ZWO ASI1120 MC-S camera and USB extension cables

[Al-man]

I am quite new to atrophotography and am having problems using my recently purchased ZWO ASI120MC-S USB 3 camera. I hope 
someone on here can help. 
To get clear views of the sky my Sky-Watcher telescope is usually mounted just over 20 mtrs from the house. 
With winter approaching I would prefer to do my viewing on my desktop computer indoors.
After a bit of research I found that some people had success with these cameras using USB 3 active extension cables 
up to 25mtrs long. 
First I bought a 15mtr active extension lead and the camera connected no problem at all, so then 
I bought another 10mtrs long and connected them together. This time it was not detected by the computer via the USB 3 port,
but was detected when the lead was connected via a USB 2 port. Any ideas why this should be ? 
The USB 3 card on the computer has a lead connected to the PC power supply and the two USB 3 leads have ports for connection 
to an external 5v power supply. Is it likely to make a difference if I connect a power supply to these leads ?

I did some further tests using Sharpcap 3.1 and found when connecting the camera with just the short lead supplied to a USB 3 port,
I was getting 58 fps with a 640x480 image and about 118 fps in High Speed Mode, I then tested the camera on a USB 2 port, first using
the short lead and then the 25mtr extension and had the same results with no dropped frames !!. This doesn't seem right to me.
I also have a 20mtr RS-232 cable, could that be used with a USB to RS-232 adapter instead of using the USB 3 cables and if so would this be a better option ?

If I just use the USB 2 ports instead would the resulting images be ok ? 
I am looking for the best & cheapest option to get the camera working at about 20mtrs from my desktop.
I noticed that the resulting captured videos all showed the same 30 fps in video properties, is this due to a slow hard drive and would 30 fps be enough to produce a fairly
good image ?. I should point out that the camera was just on a tripod indoors, so I have no idea what pictures of the moon or planets would look
like using this camera due to cloudy skies.

Sorry for the long post and many questions.

Thanks in advance for any advice

Alan

[bottletopburly]

Try a usb 3 powered hub i think that will resolve your problem . and you will benefit from a SSD drive cheap as chips at the moment even if it is only a 240gb drive 

[symmetal]

USB3 devices when first plugged in negotiate with the host computer to determine the maximum data transfer speed they can cope with. The longer the lead used to connect the lower this max speed is. At 15m the USB3 will only manage about USB2 speeds I would think. An active extension only boosts the signal gain, it doesn't clean-up the signal (regenerate the square wave shape of the original signal). At 25m the signal shape is so degraded that it can't be recovered. USB2 is more forgiving as the square wave signals it generate are a lower frequency (wider square waves) so can just at 25m but they are near their limit.

A USB3 powered hub as bottletopburley suggests placed half way between the computer and camera will improve the chances of getting a good reliable data speed but it still won't be at full USB3 speeds with those cable lengths. You can't chain hubs together to increase cable distance as the signal delays through the hubs effectively degrades the signal and eventually it can't be read as it arrives too late. 

RS-232 is much lower data speeds so will not work at all at transmitting USB speeds. The USB/RS-232 adaptors are for transmitting low speed RS-232 data over USB.

The maximum frame rate from the camera is also determined by the exposure time and the speed the camera can read the frames not just the connection speed. Reducing the frame size only increases the frame rate up to that limit.

The external power supply connections on the USB card are only there to supply more current to the USB devices if required (1A for USB3 compared to 0.5A for USB2). The camera won't be taking a lot of current so adding the external power connections won't have much effect.

30 fps is quite reasonable for getting good images. It's not the frame rate that determines the image quality available but the exposure time. The lower the exposure time per frame the better the chance of freezing a sharp image. At 10mS exposure you can get up to 100 fps and at 5mS up to 200 fps (if the camera/connection allows it). If you get a bright enough image at these lower exposure times and you're limited to 30fps by connection speed, you just have to record your video for longer. If you record for 3 minutes 20 seconds at 30fps you get the same total number of frames as recording for 1 minute at 100 fps.

Alan (Another one )

[Al-man]

3 hours ago, symmetal said:

A USB3 powered hub as bottletopburley suggests placed half way between the computer and camera will improve the chances of getting a good reliable data speed but it still won't be at full USB3 speeds with those cable lengths. You can't chain hubs together to increase cable distance as the signal delays through the hubs effectively degrades the signal and eventually it can't be read as it arrives too late. 

 

Alan (Another one )

Many thanks for all the information symmetal and bottletopburly for his suggestion, very helpful. I will try a USB3 powered hub and see if that improves things.

Alan

[iapa]

I took the route of a stick PC (£130) installed at the mount - with remote connection via WiFi.

Only need to run power out to the mount & I get to stay nice and cosy on the living room