Friday, April 18, 2014

Challenges Neverending

I spent some time this past week trying a few things to locate and eliminate the flexure I've been having with my astrophotography setup.

I'm struggling with the hassle of why there is still flexure at all!

A quick review of the way things are configured:
  • Main Scope and guide scope are resting on a side by side platform.  I originally had the guidescope mounted above the main scope but found that guiding stability improved with a side by side setup. 
  • Main scope is mounted with rings and fastened by 2 bolts for each ring on the proper holes.  I've sacrificed some of the stability of having rings separated by a longer distance to having them secured by additional bolts.
  • Main camera is supported at the tripod socket via a plate that rides on its own dovetail, providing rigidity and removing focuser sag.

Guide scope has been plagued with some flexibility and thus is now supported by a set of rings at the guide camera.  Recently found that there are a number of screws on the focuser assembly that can be tightened to improve stiffness.  At this point I think that it could be used without the rings, but it still does move a little when pushed and thus I am not ready to make this change. 

Additionally, the rings at the guidescope are used to tweak aligmnent between the two scopes.  I have started to shim the mounting of the guidescope tube to help keep the off-axis skewing of the guidecamera to a minimum.  I think that this newer method of making alignment between the two scopes will be the right way to get them pointed to the same place - the camera will remain at the center of the field and the whole assembly pivots.  At some time in the future, I could consider the ADM Max alignment device if flexure is finally beaten.

Guide star images have suffered from coma and thus I'm starting to use the Televue .8 reducer corrector.  This does drop the arc second per pixel resolution from 2.5 to 3.2 as/px.  Advantage is that the stars have much better shapes across the field and there are more to see, based on the increased field of view.  This latter effort should help on platesolves.

Before tightening up the focuser bolts per this thread, I noted that the drawtube for the SV70ED would flex readily when pressed at an unsupported guidecam.  Now that these parts are tighter (and I sheared one bolt in the process!) the drawtube is very stiff.

I believe that the amount of error that still must be corrected is small enough that it gets lost in noise from seeing and drift.  I am going to try the multi-star guiding as noted via the Maxim plugins.  I have read that the next version of Maxim will feature multi-star guiding. 

On a positive note, I was able to clean the SV4 again to remove some gunk from the inside of the lens cell.  The last time I had it apart I must have allowed something to touch the inside of the main lens and there was a smudge.  This is now clean.  I also tightened up the feather touch focuser on the flange.  Lastly, I noted that the main rings for the scope seemed to close down to touch their clasp before grasping the scope body.  Thus, I added the slips of metal the top side of the scope to build up the diameter a bit.  The additional shim was put on the top side to avoid any alignment issues.  While I'm not sure if this is a final spot for flexure, it could be a part of the issue.

Lastly, on reading the Equipment directory on Cloudy Nights, a user posted how they had repurposed plastic yard sale signs as a dew shield.  I liked this idea since the craft foam paper is not strong enough to hold its own weight.  Got a large sign (15 inches by 18 inches) and two smaller onces.  I painted the printed side flat black and left the unprinted side white.  The smaller signs fit perfectly inside the dew shield of the SV70.  The large sign wraps around the dew shield of the SV4 with a small amount of overlap.  I'll add some velcro along the overlap to secure it.  Also, the smaller sign works well to give a tube for the RACI finder or the miniguider.  If I end up using the miniguider for autoguiding, I'll need this.