FLI: 8MHz vs 1MHz

[Geoff45]

The FLI Proline 16803 has two download speeds--the fast one at 8MHz which is lighting fast and the slower 1MHz speed. FLI warns that 8MHz is noisier, so I checked this. In the graph attached the left hand plots are 8MHz downloads and the rest are the 1MHz. Eyeballing the graphs suggests that the average noise goes down from around 12.5 ADU in the 8 MHz case to about 10 ADU in the one MHz case--a reduction of 20%. This indicates to me that 1MHz should definitely be the speed of choice--obvious of course, but it's nice to see agreement with one's expectations.
Geoff

[RickS]

Interesting comparison, Geoff!

I did read noise measurements with the PL16803 my group is using at SRO and got 8.4e- read noise at 8MHz and 5.9e- at 1MHz.

The impact of this is that it would require subs twice as long at 8MHz than at 1MHz to get sky limited (8.4^2/5.9^2).

Cheers,
Rick.

[Atmos]

I did a similar test with my QHY9, 8.5e- at normal readout at ~12 at high, still incredibly slow download compared to the FLI though, 12 seconds at normal for an 8.3MP.

[alpal]

I have an interesting photo - below -to show for this thread.
You can see the difference in the graphs between
on the left - low speed readout for my QHY9 mono &
on the right -high speed readout.

At -30 degrees DARK FRAMES:
The low speed is giving a median noise of about 1025
& the high speed median of about 4200
out of a maximum of 65536.

4200 is just at the area of sky noise so obviously
the low speed readout is essential.
The high speed readout should only be used for quick captures to see the target but never for real data.
Unfortunately I did not know this & all of my pictures have been taken
with the high speed readout.
If only I would have read the manual more closely -
I would not have made this mistake.
Really - the QHY9 capture program called EZYCap should not allow you the option
of high speed for capture - except only in preview or focus mode.
I expect my pictures in the future to be a lot better knowing this
& also the fact that I have upgraded from an 8" f6 to a 10" f4 Newt. -
the difference should be huge.

cheers
Allan

[RobF]

Can relate to your frustration Allan. After loading new drivers pre-astrofest I collected days of data on the high speed setting in error. For LRGB its not ideal, but really starts to bite for narrow band. :sadeyes:

[alpal]

Hi Rob,
Really - so I won't see a massive difference now?

You know something - it's quite a surprise to me
because the system is digital -
you wouldn't think it would make any difference but my picture doesn't lie.


cheers
Allan

[Atmos]

With the large increase in ADU between low and high readout on the QHY9, all you need to do is reduce the Offset. At Normal I use an offset of 106 but during high speed (what I use for focusing and modelling) I put it down to 76.

[RobF]

The trouble with the QHY9 on high speed is you get column artifacts too.
Definitely better to always use normal speed. I'm sure I've read somewhere on the QHY forums slow speed not necessary on modern drivers.

He's an example in Ha taken with high speed - columns were prominent in dark areas, particularly bottom right until I black clipped image a bit harder. Hardly a right-off though.

http://www.iceinspace.com.au/forum/attachment_browse.php?a=187199

And final HaLRGB - all taken on slow speed. Veil is a reasonably bright object and astofest skies nice and dark

http://www.iceinspace.com.au/forum/attachment_browse.php?a=188812

[alpal]

Hi Rob,
I never noticed any columns on high speed data with my QHY9m.

cheers
Allan

[Atmos]

I have compared the read noise of both the Normal and Slow, found there was a lower read noise in the Normal readout speed, plus it downloaded 4s faster! 12s vs 16s.

[alpal]

The fast download from the QHY9m is about 2 seconds.

I don't understand why there should be any difference -
after all - it's digital.

[Atmos]

Mine takes 4s in High mode and 12s in Normal.
I cannot say why the offset needs to be changed, I can only assume that it is because of the higher read noise.

[alpal]

OK - I don't understand where the read noise comes from but anyway -
we can be sure that you need to use slow download speed
for data you want to keep & process.

[Atmos]

Normal read out gives cleaner data than the Slow, lower read noise. Pretty sure I remember reading on the QHY forums a while ago that it is supposed to be used on the normal readout for optimal results.

[alpal]

The picture I posted below shows that the readout noise is higher on high speed.

[Atmos]

Sorry, my bad! I am thinking about the ASCOM driver for the QHY9 which has High, Normal & Low readout speeds. The Low readout speed is worse than the Normal, slower download AND more noise.