Eta Car Spectrum

[Glenpiper (Bernard)]

Terry,

I should add that since last week I've switched from the C8 to C11 and may start using my C14, but that size is not needed for bright objects and is also counter productive, due to the star's larger Airey disc.

Bernard

[RobF (Rob)]

Thanks for this thread Terry - fascinating reading. Do you have data below and above the range you've shown? I was interested to see what sort of peaks existed around the 510ish and 680ish OIII and SII bands that narrowband imagers use.

[Terry B]

Originally Posted by Glenpiper

Hi Terry,

I monitor that particular window with my Lhires III+ST-8XE+C8 using a 2400 l/mm grating and a 19um slit.

Getting a decent SNR on a bright object should not be a real problem if (a) you make a long exposure and (b) you guide/autoguide well. Attached is an image of a fairly recent spectra made with my 200mm (C8) scope with a total exposure time of 50 min. I'm not allowed to show a spectra of that same region made with a +3m professional telescope but I can assure you the pro resolution and SNR was not that much better.

Bernard

Interesting.
My image is a stack of 5x300sec exposures in each region using a VC200L, 600l/mm grating with the minimum width slit that the L200 has and a ST9E with 20um pixels. Clearly more exposure time is the key here.
I am using a guide scope and seem to be able to keep the star on the slit reasonably well with this.
I would like to keep monitoring eta Car. How often do you think is reasonable to take exposures to potentially show any change?
It clearly takes time to do so with limited clear weather and other objects that I monitor.

Originally Posted by RobF

Thanks for this thread Terry - fascinating reading. Do you have data below and above the range you've shown? I was interested to see what sort of peaks existed around the 510ish and 680ish OIII and SII bands that narrowband imagers use.

I haven't tried other areas of the spectrum due to the crappy weather. I will see what I can do.

[Glenpiper (Bernard)]

Originally Posted by RobF

Thanks for this thread Terry - fascinating reading. Do you have data below and above the range you've shown? I was interested to see what sort of peaks existed around the 510ish and 680ish OIII and SII bands that narrowband imagers use.

The only emissions of any 'consequence' near those two ares are a whole bunch of FeII lines betweenH beta (4861) and 5500, and HeI at 7065.

Bernard

[Glenpiper (Bernard)]

Originally Posted by Terry B

Interesting.
My image is a stack of 5x300sec exposures in each region using a VC200L, 600l/mm grating with the minimum width slit that the L200 has and a ST9E with 20um pixels. Clearly more exposure time is the key here.
I am using a guide scope and seem to be able to keep the star on the slit reasonably well with this.
I would like to keep monitoring eta Car. How often do you think is reasonable to take exposures to potentially show any change?
It clearly takes time to do so with limited clear weather and other objects that I monitor.


I haven't tried other areas of the spectrum due to the crappy weather. I will see what I can do.

Given that you have a 20um pixel (the ST-9 isn't the bast match for higher res work) and that you need a two pixel coverage to meet the Nyquist criteria, you won't gain any resolution by using a slit narrower than 40um ... which will also give better throughput.

Normally you don't need to monitor eta Car more than once a month.

[Glenpiper (Bernard)]

Originally Posted by Terry B

My image is a stack of 5x300sec exposures in each region using a VC200L, 600l/mm grating with the minimum width slit that the L200 has and a ST9E with 20um pixels. Clearly more exposure time is the key here.
.

Terry,

Have you measured the 'minimum' slit width you are using? If it is less than 40um, as mentioned earlier, you are wasting light without gaining any resolution and that may be the reason your SNR isn't as good as it could be.

[Terry B]

Originally Posted by Glenpiper

Terry,

Have you measured the 'minimum' slit width you are using? If it is less than 40um, as mentioned earlier, you are wasting light without gaining any resolution and that may be the reason your SNR isn't as good as it could be.

No I haven't measured the slit. How do you do it?
I know the ST9E isn't the best for this use but it's what I have. I could attach my ST10XME with 6.4um pixels but I use it for photometry and would have to take the filter wheel off and on to swap the roles --Too much hassle.

[Merlin66 (Ken)]

A quick method the check the slit is to measure the FWHM width of a "good" exposure of a zero slit image.
I recommend setting the gap to 3 -4 pixel FWHM (this is not as rigorous as it should be, but seems to work pretty well for novices)

This will then at least match your pixel size better than working out the linear star size on the slit and setting to that.
You can see the impact when using a neon reference - the FWHM doesn't change much when the slit is adjusted when you have larger CCD pixels.