be nice if one had the spectroscopic resolution to determine red shift, but i guess thats pretty demanding.

Anyway, very interesting and keep us posted!

[frostynixon]

The fanned tail could for example be a result of a tilted image plane. In your case thats unlikely because than you would have a continuously growing focus from left to right. So i would guess that the spreading of the blue spectral part is actually chromatic aberration from the lens cell of your spectrometer.

[Ganymede12]

I haven't gotten out in a while. If I'm lucky, I'll get another stab at an image tonight.

[Merlin66]

Not caused by the grating....must be the chromatic aberration of the barlow.

You need to focus the spectrum not the zero order star image.

For the webcam sized chip the grating needs to be around 30mm in front of the camera CCD.

If you orientate the grating so the star trails upwards (ie makes the spectrum wider) you may have more success.

[Ganymede12]

Thanks, I'll give that a go!