Thoughts on which imaging rigs to concentrate on

[Gina]

Now 3D printing parts for the 55mm f1.8 lens - adapter ring for camera support plus focussing ring clip and quadrant gear.  I'll take and post a photo when finished.

[Gina]

Here's a photo of the new rig with 55mm f1.8 lens mounted on the EQ8.  Lens fitted with standard lens hood and then the dew shield I made for the 135mm lens added.  It wants some sticky tape to secure it though.

Edited September 20, 2016 by Gina

[Gina]

And here is the view across the valley to the far hill - full frame PNG.  Captured in SharpCap.  This is with the 3nm Ha filter.  Lens is at full aperture (f1.8).  Full resolution of 4656x3520 pixels is available on this upload - click image, click on "Full Size" in bottom left corner and click image for full size.  Visibility is poor today so the far trees are rather hazy.  The black area bottom right is the eastern wall of the observatory.

Edited September 20, 2016 by Gina

[wimvb]

That's a nice tree line nebula 

Btw, what's in the orange box on top of your rig? Focus control?

[Gina]

Yes, it is   Also, power distribution and fuses.

[Gina]

Amazingly, the sky is clear ATM   And I'm imaging at 55mm focal length - FOV about 20° x 15°.  Cygnus area.  30s exposures with gain of 500 and temperature -25°C.  Here is a single sub histogram stretched.  Full frame PNG.

[Gina]

Oops forget to flip the image to put it the right way round so here is a correction.  I've also tried a bit of noise reduction.  Rotated so that N is upwards.

[Horwig]

Now that depth is really promising with only 30 seconds, I imaged the same area in 2013 with a 55mm Pentax 67 f4 on my SX35 using 10 minute subs, whilst it did get deeper, it wasn't that much deeper.

 

Keeping my eye on this, nice work

 

Huw

[Gina]

Thank you Huw

Been having a look round the area of sky that's visible ATM and couldn't see anything more interesting than Cygnus so I decided to do a bit more on that, increasing exposures from 30s to 60s again with 100 subs in the run.  Here is just one of them - full frame.

[Gina]

Looking at the histogram it would seem that there's plenty of headroom to increase the exposures quite a lot further.  OTOH it's more a question of saturating certain areas.  Might just try a series if increasing exposures rather than just long runs at the same exposure. 

[Horwig]

Dug out my 2013 image, here's a single 10 minute sub, and a crop of roughly the same area, just histogram stretched in photoshop, no noise reduction or sharpening or anything fancy

[Gina]

Thank you for finding and posting that Huw

Here's a 120s = 2m exposure.

Edited September 20, 2016 by Gina

[Gina]

240s is too much so I'll go for collecting 120s subs for the rest of the night or until the sky gets poor/bad.

[Gina]

Actually 120s is just a touch too much so I'm going for 90s subs.

[Gina]

90s is just about alright.  Though the Crescent is just saturating on the brightest part - the rest of the image is alright.  Integration of these subs will show the best the camera is capable of with its 12bit ADC.  I think this is the reason this camera is better with many shorter subs than fewer longer ones.

[Gina]

Here is a 90s sub flipped and rotated and saved in PNG format, otherwise untouched.  This is the straight linear image as captured, just converted from FITS to TIFF, flipped and rotated then changed to PNG for upload.

[Gina]

A heavy mist has rolled in now so I guess that's it for tonight!  Oh well, I've done quite well - can't grumble   I'll shut up shop then and go to bed.  Processing will be tomorrow.

[Demonperformer]

Those images are showing really well how faint the veil is compared to NAN/Pelican. No wonder I wasn't getting much with the canon.

[wimvb]

Gina, when using different exposure times, do you adjust gain as well?

My understanding is that lower gain = more dynamic range = deeper images.

At unity gain full well = bit depth 12 bit = 4095.

Otoh, how would one utilise more than 12 bit when the output is limited to 12 bit? 

I'm confused about this.

