First test spectrum of M106

[cynosure]

I have just obtained my first spectrum of M106. This was a range finding test to see if it might be possible. The results and description can be found on my web site at:

http://www.cynosurechem.co.uk/Astronomy/M106_study.html

Hopefully I will be able to improve on this in time. I would like to be able to compare the OIII lines and the Halpha in more detail. Not sure if it is possible yet, need to sleep sometimes.

Hope you like it, it has been a challenge.

jonathan

[robin_astro]

Hi Jonathan,

Wow! that is a challenge.  I think you did well to get anything. I noticed you reduced the FR of the C14 to 6.5.  Unfortunately you will get vignetting below f9 with the LHIRES III so you will be losing about half your precious photons. It might be better to work at f9 and widen the slit a bit more, though even if you keep the slit width the same I suspect you might still be better off at f9 as you can still integrate the full lenth of the galaxy along the slit. 

Good Luck
Robin

[cynosure]

Dear Robin,
could you please clarify the issue over vignetting for me as I really do not understand this. I am assuming that the image is formed at the slit, the image of the slit is then focused internally, and will be the same whatever the f ratio the telescope is operating at, as will the degree of internal vignetting of the slit image.
I read Christian Buil's comments on this and did not understand that for the same reason, I am sure that I am missing something here.

thanks,

jonathan

[Merlin66]

It may help to imagine that your objective is producing a converging cone of light, say f4, this cone reaches a focus, a point at the slit and then passes through making a similar expanding cone of light at f4....
If the collimator lens in the spectroscope is designed for a f6 cone, then when this f4 cone reaches the collimator it will be too large and greatly exceed the size of the lens; this will cause vignetting and the edge light ( around the circumference) will be lost, the efficiency of the system wil suffer.
Hope this helps.. a diagram will quickly show the issue.

[cynosure]

Thanks,
I understand now...a rather basic error on my part...

jonathan

[robin_astro]

Yes unfortunately there seems to be no way round the rule "The bigger the telescope, the bigger the spectrograph"   You might be interested in the modification made by Lothar Schanne who has the same size telescope as you. He has fitted a larger diameter collimator which allows the LHIRES III to run at up to f5
http://209.85.229.132/translate_c?hl=en&ie=UTF-8&sl=de&tl=en&u=http://home.arcor.de/l.schanne/Einsteiger/Umbau/UmbauLhiresIII.htm
Note that you also need to fit a 50x50mm grating.
This setup was used successfully on a 0.5m telescope in Tenerife as part of  an interesting recent Pro-Am campaign I was involved in.
http://www.stsci.de/wr140/index_e.htm

Cheers
Robin

[cynosure]

I have been looking at this question about f ratio today in the light of my new understanding! Just in case I have got this wrong I thought that I would pass it by those of you that understand the optic better than I do. With the focal reducer I get an f ratio of 6.55 (measured). Using the information on Christian Buils site, this gives and efficiency of 67%, which is, as I now understand it a loss of 33% due to vignetting? However, in going to f6.55 my image scale has gone from 0.044 arc sec/um to 0.088 arc sec/um, which I assume  means that 50% more light is going into the slit as the bright part of the image is still larger than the slit width (galaxy core 100um, slit 59um). The gain then, in my simplistic view of optics is 17% in using the focal reducer.
I have also been looking at my raw data again, and the data looks just as good or even slightly better after just processing the first 35minutes of exposures. The remaining 2hrs just added noise and degraded the signal to noise ratio. Live and learn.

Thanks for your help on this,

jonathan

[robin_astro]

Hi Jonathan,

My calculation was based on a doubling of the light from a uniform source due to vignetting going from f6.5 to f9. (Do you have the page on Christian's site you used, there are so many, it gets confusing) I was then assuming that the slit would be opened up to maintain the total flux and the hit would be taken on the resolution (probably not a good idea though if the source has narrow emission lines)  As you change fr at a given resolution, You end up losing light either through vignetting or rejecting it at the slit.  I still think optimising the  fr to just avoid vignetting (ie running at about f8-f9) should be optimum though for a uniform source and a given resolution. (9/6.5)^2 * 6.5/9 = 38% better (The first term is the vignetting effect, the second term is the effect of dilution at the slit due to the slower optics)

It is good to find another LHIRES user in the UK - whereabouts are you?

Robin


 

[cynosure]

There is nothing like actual tests...ran a test using Regulus as the object. The star is slightly wider than the slit at f6.55 and about twice the slit width at f11. The data shows that it is better to have the telescope at f11.

[robin_astro]

There is nothing like actual tests...ran a test using Regulus as the object. The star is slightly wider than the slit at f6.55 and about twice the slit width at f11. The data shows that it is better to have the telescope at f11.

Looks like my crude calculation method is not too far out then

Assuming vignetting starts at f9, then f11 compared with f6.5 = (9/6.55)^2 * 6.55/11 = 1.12x compared with about 1.2 from your measurements.
I don't know if you can mount the spectrograph any closer to the FR, but if you can get to around f9, you should do a bit better still.

Cheers
Robin