Which camera ?

[Mr Jack Martin]

I have an atik 314 monochrome CCD Sony ICX285 resolution1392 x 1040 pixel size 6.45 x 6.45 uM adc 16 bit readout noise 4e cooling delta -27 

and

lodestar CCD  –  Sony ICX429AL ExView monochrome CCD
Pixel count  –  752(H) x 580(V)
Pixel size  –  8.2 x 8.4uM
Download rate  –  Approx 10 frames per second in binned 2 x 2 mode

I am currently using the lodestar as a guide camera.

Which camera would work better and why, as a guide camera for my modified Lhires III ?

Regards,

Jack

Essex UK

[Robin Leadbeater]

Hello Jack

What limitations are you trying to overcome compared with the Lodestar ?  If you are looking to guide on the faintest stars, the 314 will probably go a bit fainter with a few seconds exposure. (I tried a Lodestar with my ALPY 200 but returned it as my old cooled 16ic-s would guide on fainter stars)  On bright stars though you might have to use the cropped frame feature to get a decent update speed. Also what main camera are you using ? I dont think for example you can fit two 314 sized cameras side by side on the LHIRES. Anyway since you have both why not try them and let us know which works best?

Cheers

Robin

[Mr Jack Martin]

Robin,

I am looking to guide on fainter stars and more sensitivity.

The lodestar is getting on, and I think there are better cameras for this purpose, correct ?

The main camera is a atik 460.

What about CMOS cameras would they work better than CCD for Spectroscopy ?

Regards,

Jack

Essex UK 

[Dr Andrew Smith]

Have a look at Christian Buil’s site he has evaluated several CMOS cameras for spectroscopy. Cooled CMOS should certainly be ok for guiding. Even NASA is going to fly a CMOS camera on its Parker Solar Probe mission.

Regards Andrew 

[Andy Wilson]

Hi Jack,

Another important consideration is the field of view. From memory I think you are using a 14″ Celestron or Meade. This will have a long focal length so the field of view in your guide camera will be quite small.

Also, spectrograph guide optics are not that great, though the optics in the updated Lhires III from a few years ago are much better. I think you have one of these models? The point here is I am not sure how wide a usable field of view you get from the guide optics.

The best way to find out would be to try each of the cameras you mention. The Atik 314 will give you a larger field of view, so that can be used to find out if there is a usable wider field than you get in the Lodestar.

The frame rate should not matter. I have never used a frame rate faster than 1 image every 5 seconds for guiding, and up to 30 seconds in a field without a bright star. Anything faster and you risk your mount chasing the seeing rather than guiding. I think you have a good quality mount, so guide exposures of 10’s seconds should not be a problem. So you don’t need the high frame rate from CMOS chips. However, a fast download speed (CMOS or CCD) is an advantage as you can start the next guide exposure quickly.

My personal experience is the Lodestar is very good for guiding. It shows a few hot pixels as it is an uncooled camera, but they’ve never caused me major problems. If replacing the Lodestar with another camera, then as pointed out by Robin you need to be careful that the guide camera is small enough to fit next to your spectrum imaging camera. You might also want to consider the weight depending on the kind of focuser you are using.

Best wishes,

Andy

[Mr Jack Martin]

Andrew,

Thanks for the recommendation I will check it out.

Regards,

Jack

Essex UK

[Mr Jack Martin]

Andy,

I use a Celestron C14.

The Lhires guide optics were updated a couple of years ago by Olivier, so the guiding is good.

The mount is a Paramount MX.

I use PHD or PHD 2 guiding looping between 0.5s or 1.0 s exposures.

The 314 will fit ok next to the 460 imaging camera.

I am using a Moonlite focuser, but don’t know if it will take the weight ?

Regards,

Jack

Essex UK

Testing both cameras is a good idea.

[Andy Wilson]

Hi Jack,

You have a high quality mount, so as long as it is reasonably well polar aligned you should be able to use exposures of 5 to 30 seconds for guiding, or possibly longer.

I never guide with exposures less than a 5 seconds as otherwise the mount is “chasing the seeing” due to atmospheric turbulence. This is the small scale scintillations that cause a star image to move and change rapidly. Any command sent to move the mount to chase this ‘seeing’ will be too late as by then the atmosphere will have changed and the star moved to a slightly different position. To get around this for spectroscopy would take professional grade adaptive optics, but it is not necessary for the spectroscopy we do.

Best wishes,

Andy

[Mr Jack Martin]

Andy,

The polar alignment is good enough.

Are you saying that I need to use exposures between 5-30s on PHD guiding ?

Not sure if PHD guiding goes up to 30s.

Regards,

Jack

Essex UK

 

[Andy Wilson]

Hi Jack,

That is a good point, I think PHD only goes up to 15 second exposure times. I would suggest 5-15 seconds is fine, unless you are trying to guide on a very bright star.

I expect some people guide with shorter exposures, but I doubt there will be anything to be gained from guide exposures shorter than 5 seconds on your setup.

The longer exposures times of course have the benefit that they allow you to guide on fainter stars.

A good way to find out is to experiment. You can take spectra of the same star with guide times varying from 1 second to 15 seconds and see if you spot a decrease in the signal or an increase in noise.

Best wishes,

Andy

[Dr Andrew Smith]

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

 

[Paul Luckas]

Also, if you use TSX for guiding you can view the guiding corrections sent to the mount (right click on the auto-guiding graph and select “graph corrections sent to mount”).  Aggressiveness can be adjusted while guiding in TSX, so you can view the effect of changing the values by viewing the graphed corrections in real time (the green lines on the screen shot below).

Paul

 

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