Tuesday, April 22, 2014

Beating Flexure

Backyard Trials Show Promise

One more night under the backyard stars shows that the modifications made for the astrophoto system on the SV4 are proving to be more than a fluke.  The results from a second night give very good values on differential flexure.  Once I can replicate these results in the field, I will say that the challenge was beaten.  For now, I'm documenting the success so far.

Sunday, April 20, 2014

Best Results Ever

Flexure Possibly Fixed?

The image shows the results.  Pretty good and it blows away previous work.  There's still a tiny bit of wobble in DEC, but that's another task altogether and it's fully within the range for acceptance.

Finally got the scope set up last night after work done earlier in the week.  Spent extensive time getting good polar alignment with PHD2.  The readings on the drift tool had results under 1 arc minute for Alt and Az.  Drift was within the 1 arc seconds per minute range as needed for precise work.

Everything worked smoothly.  I was able to get the SV70ED with the Televue FR to a precise focus and then locked down.  Moving the star to the center of the field didn't require much effort and the star images remained nice and round - no oblong smears like I'd been struggling with in the past.

Used the mount at the default sidereal rate rather than the custom rate I'd tried before.  With autoguiding, I'm not certain that the custom rate was effective.

Focus on the DSLR did shift slightly over the course of the evening.  This is something that I expect to happen and it's not surprising.  In the field, I'd check focus a few times during a session and it's only because I'm at home where I want to go to sleep is this a missed step.

I tried using the Pinpoint Guider Calibration plugin on Maxim and was annoyed that the Imaginova control window for the SSAG was overlaying the plugin control panel.  This is unavoidable as the two panels probably come from the same resource, so the positions are locked together.  I also tried the Multistar guider and was unable to get it to work for me.  I think that the SSAG doesn't have clean enough star images to make it worthwhile to use.  Many times I am unable to see the stars that Maxim ends up using to guide.  The stretched field is just too noisy to pick out the proper stars.

I've also re-enabled pulse guiding over ASCOM.  I'm ditching the ST4 cable from the SSAG to the mount.  The way I read the information about pulse guide is that it gives specific guide information to the mount without having to wait for start/stop signals that could be interrupted.

Great results in the end!  With the guider chugging along and giving super results, everything just worked!  Looking at the graph above, the jump after the first few images is because of playing with the multistar guider plugin.  For a while the scope was unguided.  Everything afterwards seems to be as good as I've ever seen.

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.