ALPY 200 first light (yes 200 !)

[Robin Leadbeater]

For some time now I have had an aim to build a spectrograph capable of identifying supernovae discovered by amateurs. This needs high sensitivity (>mag17) but only at low resolution and SNR (R~ 50-80 at SNR ~20 would probably be enough) To follow this dream I have fitted a 200 l/mm grism to my ALPY, replacing the standard 600 l/mm version. (see attached photos)

comparing ALPY 600 and 200 grism

ALPY_grism_600-200.jpg (32.76 KiB) Viewed 8946 times

As well as concentrating the spectrum ~4x due to the lower dispersion, the 200 l/mm grating (made using the same master used for the Star Analyser 200) is significantly more efficent than the original 600l/mm grating (This is typical of transmission gratings where increasing number of lines significantly affects the throughput of the grating)

The resolution is only R~130 at 6000A, similar to that of the Star Analyser but sufficient for indentifying supernovae for example. Being a slit spectrograph, it has important advantages over the slitless Star Analyser of no intereference from other stars and spectra in the same field, a very reduced background from the sky or parent galaxy and what remains is easier to remove.

Attached are first light spectra of SN2014J and Nova Cep 2014 (at around mag 12.5-13) compared with the same objects a few days earlier with the ALPY600. The ALPY 200 exposures were 15min compared with 1 hr for the ALPY 600. To go much deeper I am going to need a better guide camera than my current DSI Pro


Cheers
Robin

ALPY200_sn2014J.png (8.43 KiB) Viewed 8946 times

ALPY200_novacep2014.png (7.2 KiB) Viewed 8946 times

ALPY200_novacep2014_detail.png (8.35 KiB) Viewed 8946 times

[Robin Leadbeater]

This is what a single 300 sec raw exposure looks like (SN2014J, ALPY200 C11 ATK314L+ 2x binned)

Robin

[Robin Leadbeater]

Here is the spectrum of the unidentified star accidentally caught in the slit just above sn2014J in the image above. Overlaid is a K0v Pickles spectrum for comparison. Based on the counts it is ~10x fainter than sn2014j so ~mag 15.5. The SNR is ~25 in 15min total exposure.

Cheers
Robin

[Ken Harrison]

Well done!
Another great addition to the amateur's arsenal.
I assume you used an Edmund Optics prism?

[Bernard Heathcote]

Excellent initiative Robin ... well done!
Assuming that replacing the ALPY grating is a feasible field modification (is it?) then the obvious question is why Shelyak haven't already offered this as an option, given the clear need for 'deep' spectra?

Cheers, Bernard

[Robin Leadbeater]

Hi Ken, Bernard

It is a bespoke piece of glass from Paton Hawksley matching the dimensions of the standard one in the ALPY but with the appropriate angle and with the 200 l/mm grating added using the SA200 master. It was mounted in a blank holder from Shelyak. I guess if there is enough interest it would be straightforward for Shelyak to offer it mounted in the holder as an alternative to the 600 grism. It means stripping down the core module to change it (you just have to make sure you dont tip out the other optics when you remove the grism and not to touch the grating of course) but this is straightforward enough and there is no difficult retuning to do for example.

Cheers
Robin

[Paolo Berardi]

Great instrument Robin! Did you choose the new guide camera for going deep with targets? I love SX Lodestar camera for its extreme sensitivity (see also the new model Lodestar X2). May be you should prefer a cooled cameras given that the exposure required to show mag 16-17 stars is a few tens of seconds...

I wish you a lot of spectroscopic confirmations!

Paolo

[Robin Leadbeater]

Hi Paolo,

Looking around there is a lot of choice for guiders these days. I tried my ATIK 16IC-S for guiding when I first got the ALPY. It has the same CCD as the Lodestar, has low noise read and is cooled. This would be ideal but unfortunately it is only USB 1.0 and very slow to download so difficult to use as a guider. I might try it again though, perhaps with cropped subframes if possible, before spending more money. (In the end I might sell both the 16IC-S and the DSI pro to fund a new camera.) I doubt I will be able to guide mag 17 stars on the slit though so I will need field stars for the faintest objects.

Cheers
Robin

[Robin Leadbeater]

I am now using the ATK16IC-S as a guide camera on the ALPY200. By using the subframe function in PHD2 I can get a fast enough update for guiding.

First test was on the famous gravitationally lensed QSO APM08279+5255 at mag 15.2 and z =3.87 which I had measured some years ago using the Star Analyser
http://www.threehillsobservatory.co.uk/ ... tra_22.htm

I could only see the target in >10 sec exposures in the ALPY guide camera so I first moved the target close to the slit using full frame long exposures and then started guiding using 3 sec sub frame exposures on a ~mag 13 star in the same field. I then moved the guide position 1 pixel at a time until the target could be seen in the spectrograph slit in 60 sec exposure images. The guide position was adjusted to maximise the signal in the slit and then a series of 600 sec exposures were recorded.

Attached is the result (a single 600 sec exposure and a sum of 9x 600 sec. click to view the 600 sec raw image)

apm08279+5255_20140418_ALPY200.png (32.87 KiB) Viewed 8734 times

This is also compared overlaid on the discovery spectrum from 1998 using the 2.5m Isac Newton Telescope from this paper
http://iopscience.iop.org/0004-637X/505/2/529

apm08279+5255_INT1998_20140418_ALPY200.png (44.9 KiB) Viewed 8734 times

The main features are clear even in a single 600 sec exposure and the 9x 600s spectrum is in good agreement with the professional spectrum, including features in the IR

Cheers
Robin

[Ken Harrison]

Robin,
Well done.
Some excellent results there.....
Impressive performance from your modified ALPY.