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Hi Robin could you try the filly lamp at 90 deg to the correct position to see if the off axis lines mirror the behaviour of the star.
Regards Andrew
The only thing I can think of is the the dispersion direction of the prism and grating are not aligned in such a way that the path differences with a fully illuminated entrance pupil and a partially illuminated one results in more dispersion from the prism in the fully illuminated case.
Clutching at straws!
Regards Andrew
I would avoid using the star plus lines method as it is mixing two different dispersion if the light path is different between the two.
I think it would be best to use the lines to get the dispersion then adjust any offset with an A star. The ALPY with Ne Ar has enough lines without the star lines for a good fit.
Regards Andrew
Simple answer is I don’t know.
Can you process a calibration frame as if it were a star? Rename it so ISIS is not confused. Are the lines in the right place?
If they are then I can only think it is the fact that the calibration light is not illuminating the spectrograph in the same way as the star.
If the are not then it may be other issues like undersampling.
Regards Andrew
The error is about 1/2 a pixel. I wonder if it is due to undersampling in which case it will be a practical limit unless you use smaller pixels.
Somewhere I have a paper on this and I will try to find it.
Regards Andrew
Hi Robin the 3rd order fit has 0.57 and the fourth 0.34. The fourth is as the ISIS fit. Though looking at this again I don’t think this is an issue.
DAVID makes an interesting point about the line wavelengths though.
Regards Andrew
I will need to do this again as I used Robin’s figures!
Hi Robin, no I just used the trend function in Excel which just reports the R value. I will see if I can just fit it in Excel proper rather than on the graph.
I nitially I was refering to wavelength then I was referring to the colour on the plots not wavelength. I should have changed the colour to avoid the confusion. Just used the defaults without thinking.
Regards Andrew
I had a play with Excel and fitted the data to different orders all were good fits but you can see the difference in the extrapolation into the blue. Order 1 fitted at R=0.9998 ,orders 2,3,4 fitted to R=1 the graphs are order 1,3,4. The blue line is the fitted data and red the extrapolation using the ISIS fit.
Regards Andrew
It did occurs to me that with a non-linear dispersion you might get errors due to more lines being fitted at the red end compared to the blue. This would then give a better fit in the red while a poorer one in the blue even if the overall fit looked good.
Regards Andrew
James, in addition to the points Peter has made on the size of the exit pupil another constraint is the size of the field stop that can fit into a 2″ focuser. You need a larger field stop at longer focal length.
By the way if you do get the exit pupil larger that that of your eye you just lose some light and have to shift your head to look around the field.
Regards Andrew
I agree with Robin’s comment in #5 . You can easily add noise or worse systematic error from the reference.
I simply use the W lamp to remove the blaze from my echelles spectra and then rectify just those orders I wish to measure. The echelles has a more complex set of IR issues than the Lhires III.
Regards Andrew
Tony are you using a single doublet or a pair of doubles for the transfer of the slit image to the guide camera?
What guide field of view are you aiming for?
Looking forward to the results, especially st ability of the printed component.
Regards Andrew
Yes Robin is right. I use a Shelyak guide head and it is a fine bit of kit. They can customise it’s focal ratio and hole size to better suit your telescope within a certain range.
Regards Andrew
If you go this way “What has come to mind are builders adjustable acrow supports. They have metal bases and heads that can be secured at the base to a concrete pad and at the head directly to the timber out-runner.” you will need to ensure they can’t tip over by bolting down he base.
Regards Andrew
Yes thanks to Andy and Dominic for their expert work.
May I ask what are the rules and or copy write issues on sharing these with other either directly or via other astronomy forums?
Specifically with me sharing the talk I gave but a more general answer would be appreciated as well.
Regards Andrew
I attended the joint BAA/AAVSO meeting stream and enjoyed a wide range of stimulating input. Made some new contacts and generally had an excellent time.
Regards Andrew
I use TSX for quiding on the hole in my fibre guide head. For the relatively bright stars I am observing I find 1 sec is often the max exposure to avoid saturation, even when the star is centered on the hole.
To avoid chasing the seeing I run the guider with a minimum move of 0.01 arc secs and very low aggressiveness setting (3 or below). This in effect integrates up the individual guide exposures and averages out the move. With the Paramount ME II and Protrack very little guide action is required. The guide graphs looks bad as the star edges the sides of the hole with my poor seeing conditions. The image has a strong stretch.
Regards Andrew
