Tracking or guiding??

[Merlin66]

The attached image is an extract from a full frame Canon D1000, 1 min image with no tracking or RA drive. The length of the trail is in RA.

The interest is the fact that although the bright star was slightly out of focus (to focus on the spectrum being obtained) the track shows the amount of atmospheric movement recorded over a one min period.

The Air Mass at the time was around 1.6

Good tracking would have compressed this wobbly line, hopefully into a "blob" which contains the extreme edges....good guiding, it is hoped, would effectively follow this "wobble" and result in smaller rounder images.

An interesting exercise.

[LewisM]

Do fluctuations in the guiding/tracking affect spectrographic readings Ken?

[Merlin66]

Good question!

For transmission gratings working "slitless" the resolution is 100% controlled by the seeing disk. The better the seeing the better the results.

For serious slit spectroscopy we use a reflective slit plate and guide on the slit gap/ target star. With good guiding (PHD2 or AstroArt) it's possible to hold a target star image on the 20 micron slit gap almost indefinitely - ten minute subs are not unusual. Total exposures can be up to two hours on faint targets.
The key is to get enough signal to give a SNR >100.

[alex-borek]

Wind and some vibrations from walking around etc can do that to the image, I guess..

[Merlin66]

Alex,
There was no wind, and during the exposure I was sitting quietly.

Comes down to Seeing and atmospherics.

[astro_nutt]

Hi all. Now here's something to try when testing atmospherics before imaging. Brilliant!
Cheers!

[alex-borek]

What was your setup? I don't think seeing would impact that much to the image... unless it was under REALLY bad atmospheric conditions!

[Merlin66]

Alex,
I had the ED80+transmission grating + Canon DSLR mounted on the HEQ5.
No drives operating. A 1 min exposure.
I do admit later similar images showed a much better, quieter trace.

The exercise I hoped was to give some insight into what could be determined from just a 1 min un-driven track.

[alex-borek]

It seems a bit heavy for the SW standard focuser, so I am gonna stick to the wind theory! If we estimate that the "blob" was around 6" wide and the bad seeing moved it 3" (should be way more than 6" though, I get 1.36"/pixel), it would make it impossible to do any imaging, since the smallest stars you would be able to image would be just dancing around :cool:.

[Merlin66]

Alex,
The target star was out of focus - to give a better focus on the spectrum....

I was holding the DSLR remote and having problems with the 2" adaptor - the "safety recess" (definitely the Devil's work) so I probably could have contributed to the vibration.