28 December 2017 at 11:46 am #573909
having reset my whole setup, as things needed cleaning, I set out to use it briefly prior to the clouded rolled in. I started with alpha cyg, then beta peg, alpha peg, zeta peg then it got cloudy. The problem I’m trying to sort out is the alpha cyg and alpha peg were very noisy and not looking like then images I’ve picked up previously, but beta peg and zeta peg were very clean and easy to calibrate.
So far I can’t work out what I did wrong with alpha cyg and alpha peg, but if anyone has any suggestions I’d really be grateful, as I have a small window tonight to try again (if you trust the weatherman).
Kate28 December 2017 at 1:31 pm #578894Dr Andrew SmithParticipant
Hi Kate, without more detail on your system I would go with one or more of the following
Poor guiding with the star not staying on the slit
Regards Andrew28 December 2017 at 1:55 pm #578895
Thanks for the suggestions. Guiding was doing well as I was watching it, seeing was good when I imaged these two and Im sure they were the right stars as I used sync before moving on to next target. I did wonder about trees so checked height and tree tops but they appeared clear.
i use an Atik camera controlled by Artemis, the telescope runs with Cartes du ciel with a costar for guiding in phd2.
if we get clear skies I’ll try again tonight
kate28 December 2017 at 3:13 pm #578896Robin LeadbeaterParticipant
How do the pixel ADU counts in the raw spectrum images compare ? ie are the counts in the spectrum image much lower in the noisy spectra? If so we can discount some sort of problem with data reduction.
If the counts in the noisy spectrum images are lower, are they lower in all the sub images or is there a lot of variation between images?
Robin28 December 2017 at 6:53 pm #578897
if I’m looking at the right thing, all the ones on the two not so noisy spectra are the same, whereas there is a difference between the subs for alpha cyg and alpha peg.
The weather has defeated me for tonight, hopefully will get a cleat patch next week to try them again.
Thanks Andrew and Robin.
Kate28 December 2017 at 7:03 pm #578898Hugh AllenParticipant
It is strange that the two brightest stars gave the ‘noisiest’ spectra. And also strange that you got a good spectrum of beta Peg in between the two noisy ones so presumably the problem wasn’t due to some step change in the set-up. All but beta Peg are hot B-type stars so again, no correlation there either. Maybe there was patchy cloud on the night which reduced the light from the two noisy stars? The fact that you obtained good quality spectra from two of the stars, event the faintest, suggests that fundamentally all is well with your set-up
Hugh28 December 2017 at 8:33 pm #578899
i did wonder about high cloud wafting through which I didn’t see, I also wondered if I used to long an exposure. Next clear skies I’ll try both stars again with different length exposures.
Kate29 December 2017 at 1:36 am #578900Robin LeadbeaterParticipant
Large variations between sub exposures can be a result of a seeing/scintillation affecting the guiding. (The star is jumping in and out of the slit). Guiding on very bright stars can be problematic. You need a well exposed (ie not saturated) star image to guide on but to avoid saturation the exposures end up being very short. The guider then makes too frequent adjustments on seeing variations. PHD is also prone to hunting about the slit position when guiding using the overspill from the slit. (PHD tends to lock onto the brightest half of the split star image rather than the centre of the star) Using one of the alternative algorithms in PHD eg hysteresis or slow pass filter can sometimes help in these cases.
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