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Old 16-04-2013, 10:09 AM
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SX icx694 first impressions

Have been working up the first element of a galaxy imaging system – a Starlight express 694 camera. Am still waiting on a coma corrector and for the EQ8 to appear and have not yet finally decided on the scope, so initial testing was with an 8 inch f4 Newtonian on a tripod mounted EQ6, without coma corrector and with a 50mm guidescope.

The EQ6 was out of its comfort zone at 1.17 arcsec per pixel in wind. Modelling had shown that the optimum sub (for maximum SNR) was 2 minutes, but that the system should still be effective with subs down to below 20 seconds, so short subs were used to help overcome mount deficiencies. The attached image is a native scale central crop with a bit over 30 minutes total imaging time using both 20 and 35 second subs in about 2.7 arcsec FWHM seeing+tracking. The only processing used was a stretch and modest deconvolution/sharpen to remove the worst of the remaining tracking excursions (decon pushed some stars into saturation and upped the apparent noise). No calibration (dark, bias, flat) of any sort was used and the dark current can be seen as a few horizontal drift lines from individual warm pixels (deliberately retained in stacking) – that really is the total extent of obvious thermal noise straight off the chip. There is a large amount of headroom with short subs and only three stars in this image reached saturation. I clearly needed more than 30+ minutes of data, but clouds and wind ruled that out – the results are still promising for first light.

Overall, am very pleased with the camera - works exactly as advertised. The modelling was also spot on and accurately predicted how it would perform in a system. Very short subs are practical – sensitivity is high and noise is very low. Image scale is OK with the 200mm f4 and will be about right with the final scope (10 or 12 inch f4). This fairly crudely processed technical image is the first step towards the new system – there will be lots of hurdles to come, but so far so good….

Thanks for reading. Regards ray
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Last edited by Shiraz; 20-04-2013 at 07:36 AM.
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Old 16-04-2013, 10:29 AM
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That's very impressive Ray.

Greg.
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Old 16-04-2013, 02:10 PM
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That's very impressive Ray.

Greg.
thanks Greg - quite a way to go yet, but it is looking pretty good as a first step. Also, thanks very much for your thoughtful advice earlier on, when I was trying to work out what to do - much appreciated. regards ray

Last edited by Shiraz; 16-04-2013 at 06:24 PM.
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Old 16-04-2013, 04:21 PM
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A very decent result given short subs, and a total of half an hour, very decent indeed.
Bodes well.
Gary
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Old 16-04-2013, 08:27 PM
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Nice image Ray, well done
and nailed the processing to.
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Old 16-04-2013, 11:18 PM
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Hi Ray,

That looks great. Are you able to post a full frame image?
The specs say that full well capacity is greater than 20k. Does that mean it can't register anything over that? Is this related to the number of shades of grey that can be captured and represented?

How would it perform with objects such as the horse head or Orion that have a wide dynamic range?
Was that on a stock standard gso 8" F4?

Cheers
Alistair
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Old 17-04-2013, 08:34 AM
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Originally Posted by gbeal View Post
A very decent result given short subs, and a total of half an hour, very decent indeed.
Bodes well.
Gary
Hi Gary. Yes, this camera allows short subs because of the low read noise and high QE. with the f4 scope, the optimum sub is 2 minutes for broadband imaging.

I am tossing up between a steel and CF scope - do you find that CF is worthwhile, particularly for focus stability?

Quote:
Originally Posted by Troy View Post
Nice image Ray, well done
and nailed the processing to.
hi Troy - thanks for the comment

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Originally Posted by alistairsam View Post
Hi Ray,

That looks great. Are you able to post a full frame image?
The specs say that full well capacity is greater than 20k. Does that mean it can't register anything over that? Is this related to the number of shades of grey that can be captured and represented?

How would it perform with objects such as the horse head or Orion that have a wide dynamic range?
Was that on a stock standard gso 8" F4?

Cheers
Alistair
Hi Alistair
.
resampled full frame posted - hope you like coma.

well capacity is a measure of how many electrons can accumulate in each pixel before the ABG overflow mechanism kicks in and drains excess charge. the number of shades of grey is determined by the free space between this upper limit and the lower limit set by the read noise (this free space is the dynamic range). As a rough generalisation, most chips in wide use produce 16 bit signals, but have similar dynamic ranges of about 12 bits (4096 shades of grey) - the Sony chips have relatively low well depth, but low read noise - the Kodak chips generally have deeper wells, but higher read noise. To put this in perspective, the icx694 has more dynamic range than a K8300, about the same as a K11002 and less than a K16803.

This camera should perform about the same as most others on any target, except that, at the same pixel scale, it will take less time to get to a given signal-to-noise level, because of its high QE and low noise.

