P Cygnus with Star Analyser 100

[ejp1684]

I've been playing around with the SA 100 for a while, trying it out with a couple of focal reducers. In my innocence I thought that just screwing an F6.3 focal reducer onto my Evo 8 and attaching the camera and SA via a flip mirror would give me F6.3. Not so. It depends on how far the camera chip is from the focal reducer. After reading a lot about the subject on the forums I realised that I was in fact achieving F3.9, and when I plugged that into the TransSpecV2.0 spreadsheet the resolution was not the best.

I did a number of tests using the Celestron F6.3 and a (much) cheaper 0.5 focal reducer. (I did read somewhere that the Celestron F6.3 is not actually a focal reducer, which left me slightly confused...) Anyway, the result was that the cheap 0.5 when set to give a focal ratio of 6.3 seemed to produce the best results. Here's a comparison of Vega with both, with the Celestron F6.3 set at 110mm from the camera chip which I understand is the correct distance to place it.

The 0.5 reducer gives more pronounced dips than the Cel F6.3 especially with H alpha where the Cel F6.3 is very half-hearted in its efforts. This was somewhat annoying, as the cheap fr was a quarter the price of the Cel.

I then decided to have a go at some WR stars in Cygnus. I haven't finished processing them yet, but I also took a spectrum of P Cygnus (as it was close by), and here it is.

I'd got hold of the 3.8 Prism by the time I took this and it certainly made it easier to focus. I took the info from Richard Walker's excellent 'Spectroscopic Atlas for Amateur Astronomers'. I hope I've got it all correct, no doubt more knowledgable folk will tell me if I haven't. All the processing (hot pixel removal, binning region, etc) were done in BASS, and the image captured using SharpCap.

I found that the best results were obtained by placing the elements thus: Camera - FR0.5 - SA100 - Prism3.8. Patton Hawksley say you can put the prism either before or after the SA, but I found the image was cleaner when the prism was placed after the SA.

Eric.

[Tiny Small]

An excellent bit of empiricism. Wish we saw more of this stuff about as it could save people time and money.

I'm assuming that both reducers were set to give exactly f6.3 during the test so as not to advantage/disadvantage one over the other? I ask as the spacing is different for each one.

[ejp1684]

Yes, both set to give f6.3, or as close as I could estimate. I wasn't sure whether the 110mm for the Cel is from the front, the middle or the rear of the lenses in the reducer. I read somewhere that the last element is the flattener. I took the measurement from the middle of the lenses.

Eric.

[robin_astro]

Hi Eric,

Interesting experiments. There are quite a few competing factors going on here which will impact the resolution, though I am not sure if they will explain the difference or not.

The size of the star image relative to the dispersion directly impacts the resolution and defines the maximum possible resolution for a given setup.

The dispersion depends on  the distance from grating to sensor but note the effective distance is less than the physical distance if you place the focal reducer between the grating and the sensor

The star image size depends on the focal length so the focal reducer will potentially increase resolution.

However.. because the converging beam produces aberrations the actual resolution is lower than the maximum possible. The faster the beam, the more the aberrations so when adding a focal reducer you get two competing effects - a smaller star image but more aberration.

A way round this is to place the grating before the focal reducer where the beam converges less which as I understand is what you have here for the best arrangement. 

Note you can only really compare directly if both the dispersion and the focal ratio are the same for both cases. (The focal ratio can be checked by measuring the distance between two stars in the field or measuring the rate of drift through the field)

it might also be worth checking the size of a focussed star image in each case without the grating which would check for any inherent optical problems with the celeston reducer

 

Cheers

Robin

[ejp1684]

On 14 October 2016 at 18:01, robin_astro said:

it might also be worth checking the size of a focussed star image in each case without the grating which would check for any inherent optical problems with the celeston reducer

Thanks for your reply, Robin. I'll check this the next chance I get. I did make sure that I was at F6.3 in both cases, and tried to get the dispersion the same as well.

I have to say I'm quite pleased with the results using the setup with the 0.5 focal reducer. It's easier to get a good focus, and I can use my flip mirror which is very helpful, which I can't use with the Cel focal reducer because the flip mirror sets the camera chip too far from the reducer to get F6.3. The seven WR star spectra I obtained look promising. Here's WR134, I still have to check I've got the wavelength correct and look up the data for these emissions.

 

I still find it amazing I can get all this information with such a simple setup, particularly from a Mag 8 star.

Eric.

 

[robin_astro]

Hi Eric,

WR stars make fascinating spectroscopic targets at all resolutions. I am just gearing up to follow the binary system WR140 through periastron again as part of a Pro-Am study. This is what we saw last time    http://www.threehillsobservatory.co.uk/astro/spectra_41.htm

There are a couple of references I find useful for identifying WR spectra

An atlas of optical spectrophotometry of Wolf-Rayet carbon and oxygen stars      http://adsabs.harvard.edu/abs/1987ApJS...65..459T

Spectral atlas of the Galactic Wolf-Rayet stars (WN sequence)   http://adsabs.harvard.edu/abs/1995A%26AS..113..459H

and new ones are still being found occasionally, one even with the help of a Star Analyser!   http://ukads.nottingham.ac.uk/abs/2012AJ....143..136L

Prof Paul Crowther at Sheffield University maintains an up to date  list of WR stars here    http://pacrowther.staff.shef.ac.uk/WRcat/

Cheers

Robin

 

 

[ejp1684]

Hi Robin, thanks for those links, very helpful. I'll be able to get on and study the spectra of the WR spectra I've taken in more detail.

Eric.