Star analyser and C11

[bojan]

I am planning to use my new C11 for spectroscopy...
The idea is to use existing GRISM
I already used it with MTO_10_1100A, but because I could not go deeper into tube (field element was in the way), the light cone at SA was ~5mm in diameter, so the resolution was not great.
Now, I can go deeper inside the baffle (150~200mm away from camera sensor), to fully utilize the resolution of Star Analyser (calculator is here: http://www.rspec-astro.com/calculator/)
There will be a lot of 3D printing and turning plastic tubes on lathe... :D

[Merlin66]

Bojan,
Sounds good.
I think you'd get better performance using the grating in an f6.3 beam than increasing the spacing in an f10 beam.

The size of the seeing disk determines the resolution. Smaller disk when using f6.3

[bojan]

Yes, I know.. but reducer is out of budget at the moment..

[sheeny]

With my C8 and my C11 I found there was enough backfocus to have the SA mounted in the front of a barlow body and still achieve focus. Is it the weight of the DSLR that you don't want hanging off the barlow?

I used a DMK mostly, but tried my SBIG a couple of times which would be similar weight to to a Canon I would think.

Al.

[bojan]

Al,
The resolution of diffraction grating depends on number of lines engaged to disperse the light beam of the star under test. More lines in the beam, the better resolution.

With f/10, the distance between SA and focus should be ~200~300mm, so if I place SA inside C11, I can use the same camera adapter and have the optimised spectrum resolution and dispersion

[Merlin66]

Bojan,
Sorry mate, that's only the theoretical resolution not the actual...

The final result when using a transmission grating "in the converging beam" will be influenced by the chromatic coma, field curvature etc.
My TransSpec V3 spreadsheet may help.

[bojan]

Ken, thanks for the spreadsheet, it will be useful :thumbsup:

[bojan]

Some more on grating resolution (theoretical) is here (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratint.html).

[Merlin66]

There are many good websites which cater for amateur spectroscopy.

I have provided many interesting links on my webpage
http://www.astronomicalspectroscopy.com/

Christian Buil's page is highly recommended.
http://astrosurf.com/buil/

[bojan]

Discussion of grating resolution as function of number of grooves, from Christion Buil's page, see here (http://www.astrosurf.com/buil/us/spe1/spectro2.htm).

From the formula below, (theoretical) resolution is directly proportional to product of m*L, which is number of grooves, engaged by (converging) beam.

All this above is rationale behind my decision to place the grism inside the back-end tube of my C11... and why I expect better results with this setup.

But first I have sort out burning C11 mounting issue om EQ6... losmandy flange is on its way (I hope).

[Merlin66]

Bojan,
Read to the end of that paper.
You'll see in the "grating in converging beam" that the actual resolution is severely compromised by the aberrations.
The same formula is used in the TransSpec spreadsheet.
ie the dispersion may be 4A/pixel, but the resolution 35A......

[bojan]

I did read the paper.
What I am saying here is, I am trying to optimize my setup the best I can, by eliminating limitations I had earlier.
I do not have any dispute with you.