The scope is a standard GSO with the following mods:
- tube coated in adhesive Al foil
- partially flocked
- custom secondary holder with larger secondary
- strengthened tube around focuser
- strengthened GSO focuser with SharpSky drive
- primary fixed with adhesive tape and clips removed

regards Ray
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Last edited by Shiraz; 17-04-2013 at 09:42 AM.
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Old 17-04-2013, 09:07 AM
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Ray.
Steel Vs C/F? I have the AstroTech 8" f4 Imaging Newt, it is steel, and heavy too. It would definitely be "nicer" in C/F. But would it be better? Not sure.
The home-brew 10" F5 Newt I do have in a C/F tube, home-spun tube. Better? Again, not really sure, but t my mind t's better than the sono-tube it started out in.
If focus stability is the concern, then maybe. But I'd be looking to train my system and use temp compensation, and the appropriate software. I have the relevant bits for the temp-comp, for the F/T focusers I use, but have never used it. Too hard for me.
Might be an option for you though.
Gary
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Old 17-04-2013, 09:37 AM
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Ray.
I have the relevant bits for the temp-comp, ....but have never used it. Too hard for me.
Might be an option for you though.
Gary
I also have a temp compensating focuser, but have been putting off trying to calibrate it - as you say, looks hard. Will give it a try next clear night

thanks, Ray
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Old 17-04-2013, 10:53 AM
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My testing with my SXVR-M25C and an NEQ6 have shown, as you have, that subs below 5 minutes are optimal - my optimal figure was around the 3.5 minute mark in nil wind - to avoid slight elongation.

The image below of M83 I did was similar to yours in many respects. You can notice very slight - yet to me unacceptable - star elongation as these are 5 minute frames.

Having switched to a Vixen GPD2, I will have to retest to achieve optimal, but with a PEC of apparently sub-15, I am anticipating at least 4 minutes per frame (fingers crossed). Also using a 50mm guidescope.
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Old 17-04-2013, 12:09 PM
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Originally Posted by LewisM View Post
My testing with my SXVR-M25C and an NEQ6 have shown, as you have, that subs below 5 minutes are optimal - my optimal figure was around the 3.5 minute mark in nil wind - to avoid slight elongation.

The image below of M83 I did was similar to yours in many respects. You can notice very slight - yet to me unacceptable - star elongation as these are 5 minute frames.

Having switched to a Vixen GPD2, I will have to retest to achieve optimal, but with a PEC of apparently sub-15, I am anticipating at least 4 minutes per frame (fingers crossed). Also using a 50mm guidescope.
Thanks for that info Lewis.

The “optimum” sub exposure that I use is based on maximising the signal to noise ratio – it is not dependent on the tracking. But of course there is no point in using long subs if the tracking goes to pieces.

The basis for the procedure is summarised very well on the Starizona website, but the underlying idea is that you can minimise the read noise contribution to the total noise by making subs as long as possible (to minimise the number of read events). The accepted rule of thumb is that subs are “long enough” when the read noise contribution is below ~ 5% of the total noise.

I ran your 102 system through my spreadsheet – purely on the basis of signal to noise ratio, you will do best with subs of about 20 minutes for the native focal length and a bit under 10 minutes with the focal reducer. You can use shorter subs if you increase the total imaging time. I have a QHY8 which has the same chip as your camera and with my f4 scope it works best with at least 5 minute subs - this is based on practice as well as theory.

Hope your new mount works out well - specs are good.

regards Ray

ref:http://starizona.com/acb/ccd/advtheoryexp.aspx

Last edited by Shiraz; 17-04-2013 at 12:26 PM.
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Old 17-04-2013, 12:18 PM
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Ray,

Thanks for that.

I have - on the odd occasion - been able to handle doing 20 minute exposures with the NEQ6, after PEC training etc. That was with an EOS DSLR.

I have not tried long subs with the SXVR-M25C, but - fingers crossed - if the weather holds here tonight, I will be trying 10 min subs using the focal reducer (assuming I got the FR to chip distance right!)

With all my prior imagery - since the SX is new to me - I merely kept at 5 min subs and increased the overall time on target proportionately.

I must say, SX cameras impress me significantly.
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Old 17-04-2013, 12:32 PM
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Lewis - I should have added that the sub times also depend on the sky brightness - the figures I gave are for dark skies. If you have bright sky, you can use shorter subs and not mess up the SNR - it will already be bad and a bit more read noise will not make much difference. Regards ray
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Old 17-04-2013, 01:20 PM
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My light pollution is SEVERE, so yes, I use shorter subs on average, which means even more subs in the long run. Fun juggle act.
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Old 17-04-2013, 02:06 PM
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Quote:
Originally Posted by LewisM View Post
The image below of M83 I did was similar to yours in many respects. You can notice very slight - yet to me unacceptable - star elongation as these are 5 minute frames.

Having switched to a Vixen GPD2, I will have to retest to achieve optimal, but with a PEC of apparently sub-15, I am anticipating at least 4 minutes per frame (fingers crossed). Also using a 50mm guidescope.
Hi Lewis, Looking at your test, your elongation appears to me to be on the left hand side of the image but not on the right so I dont think thats your mount. Tilt?
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Old 17-04-2013, 02:41 PM
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David,

It is right through. There is likely MORE tilt left than right, but still present. The SX cameras have a "collimatable" front face, so I will get to do that if the problem persists.
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Old 17-04-2013, 03:52 PM
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Originally Posted by Shiraz View Post
The basis for the procedure is summarised very well on the Starizona website, but the underlying idea is that you can minimise the read noise contribution to the total noise by making subs as long as possible (to minimise the number of read events). The accepted rule of thumb is that subs are “long enough” when the read noise contribution is below ~ 5% of the total noise.