[Gina]

At present I'm using a constant gain of 500 but this is something I want to investigate.  It's very unlikely that I have picked on the best gain setting by mainly luck and a little judgement.  A first thought is to match the Full Well with the ADC.  So we have Full Well of 20Ke and ADC of 12 bit.  12bit = 4096 ADU giving a gain of 20,000/4096 = 4.88 electrons per ADU.  The graph shows that a gain of 0dB gives 5 e/ADU so maximum dynamic range in each sub occurs with a gain of a little over 0dB, though I suspect that the graph is approximate and related to the gain that matches FW to ADC.

However, this is not "the whole story".  Image integration results in adding the data per pixel from all the frames stacked.  If we work in 16bit unsigned arithmetic the resultant ADU must not exceed 2^16 = 65536 which is 65536/4096 = 16.  In other words, if we were to use 0dB gain we would saturate the whites with 16 frames of exposure giving 65536 somewhere in the frame using optimum exposure per sub.  If we stack say 96 subs we can increase the gain by 6x = 7.78dB or to put it the other way round, a gain of 8db gives a ratio of 6.3 corresponding to a stack of 101 subs.  So for 100 subs we can use a gain of 8db.

All the above assumes we can use an exposure that will attain full well for a few of the pixels in the image (ignoring stars which we allow to saturate).  In the case of the data I have obtained for the Cygnus area with the 55mm lens, I estimate that 60s will just saturate some pixels in the Crescent Nebula - the brightest DSO in the FOV.  Now if I were to reduce the gain to 8dB compared with 50db I would need 42dB of extra exposure = 19953 times.  This would clearly be ridiculous.

That's enough for one post - I'll look at this from a practical point of view in another post.

Edited September 21, 2016 by Gina

[Gina]

If I'm right the S/N depends on the total exposure time of all the subs but it is limited by the read noise which is reduced overall by using a large number of short subs because the signal is additive whereas the noise increases by the square root of the number of subs.  Optimum is attained where the read noise equals the thermal noise which in turn depends on the temperature.  The ZWO CMOS sensors have a lower read noise compared with thermal noise than CCD sensors and hence benefit from more and shorter subs than CCD cameras.

Calculations on the theoretical amount of thermal noise is beyond me but I have read that there are ways of measuring it for the camera concerned.  I am going to look into this further and see what I can get for my ASI1600MM-Cool at a reasonably achievable set-point temperature.  So far this seems to be -25°C but I should be able to go lower when winter comes.  The limit for this camera is said to be -30°C so I'm already approaching that.

Edited September 21, 2016 by Gina

[Demonperformer]

FLO website states that the camera has a 'delta T' of "40-45 below ambient". So, come the winter when nights get down to about freezing, won't that mean that we should be getting down to below -40°C rather than -30? Or is there some other limit I am missing?

Not that (I suspect) this extra 10° will make an enormous difference at that point - law of diminishing returns?

Thanks.

[Gina]

I got that info from the ZWO manual.  Here's the relevant part of the manual - note the last line of what I've snipped :-

Edited September 21, 2016 by Gina

[Gina]

The manual doesn't show the noise below -20°C as shown in my next clip below, but my understanding is that thermal noise decreases all the way down to absolute zero, though the camera will cease to function well above that   I've certainly seen a reduction in noise between -20°C and -25°C at the gain I've been using.  The general rule is that the noise doubles with every 6°C rise in temperature, or thereabouts.

[Demonperformer]

Thanks, Gina, I hadn't seen that manual.

In truth, I never really did much exploration of the "cooling" aspect of the SX camera, so getting down to -30° will be a great step forward for me. But there does seem to be some sort of discrepency between "40-45 below ambient" (p7 of manual) and only down to -30° for cold winter nights. We shall no doubt find out over the next few months!

Also I note that I will be able to use fast speeds on USB2 for planetary, as long as I cut down the capture size. But, as most of our planets will fit on a 320x240 frame when taken through my 8" scope, that should not be too much of a restriction. I will have to do some experimenting, but we are more or less losing Mars & Saturn now and it will be a couple of months before Jupy is viewable in the evening.

Thanks.