Hi Kevin
The UVEX is a great instrument. It gives a huge range and is quite uniform from the UV up to the NIR. As I operate mine remotely, I don’t change the grating angle often, usually when I have a particular target that requires it - mine is set to cover the range from just above H beta to just below H alpha.
The motor unit from Shelyak is some months away. I spoke with Francois Cochard recently - he tells me that the hardware is designed and being finalised and they are tweaking the software. I am very keen to see it in action as it will give me the chance to use a higher resolution grating and take spectra at different intervals to get a better view of areas of interest.
Focus is an interesting issue. The UVEX is a pain to “tune” initially - there are 2 mirrors and a spherical lens to adjust when setting it up and an adjustment to one often impacts on another so you need to switch back and forth when tuning it. Once that is done, the focus is quite solid. I’m keen to have the ability to tweak the focus of the M2 mirror with the motor unit to make some fine adjustments but for the last 6 months I have not had to make any change.
The one issue that I know Shelyak are looking at is thermal stability. For some reason, the calibration shifts with changes in temperature. This doesn’t affect focus as such. Let me explain - its a bit detailed.
When I take an ArNe frame, I have a set of emission line wavelengths listed in a file. I use ISIS for processing. When a single line is selected in the calibration frame and the correct wavelength identified, ISIS finds a solution for all the other lines and this solution is then applied to all subsequent frames. The solution is a polynomial with several elements - the first 2 are critical. The first element (described as A0) sets the wavelength for the first pixel in the image. The next (A1) is the dispersion ie Angstroms per pixel.
What I find with the UVEX is that if there is a change in temperature between taking an ArNe frame and the first frame of the science images, A1 remains the same but A0 shifts. A real life example - I take an ArNe image then spend 10 minutes on the reference star. I then start taking the spectra images for the target. If the ambient temperature has dropped by say 0.5 degrees C, spectra calibrated with the ArNe frame will be out because the starting position for the solution doesn’t match the target - I will have a perfectly formed spectrum but it will be shifted to the red by a few Angstroms.
I overcome this by taking ArNe frames before each set of images. I avoid starting imaging till the equipment has properly cooled down (about an hour). And I use the wavelength calibration tool in ISIS - this addresses the shift in calibration during an imaging session by matching prominent features in each individual frame and adjusting the A0 element in the solution for that frame. Without this last step, you lose resolution as the tight lines in each frame smear out when stacked if they have shifted slightly.
As I said, Shelyak are trying to pin down the cause to address it with hardware. I don’t see it as an issue as it can be addressed with good technique and software.
Importantly, the ability to adjust the focus of the M1 mirror with the motor proposed will not affect this - that only addresses how sharp are the lines in each individual frame. Let me know if that doesn’t make sense.
If I were you and were looking at the UVEX, I don’t think the motor should be a precondition to diving in.
Another point to consider - if you are looking to extend into the near infrared, you will need a blocking filter. Without it, the second order of UV and blue light overlaps with the first order spectrum in the NIR which contaminates your result. This is why Shelyak supply a blocking filter which sits under the slit. This is not viable for me as I won’t drive 6 hours just to change a filter. The solution is to set up a filter wheel ahead of the UVEX and include a blocking filter there. I will be doing this when the motor is available. Let me know if you want details of the filter.
You can see more on this here: http://www.astrosurf.com/buil/download/asw2020.pdf
To your questions:
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I didn’t tinker with the CDK - I’ve managed to get down to about 3600A with the current set up - the response of your camera is probably more important than the scope itself. I have read that the CDK is a good match, particularly at f/8. The coma comes from the guider optics. It really isn’t designed for as wide a field as I use. I wanted a wide field to maximise prospects of plate solving and that works fine.
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See above. I do know that the motor is included in UVEX4 which is being developed. I have neither the skills nor the patience to involve myself in 3d printing or electronics - I’m a lawyer not an engineer
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I have borrowed an old Takahashi Sky90 from the operator of my observatory. That gives me a wide field (over 1 degree) and so plenty of scope to find comparison stars. I don’t use any reducer as I wanted to keep the focal length longer so there is some aberration at the edges of the field - but I’m not taking pictures for display so I don’t fuss with that.
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I am using the Alpy calibration unit as well as the Alpy guide unit. If you send me your email address I’ll shoot some pictures to you. I suspect the tweak you refer to is to stop down the reflective plate on the Alpy unit so that the ArNe light presented to the UVEX is closer to the focal length of the scope - the Alpy is optimised for f/5. I understand that this is done by adding some non-reflective black felt to the reflective plate. I had considered this - it would require longer exposures for the ArNe lamp and as these lamps have a finite life, this reduces their usable lifespan. And I’m impatient. In any event, I don’t think it would make that much difference in calibration.
An externally mounted calibration lamp would be better - as C Buil’s work shows. But its too fiddle for my set up at this stage. A remote rig needs to be as simple as possible and its already a jumble of wires and equipment. That said, I do plan to play with SpecInti soon and that opens up a range of other processing options. One method is to use atmospheric lines for calibration eg 5577 - which would mean I don’t need the ArNe lamp at all.
An eShell would be a better solution for a remote system. But to make it really worthwhile, I’d want to use the fibre optic system and then I’d need to buy a wine fridge to keep the instrument thermally stable. And after that I’d need to rig up some way to wriggle the fibres continuously. So with that and the modest size of my scope, its not really a fit for me. I prefer to go for dimmer targets where I can check out transients rather than look in depth at Ha lines in Be stars - I’m a low resolution fan I guess.
If you are looking at an eShell, you might want to look at the NOU_T and chat to Joan Guarro Flo. Its a lower cost option than the Shelyak instrument and seems to produce good results. Details are here:spectro-aras.com • View topic - First starlight for my new echelle
Ping me an email if you have any follow up questions - my email address is peter@oblaw.net.au I’m in Sydney Australia so if you wanted to have a chat by Zoom/Teams we’d need to find a time that works for us both.
Pete