130 replies to this topic

[Der_Pit]

The read noise probably not, though it helps when you crank up the gain to compensate

I'm using one with an ZWO OAG.  It's location relative to the main cam is fixed - I just adjust the main cam FOV, then start the guide cam and pick a star.  There's so far always been (more than) enough to pick from.

 

And don't forget guide camera pixel size limits the accuracy of your guiding.  A rough estimate would be getting a guide RMS of at best 1/3 pixel.  With notoriously short FL guide scopes that can make quite a difference when converted to arc sec.....

[rgsalinger]

The 290 has a wider field of view when compared to the 1120. Given the equal sensitivity of both cameras the wider field wins every time in my experience.

 

Now this 1/3 pixel thing bothers me. I'm not sure where it's coming from mathematically at all. This is from an Craig Stark's website (author of the original PHD)

 

"Without any noise, PHD is accurate down to (on average) 0.004 pixels or 1/250th of a pixel. With a low amount of noise, the accuracy goes to 0.018 pixels or 1/56th of a pixel and with high amounts of noise it goes to 0.18 pixels or 1/5.5th of a pixel. Better stars and/or better guide cameras will get you more accuracy, but even with this very noisy star, we're still at 1/5th of a pixel accuracy." ---

 

http://www.stark-lab...xelAccuracy.php

 

Long before you are going to get down to .1 pixels the mechanics of your mount and the seeing is going to be a problem. I can barely get 1/2 pixel accuracy when imaging a .7 arc seconds per pixel but I'm happy with it as that's .35 arc seconds. That's not going to ruin my images and it doesn't. On the other hand when I'm imaging with a guide camera a 2.28 pixels per arc second I can get down to this same accuracy which is near 1/8 of a pixel. So, that rule of thumb seems at odds with my own experiences recently.

 

Rgrds-Ross

[ChrisWhite]

Additionally, number aside, I have extensively used guide cameras with both the sensor from the 120 as well as the 290.  The 290 outperforms the 120 whether you are using a guidescope or OAG.  You will have more stars, fainter stars to choose from, cleaner images and better guiding.  Just my take on it.  

[suvowner]

Additionally, number aside, I have extensively used guide cameras with both the sensor from the 120 as well as the 290.  The 290 outperforms the 120 whether you are using a guidescope or OAG.  You will have more stars, fainter stars to choose from, cleaner images and better guiding.  Just my take on it.  

thanks chris, even though you just cost me more money 

[premk19]

Additionally, number aside, I have extensively used guide cameras with both the sensor from the 120 as well as the 290. The 290 outperforms the 120 whether you are using a guidescope or OAG. You will have more stars, fainter stars to choose from, cleaner images and better guiding. Just my take on it.

+1

Have used both and the 290 wins hands down.

[Der_Pit]

Now this 1/3 pixel thing bothers me. I'm not sure where it's coming from mathematically at all. This is from an Craig Stark's website (author of the original PHD)

 

"Without any noise, PHD is accurate down to (on average) 0.004 pixels or 1/250th of a pixel. With a low amount of noise, the accuracy goes to 0.018 pixels or 1/56th of a pixel and with high amounts of noise it goes to 0.18 pixels or 1/5.5th of a pixel. Better stars and/or better guide cameras will get you more accuracy, but even with this very noisy star, we're still at 1/5th of a pixel accuracy." ---

 

It's of course difficult to justify those things mathematically.  But even the 0.004px claim has boundary conditions that are not mentioned.  How many pixels does the star actually cover?  What is the dynamic range/bit depth of the sensor?  All this will change the numbers, even without noise added.

 

So the real gauge would be what people actually see in their guide logs.  There's probably not many that guide at 0.1px accuracy.  Now I agree that this can be selection bias because people 'design' their guiders to get good enough guiding with a 1/3px accuracy.

 

One test you could do with your setup is guiding, then switching the guide cam to binning, and see if you can maintain the (absolute) accuracy of the guiding.  My prediction would be it goes down.  Not a factor of two, but noticeable.

 

(And whether its 1/3 or 1/5 is hopefully not the real issue?)