Back to forums

Doubled bhatinov spikes.

Acquisition Deep Sky Troubleshooting Collimation
8 replies•179 views
Sean Mc avatar
Sean Mc avatar
An interesting thing happened last night. I got double bhatinov spikes on my edge hd 8. 

my backspacing was previously 104-ish, and everything was ok for the most part but i figured i would try to increase backspacing to the more reported 105ish. To do this, i bought an aluminum anodized set of extension tubes off amazon. 3mm, 5mm etc. it came with a 12mm, so i used it to replace the 11mm celestron extension tube I had been using in my imaging train. 

noting else was changed. 

well.. it looked like collimation was off ¯\_(ツ)_/¯   Except… i was getting double spikes with the tri-bhatinov mask. Also, i couldn’t get the scope to collimate. The star shapes were horrible, and bright ones had triangular skirts…without the bhatinov.

2 hours of fiddleing, focusing, recentering…. I gave up and went back to the celestron 11mm tube. Instant round stars (needing a little collimation because i was messing with it).  No more double spikes with the bhatinov. 

1mm difference of backspacing isn’t going to do that right?  I’m assuming it was the fault of the extension tube?  Maybe the treads are off axis slightly?
Helpful Insightful Respectful Engaging
John Hayes avatar
John Hayes avatar
Does your scope have fans?  I once had distinct double imaging from a C14 due to thermal layers inside of the tube.  Turning on the fans immediately cleared the double imaging and produced a pretty good image.

John
Well Written Concise
Sean Mc avatar
Sean Mc avatar
No fans.  like I said, the only thing changed was the spacer.
andrea tasselli avatar
andrea tasselli avatar
That's something quite wrong in all this. I wonder what it is.
John Hayes avatar
John Hayes avatar
Nothing you did should have caused a double image!  Without fans, there is no way to tell if you had an index gradient inside the tube.

John
Sean Mc avatar
Sean Mc avatar
The scope was outside for weeks. It’s an edge hd, and it has the reducer on it, so no air changed inside the scope.
John Hayes avatar
John Hayes avatar
Sean Mc:
The scope was outside for weeks. It’s an edge hd, and it has the reducer on it, so no air changed inside the scope.

Right…that might present just the right conditions that you need to develop a stable gradient index variation inside the tube.  When I saw it happen, the scope had been outside for many hours and it was very well stabilized.  I don’t know what happened with your scope that it would produce double images, but a stable thermal gradient inside the tube is one way for that to happen.   Remember that radiative cooling to the sky can cause such a gradient.  The best way to test for it is to turn on fans to mix the internal air to see if the double imaging quickly disappears.

John
Well Written Helpful Insightful Respectful Supportive
Sean Mc avatar
Sean Mc avatar
But how would it disappear completely just by changing the extension?  It wasn’t there before I changed the extension either.
Well Written
John Hayes avatar
John Hayes avatar
Without knowing exactly what you did, I can't explain how it came and went.  IF it were due to a internal thermal index gradient, that condition is pretty fragile.  Merely opening up the tube could cause the gradient to rapidly dissipate.  Again, I'm not saying that is for sure what happened.  I am merely pointing out that I've seen it before and it's a possiblity.

John