Should the guide scope be on its own USB cable?

Fri, 12/10/2021 - 21:45

I am refining my telescope configuration and doing some re-cabling.  I have put the main camera on its own USB cable to keep all that traffic from interfering with other functions of the telescope.  For exoplanet work where precision of the guilding is of primary importance I was wondering if the guide scope should also be on it's own USB cable going to the computer.  What do other exoplanetary observers do?  Any thoughts?

guide scope

Hi Ed,

What is your equipment configuration now?  Note that a separate guide scope will have flexure and the stars on the science sensor will not necessarily fall on the same pixel during a long run.  Also, a GEM flip will negate the same-pixel requirement, so forks or alt-az-rotator systems are normally preferred if wanting to meet the guiding requirement.

I know that on the old SBIG cameras, guiding was disabled while the shutter was closed since the guide chip was adjacent to the science chip.  For other configurations, does MaximDL continue to guide throughout the exposure sequence?


Hi Arne,I used the wrong…

Hi Arne,

I used the wrong terminlolgy..  I meant to say "guide camera".  Right now I am using a standard off-axis guide camera, but I am replacing that with an on-axis guide camera from Innovations Foresight.  Here is a picture of the current setup:  

Originally, Planewave had plugged the USB for the main camera into a local hub on the telescope and that hub led to a second hub before it reached a USB port on the computer.  I pulled all of that out and just ran a single USB cable directly from the main camera to a single USB port on the computer which is issolated from other USB traffic.  That was to prevent a traffic jam on the USB cable every time an image downloaded.  In particular, guiding signals could not be compromized during the 8 seconds it takes to download an image.

The other USB subsystems on the telescope are very light users of USB (focuser, heater control, telescope control, mount control) so I left them in the local hub that Planewave had installed on the telescope and I ran all of them down to a single, issolated USB hub on the computer.

It would be easy for me to connect the guide camera to that local USB hub also, but that camera does download .fit files on a constant basis so it might cause some significant traffic on the cable.  I am wondering if it would be best to also put that guide camera on its own USB connection.


Guide camera

Hi Ed,

It would not hurt to route the guide camera USB separate from other USB devices, but I doubt that interference occurs very often if at all.  Usually guide cameras are small format sensors that read out very quickly.  Images are downloaded in native format with the driver, and then some higher-level program on the computer reformats into FITS as needed.

I route my science camera separate from other USB devices, in a similar manner to what you are doing.  My cameras are CMOS and use USB3, which makes it a little tricky for extension.  Currently I'm using 3m USB3 cables and have the computer adjacent to the pier in the observatory.  The computer seems to be reliable to -20F.  The cables are the bigger problem at cold temperatures, since they lose their flexibility.

Nice setup!


Extension of the USB cables…

Extension of the USB cables has also been a problem for me.  I implemented an Ethernet connection between the telescope and the computer.  It is good up to 200 feet.  USB is only reliable for 16 feet.  So far this seems to be working fine.  This is the adapter I used: 

AmazonSmile: U-Link Ul10 by Sewell, USB 2.0 Over Single CAT5E/6 Extender, 200 ft, 480 Mbps, 4 Port - V2.0 : Electronics

The adapter was made for USB2 but it will probably work for USB3.  I decided to put the main camera on a single USB connection and the guide camera on its own separate USB connection (both implemented with the Ethernet adapter for distance).  I also put each camera on its own USB interface on the computer.  It is important to remember that a single USB root hub inside the computer provides two USB interfaces.  So you will want to make sure to put each camera on its own USB root hub.  Then there is no possibility that anything on the telescope could interfere with either the main camera or the guide camera.



American Association of Variable Star Observers (AAVSO)
FWIW  I am trying to shoot…

FWIW  I am trying to shoot unguided.  My software bisque mount and The Sky X T point model can track quite well once I have things well tuned.  Although I am grounded by the weather lately I was able to expose 6 minutes with no drift.  Appreciate the ideas about dedicated USB cable to computer . . .this may explain some of the interference I am getting with USB hub on the mount

American Association of Variable Star Observers (AAVSO)
I use the Cat-5e also

George, over at Astro-Physics, sold me an ICRON USB 2.0 Ranger 2304 when I bought the 1100GTO. (Thankyou George) Similar to the SEWELL unit you have, which claims to drive 200 feet of Cat5. The computer is inside the house, 50 feet from the telescope, and never gets cold. The  Cat5e ICRON will drive 100 meters. ICRON also makes a fiber optic version that is rated for 10,000 meters. On Previous mounts I had relied on 32 feet of USB2, which was problematic because the splice was underground, and hard to fish through the PVC pipe.

My USB2 downloads are only 4 seconds (3.1 MBytes). MaxIm DL calculates a correction if needed and sends it back to the mount before MaxIm initiates another exposure.

Sometimes I add a delay between images, short exposures for bright stars would over-populate the hard drive. I have not seen problems with overloaded USB or Cat5 lines. On the other hand, I have not yet bothered to add up the bits and multiply by link speed and add a reasonable error correction redundancy.

My guess is that most communications error-correction stem from bad USB/Cat5 connections, always the bane of data-collectors. I alleviate that with the unplug-replug method as needed. Have yet to spritz anything on this telescope with contact cleaner. I do keep my connections under Saranwrap, electrical tape to keep out drips and dust (which fall toward the Earth's center of gravity, that is, down)

The mount is high quality like the red mounts, good enough to allow at least 3-minute exposure without drifting, Don't know if it would go 5 or 10 minutes because I never ask it to. 

Better to stack short exposures than to imperil a lot of observing time with satellites, airplanes, clouds, cosmic rays, etc,

Meridian flips are generally orderly, but, as Arne notes, the comps will flip to the other side of the images.

That results in a magnitude shift. The shift can be minimized with careful calibrations, mostly flats, and by selecting comps which are near the variable at the center of the image, that, to minimize uncorrected vignetting. ImageJ fakes out, takes out the shift. Magnitude errors due to image shift are about the same size as errors due to comps that vary a bit; sort of 10 to 50 mmag. 

For exoplanet work, I have seen a 6 mmag transit despite the star center straying 1 or 2 pixels during the observing session. Other things, like refocusing, or mirror flop, can have a bigger impact than drifting off the pixel. I am not applying much rigor but getting results that seem to be about the same as those I have seen from others.

Hope this provides some guidance. You have fancier equipment and will do better than I can, once you begin doing it.