I ran your 102 system through my spreadsheet – purely on the basis of signal to noise ratio, you will do best with subs of about 20 minutes for the native focal length and a bit under 10 minutes with the focal reducer. You can use shorter subs if you increase the total imaging time. I have a QHY8 which has the same chip as your camera and with my f4 scope it works best with at least 5 minute subs - this is based on practice as well as theory.
Thanks for the info Ray.
didn't pay much attention to these aspects before. it was more winging it.
it makes sense now cause the other day, I tried a 5 min sub of M83 on my 8inch F4 with the qhy8 and then a 10min sub. There was hardly any difference, but it was less grainy on the 10min sub, I'm guessing because there was slightly more signal. I've noticed that with 2 and 5 min subs. the mild graininess disappears with the 5 min sub.
would you have a link to the SS?
Else I'll the online calc at starizona.
I have a qhy8 and an 8inch F4 and a 10inch F4.
whilst determining exposure time, shouldn't it be based on the sky background measurement as well as explained in the site?
so the optimum exposure length would vary based on seeing conditions as well?

the specs on the qhy8 are
Spectral Response:
QE max at 540um ~60%

Read Noise:
less than 11 -e RMS (typically less than 9 -e)

Full-well capacity:
Greater than 25K -e un-binned

so does this mean the dynamic range is between 11e- and 25Ke-?

Cheers
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Old 18-04-2013, 01:01 PM
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Originally Posted by alistairsam View Post
Thanks for the info Ray.
didn't pay much attention to these aspects before. it was more winging it.
it makes sense now cause the other day, I tried a 5 min sub of M83 on my 8inch F4 with the qhy8 and then a 10min sub. There was hardly any difference, but it was less grainy on the 10min sub, I'm guessing because there was slightly more signal. I've noticed that with 2 and 5 min subs. the mild graininess disappears with the 5 min sub.
would you have a link to the SS?
Else I'll the online calc at starizona.
I have a qhy8 and an 8inch F4 and a 10inch F4.
whilst determining exposure time, shouldn't it be based on the sky background measurement as well as explained in the site?
so the optimum exposure length would vary based on seeing conditions as well?

the specs on the qhy8 are
Spectral Response:
QE max at 540um ~60%

Read Noise:
less than 11 -e RMS (typically less than 9 -e)

Full-well capacity:
Greater than 25K -e un-binned

so does this mean the dynamic range is between 11e- and 25Ke-?

Cheers
Hi Alistair

I haven't made my spreadsheet available anywhere - might do so soon.

suggest that you use the Smith equation on Starizona rather than the calculator - you need to take off bias/pedestal signal to get the true background sky and the calculator assumes a pedestal of 100. as you know, you can set the pedestal anywhere you want with the QHY8 and you can also change the gain any way you want - the calculator assumes some sort of fixed gain for a given camera.

to get around these problems, I used a typical dark sky background brightness in my spreadsheet, rather than measured data on a specific night, and went back to basic CCD parameters - that way you can work out how long the optimum subs are under dark skies. If sky is brighter you can use shorter subs, but you will not stuff up by using the longer ones.

the dynamic range is indeed between 11e and 25ke. it is usually expressed as the ratio of the two measures - ie about 2200. this is a bit less than some other cameras, but still pretty good.

be wary of the quoted QE for OSCs. this is the maximum inband QE for any of the Bayer filters. At a given wavelength, the ccd can have a QE of 0.6, but if illuminated with broadband light it will only detect about <.2 of the total incoming photon flux in any pixel. This is not a fault, but makes it more difficult to compare OSCs and mono cameras on anything like an apples-to-apples basis.

as I see it, the optimum sub calculation is a guide only - you can use what you want. But it does give you a suggestion for the best combination of sensitivity and exposure times for your setup.

And on the original topic of the thread, my expensive new SX 694 camera just died - Peltier cooling stopped working. whoopee... So now I find out how good the after sales service is while all the interesting galaxies head steadily westwards.

regards Ray

Last edited by Shiraz; 20-04-2013 at 07:33 AM.
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Old 26-04-2013, 07:19 PM
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Hi Ray,

Great info, cheers for that. I too am in the process of getting my next setup together. Also waiting on an EQ8 and will probably get a GSO12" I was planning on upgrading the tube later on down the track, teleskop shop has carbon fibre tubes you can purchase to replace the steel tube. So that is always an option if required.

Best of luck,

Rusty
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Old 29-04-2013, 10:57 AM
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Shiraz (Ray)
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Hi Ray,

Great info, cheers for that. I too am in the process of getting my next setup together. Also waiting on an EQ8 and will probably get a GSO12" I was planning on upgrading the tube later on down the track, teleskop shop has carbon fibre tubes you can purchase to replace the steel tube. So that is always an option if required.

Best of luck,

Rusty
thanks for that info Rusty. The short Newt scopes open up a lot of opportunities don't they - particularly if the EQ8 arrives some time. regards ray
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