ALPY 600 limiting magnitude

[Robin Leadbeater]

For anyone intersted in purchasing the ALPY 600 I thought I would pull out the limiting magnitude figures. These are Christian Buil’s calculated figures, which seem to be matched in practise.

Note that a spectrum with 10 signal/noise ratio is pretty noisy but still useful for identifying features for identification of transients for example. SNR is roughly proportional to sqrt of the signal (neglecting camera and sky noise) so a low noise spectrum at say 100 SNR, sufficient for quantitative measurements to a few percent precision would need ~100x more signal or 5 magnitudes

Cheers

Robin

From http://www.astrosurf.com/buil/alpy600/performances.htm

Magnitude V limit  reached is based on a star of spectral A0V type, a signal to noise ratio of 10 (by FWHM) and a 1 hour integration time divided into 6-up 600 seconds each. The seeing is assumed to be 3 arc seconds. The camera is an Atik460EX model used in 2×2 binning (pixel binned 9.08 microns). Guiding module with slot 23 microns. The star is at the zenith. The observatory is located at sea level, but with a dark sky (magnitude 6.0 visual boundary to the naked eye).

Telescope	S / N 10 @ 650 nm	S / N 10 @ 450 nm
Refractor 85 mm f / 5.3	12.9	14.3
Refractor 106 mm f / 5.0	13.4	14.8
Newton CN212 (D = 212 mm f / 3.9)	14.8	16.2
Schmidt-Cassegrain 8-inch f / 10	14.3	15.7
Schmidt-Cassegrain 8-inch f / 6.4	14.6	16.0
Schmidt-Cassegrain 11-inch f / 10	14.8	16.2
Schmidt-Cassegrain 11-inch f / 6.4	15.1	16.5
Schmidt-Cassegrain 14 inches f / 6.4	15.4	16.8
Newton D = 940 mm f / 4	17.0	18.4

[Mr Nicholas John Atkinson]

Hi

Could you advise the limiting magnitude for a 150 mm Refractor

Thanks

[Robin Leadbeater]

Hi Nick,

The corresponding figures for a 150mm f5 scope under the same circumstances would be ~ 14.2/15.6

Cheers

Robin

[Tony Rodda]

V.useful Robin many thanks. Received my Aply600 a couple of weeks ago and I’m familiarising myself with it – and with ISIS.

(Although at the moment I’m having more success with BASS).

I’ve ditched my QHY5 guider in favour of a Lodestar which seems up to the job.  Its now a case of optimising the platform.  C925 With 0.63fr and Atik460.  Lots of success with brighter objects.

I can identify (and track) targets to the mags you outline but the biggest challenge is just finding the slit to put those faint objects in!  Frustrating when you take a long exposure for a faint star only to find it’s not quite “in the grove”.

I’ll start posting ‘first efforts’ asap.

See you at the workshop later this month?

Regards

Tony

[Robin Leadbeater]

Hi Tony,

Yes I will be there. I have some techniques which we can discuss there which should help get your fainter targets on the slit. (Most targets with my modified ALPY 200 are so faint that they are not visible in the guider image at all.)

Robin

[Robin Leadbeater]

If you switch a calibration lamp on (or in twilight if you don’t have the calibration module) you can see the slit in the guider image. I use PHD2 for guiding and note the XY position in pixels of the point on the slit where I want to place the star. If everything  is screwed down tight the spectrograph is sufficiently stable that I can then set that location in PHD (or an offset from that location if guiding on a field star, the offset having been measured on a stack of longer exposure guider images)  

Robin

[Steve Cuthbert]

Hi Tony

I don`t know if you have tried Astro Photo Tool (APT) which was primarily meant for dslr`s but now has ccd option. I have it running twice on screen to show 1- my Atik 314 with the Alpy and 2- My Lodestar with the guide module. Apt has a Histogram option with the Lodestar live view and can stretch the intensity to show the slit under fairly dark conditions and can auto lock the intensity so the slit is always visible. It also has a `single pixel` intensity option so you can tell if over exposed or not. I`m currently having luck with the French control software `Audela` too!

Steve

[Tony Rodda]

Hi Robin, Yes thanks, I did something similar by flashing a red torch into the scope whilst taking a sighting exposure.  The slit showed up fine and I marked it with the PHD mask as suggested.

I’ve noticed a few shadows and elongated stars in exposures and the Lodestar field seems slightly offset. I guess that it’s simply down to the OAG mirror(s) alignment.  I took a quick 15 sec exposure of M15 to ‘see what I could see’ and the attached frame clearly shows the dead space.  Is that common or do I have to work on the alignment and focusing to perfect?

Regards

Tony

[Tony Rodda]

Thanks Steve.  I’ll try both.

Regards

Tony

[Robin Leadbeater]

Hi Tony,

I can’t see the attachment for some reason (was it below the maximum 640×48 size?) but my field has significant off axis coma, something I plan discussing with Francois at the workshop. I believe it is a feature of the guider optics and it is ok close to the axis where the slit is but I would be interested to explore if it could be improved as sometimes I would like to be able to use guide stars well off axis. I have attached an example  guider frame from my setup

The shadowing might be because the mirror slit in the ALPY module is not quite aligned at the optimum angle relative to the guider. You can rotate the ALPY module to optimise the illumination and then rotate the guide camera to align the slit in the field.  My slit is slightly offset. I am not sure where this arises, possibly in the camera coupling.  These will be good subjects for the workshop however when we have Francois to demonstrate how to set the spectrograph up optimally.

Cheers

Robin

[Tony Rodda]

Thanks Robin.  I’ve got something very very similar.

Regards

Tony

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