UVEX - Shelyak Motor Unit

[Peter Velez]

I recently received the motor unit produced by Shelyak for the UVEX and have been busy testing and integrating it into my system. There may be other users considering this kit so I've set out below my initial thoughts on it.

I operate my system remotely - my rig is housed in a commercial observatory about 6 hours drive from me at Siding Spring Observatory, NSW Australia. While there is a technician onsite, he has over 25 systems to monitor and so I do need to work independently as much as possible. A key selling point for the UVEX for me was its ability to take spectra from the UV right through to the IR. However, to date I have made limited use of this range as changing the grating angle required the observatory tech to wander down to the shed and adjust the grating manually as I took images to check the adjustment.

So I was very excited to learn that the Shelyak motor unit was available a few months back.

I now have the unit installed and running smoothly. Shelyak has designed a program to operate the motor unit but as I have an automated set up, I use some basic Python scripts to run the motors. I have settled on 4 wavelength ranges - UV, Visual, NIR and IR with some overlap between them. If I want to adjust the grating, I change it, take a sequence of flat frames, slew to my target, take ArNe frames and start imaging. The motor unit is also able to control the Alpy calibration unit - my scripts allow me to take flats, ArNe and dark/bias images so these are also managed by my automation system.

The weather has not been kind to us in Australia this year (again!) and imaging time has been limited. However, so far I have been able to take 2 sequences of the same target at different grating angles - with my set up, this gives me a range of over 4,000 A at resolutions of up to 1500.

To illustrate, I attach a plot of 3 spectra taken of V618 Sgr - my current target of choice. The spectra were taken on 3 consecutive nights. My system controlled all aspects of the imaging process eg set grating, take flats, take ArNe and take the reference star and target images - my only task was to set it running each evening. The NIR spectrum's continuum doesn't quite match the other 2 - likely a flaw in my processing - so I have normalised the NIR to the visual. I have another spectrum of this target in the range 7500 - 9600 A but didn't add this as I have yet to determine how to normalise it correctly.

To say I am a happy camper is an understatement. This add-on now allows me to use the full capabilities of the UVEX.

The USIS Controller software allows the user to set the grating angle either by reference to the angle or wavelength. The wavelength is in turn set by the grating specification (ie 600 lines, 1200 lines etc) input by the user. What I found was that while the motor operation was repeatable, the reported wavelength didn't necessarily reflect the actual wavelength. For example, if directed to centre the grating on wavelength 4700 A, the unit in fact took the grating to 4554 A. If I moved to another wavelength and then back to 4700 A, it always returned to the same position - in this case 4554 A. I understand from Shelyak that this is because the gratings themselves are not quite accurately reported. So a 600 lines per mm grating may in fact have 580 or 620 lines per mm hence the error. For me this is not an issue. I notionally 'calibrated' my Python scripts by determining the required centre position in my script to get the wavelength where I want it.

My next task is to test the system out with the 1800 lines/mm grating. This would enable me to take mid-resolution spectra of particular regions eg Ha and Hb in the course of the same evening.

A key element of all this is the automation system I have built up in the last 6 months. It allows me to set my targets for the night and then head to bed knowing that (if the weather gods play nicely) I will have a hard drive full of data to process the next day. Its nowhere near as simple or elegant as the solution developed by the 2SPOT team but it works for me. As I am not a software engineer, I needed to use off-the-shelf software and have turned to Voyager to get the system running. But it works. I plan to prepare a separate post on that soon.

Pete

[Hamish Barker]

is there any facility for an IR-low pass filter for order sorting (to avoid UV 2nd order aliasing into the IR)? Or would one need to have a filter wheel between the telescope and uvex? i.e. upstream of the slit? I assume there would be insufficient backfocus between UVEX camera flange and science camera for a filter.

[Peter Velez]

Hamish

Shelyak supply a filter that sits behind the slit for IR and NIR observations. That doesn’t suit me as I want to be able to switch wavelengths without disassembling the instrument. So I have installed a FW between the telescope and UVEX (immediately before the guide unit) with a filter blocking shorter wavelengths than 5300 A.

It does the trick

Pete

[Thibault N]

Congratulations for setting this up !