eta Carinae spectrum

[bojan]

After a long time I tried some spectroscopy with my "new" spectrometer, based on Star Analyser and 4° prism, obtained from BXoptic (China) for less than half the price of Edmund Optic, but similar/same quality (will post the construction detals later).


Balmer series and He line are very easy to identify as they are dominant emissions in the etaCar spectrum.
Interesting detail is the Helium emission (or, at least the wavelength if the "bump" between Halpha and Hbeta suggests it is He).
This spectrum is quite different from spectra obtained by others in different time.... it seems eta is very volatile place.

Relatively low resolution is probably the consequence of the grism place (between MTO lens and Canon camera), where the beam diameter is relatively small (~5mm only). Placing it further away from sensor would increase the resolution, but this will be difficult to achieve because of the diameter of the prism (38mm).
Also, MTO 10_1100 is probably not the sharpest lens there is... my 10" Newtonian would be a better choice for this job. Unfortunately, I haven't finished yet the modification I started some time ago.

Spectrum attached was extracted from 16x10sec stack, ISO400, taken by Canon 450D (FSM) through MTO 10_1100, all on EQ6 mount from my backyard in MtWaverley.

[Atmos (Colin)]

Very nice Bojan! I have often thought about getting into some spectroscopy but just never got around to it.

[bojan]

Colin,
Thank you!

On the attached images you can find more design details - 3D printer was very useful here. (Probably the best and most versatile DIY project I embarked recently).
Prism (obtained from BXoptic) is blue, SA is purple.
White components were 3D-printed.
The whole plastic assembly (grism holder) rotates inside the holder (M42 extender), the movement is limited by the slit cut into the wall of the extender.

[sharptrack2 (Kevin)]

Nice project Bojan! Very professional looking. 3-D printer on my wishlist now.

I do have a curiosity question... I fundamentally understand spectroscopy and would like to know why you put a prism in front of the grating? Is that to concentrate the central target light into a smaller area, or spread it out to get better resolution?

[bojan]

Originally Posted by sharptrack2

Nice project Bojan! Very professional looking. 3-D printer on my wishlist now.

I do have a curiosity question... I fundamentally understand spectroscopy and would like to know why you put a prism in front of the grating? Is that to concentrate the central target light into a smaller area, or spread it out to get better resolution?

Kevin, this is for two reasons:
- to reduce chromatic coma
- to bring the first order spectrum closer to optical axis of the system
Those reasons are very nicely discussed on Christian Buil's webpage here:
http://www.astrosurf.com/buil/us/spe1/spectro1.htm
http://www.astrosurf.com/buil/us/spe1/spectro2.htm

However, prism is not necessary to build a good performance spectrograph with Star Analyser.

[sharptrack2 (Kevin)]

Thanks Bojan,

Appreciate the links... some more reading for me on the train!

[Merlin66 (Ken)]

Bojan,
Neat nice solution to mounting the SA grating. Well done!
I think you'll find that adding a grism to the SA100 only give a marginal improvement in resolution. The grism is more effective on gratings with 200 l/mm and above. (The 600 l/mm ALPY design gives a good performance with the Grism.)
As mentioned on other threads Richard Walkers Spectral Atlas is a must have for novices in spectroscopy.