Sunday, May 27, 2018

Collimating and aligning the RH200

So, I finally collimated and aligned my RH200 scope good enough (still not perfect, but good enough to start imaging).

This post should really have been a series of posts to describe all the avenues I went - but I compress it into one ...

Let me start with what steps finally worked:

  1. Good (not perfect) collimation with Mire de Collimation
  2. Perfect collimation and very good alignment with Hotech Laser Collimator
  3. Final alignment with CCDInspector
1. Good (not perfect) collimation with Mire de Collimation
I was very much surprised how a simple tool like this can be so helpful. I used it together with TheSkyX and overlayed it on the image view. I used the continuous imaging and tried to size the rings in Mire de Collimation such that one ring was just inside the out-of-focus donut:

First, I worked on the primary mirror collimation. I experimented to understand which collimation screw has which effect. This helped A LOT when I then tried to get the out-of-focus donut concentric.

One problem was that I couldn't reach the collimation screws because the Atlas focuser was in the way:

The solution was to turn the Atlas focuser around, so that it extends from the scope when I move it out of focus.
I had the best luck with bright (not THE brightest) stars. This gave a good donut but allowed me to use short exposure times (1 seconds) which made it easier to see the effect of collimation adjustments.
My workflow was:
  1. center out-of-focus star
  2. overlay Mire de Collimation perfectly concentric
  3. adjust one(!) collimation screw
  4. re-center out-of-focus-star
  5. <go back to #2>
Once I figured out the workflow it took just a few nights to get the collimation of the primary mirror.

Once the collimation looked good, I moved onto the front plate. My front plate was initially WAY too tight. The in-of-focus start showed really bad aberrations. I loosened it and used the same process as above to adjust the plate. Because of limited focus range, I couldn't get the star as good in-of-focus as I could get it out, but it was good enough:

After I changed the plate, I also had to redo the primary mirror collimation. It took me a few iterations to get both good.
  • using CCDInspector to collimate main mirror and/or front lens - didn't work well
  • using CCDInspector to collimate front lens (ask on forum!!!)
  • tried collimation mask - didn't work (plus: front or back?)
  • posted in OS forum
  • Miere de Collimation - yes, simple and works!!!
2. Perfect collimation and very good alignment with Hotech Laser Collimator

This step took me the longest to figure out (see below all methods that I tried and that didn't work). I could get the collimation good - but not perfect. I ended up forking off the $$$ to buy the Hotech Laser Collimator - and it was SOOO worth the money!

First, I put the laser collimator head on my very sturdy DSLR tripod:

Then I stepped through the process to collimate and align the scope:

  1. Align the scope with the collimator
  2. Collimate the primary mirror
  3. Align the imaging train
The key in getting great results was #1. I ended up spending 2 nights just doing this. I needed to put on my reading glasses to see very exactly if the three laser dots were in the same position all around:

Again, not difficult to understand but needed a lot of patience and many iterations to get really good.

Then I moved on to collimate the primary mirror. Because it was already well aligned, this didn't take too long, but I could improve it.

The alignment caused me most headache. After I did it the first time, I rotated the special mirror piece around:
And then the imaging train wasn't aligned... ... I figure out that this was because the mirror at the end of train is not necessarily completely orthogonal to the optical axis. What I ended up doing was to rotate the eyepiece around and mark the inner- and outermost position of the 3 laser points. The middle point was where I needed to get the laser point to.

Excluding setting up and learning about the laser collimator, this took me 3 nights.

3. Final alignment with CCDInspector
Now, I moved the scope outside and did the final alignment using CCDInspector. I used the flatness view to see which corner/side was too far in the front / back (I always had to experiment if I needed to move this corner in or out to correct).
At this point, I made tiny corrections (1/8 of a turn of the alignment screws) - always checking if and how much the plane changed.
Once the plane looked fairly plane, I used the average of first 5 and then 10 images for final adjustments to make sure that didn't end up chasing seeing.

One weird thing happened was that after I had the alignment almost perfect, I moved the scope to the other side of the pier and got this:

On closer analysis, I realized that one of the lock screws wasn't completely fastened. That fixed it. But it also showed how incredible sensitive the scope is to even slight errors.

So, yei! I finally had it. Took me only 3(!!) months ...

Things that I tried that didn't work:

  1. Using CCDInspector for collimation
    I really don't know how this works (I tried both the in-focus and out-of-focus method). I could see with my naked eye that the donut wasn't concentric... Also, for rough collimation the adjustments were so large that I had to refocus A LOT - which made the process really slow.
  2. 4 corner view in TSX
    The visualization was good:

    But I found it too difficult understanding from the picture what adjustments I had to make
  3. Collimation mask
    This only worked for rough adjustment and I found it easier and faster to use Mire de Collimation.

Sunday, May 13, 2018

Heart Nebula (IC 1805)

I took the data for this image October 2016 (!!!) It's a mosaic of 4 images and I always had problems stitching them together. Especially the OIII data which had very different noise levels in the 4 panels. But I finally figured out a way to minimize the effect! As always an awesome tutorial from Light Vortex Astronomy helped me!

(click here for a full-resolution view)

The Heart Nebula is at a distance of 7,500 light years from earth in the Perseus arm of our galaxy. It spans 200 lightyears! The nebula is ionized by the relatively young stars at the center of the nebula (their open star cluster is known as Melotte 15). The nebula is 150 arcminutes in size (the moon is 30 arcminutes!) in the constellation Casiopeia.
It was discovered by William Herschel on November 3rd 1787 (he first discovered the brightest part in the lower left - NGC 896).

Being a mosaic there is SO much interesting detail here:

1. The bright NGC 896

2. The open star cluster Melotte 15 at the core

3. Beautiful dust pillars (created by the energetic light from the young, hot stars at the center)

Each of the four panels has and integration time of 10 hours (10xHa, 20xOIII, 40xSII - each 10 minutes). I think this is the longest integration time I ever had.

Saturday, May 12, 2018

Mosaic with different background levels

Taking the images for my mosaic of the Heart Nebula took a long time (several weeks). And as a result the background levels of the different panels were quite different (mostly because of the moon). This was most noticeable in the OIII channel:

Here is mosaic of them without modification:

The first advice that I found was to remove backgrounds as much as possible using ABE or DBE. That made it better, but the differences were still very noticeable.

On one thread in the Pixinsight forum, somebody recommended to use LinearFit to equalize the levels - but that did not work at all for me:

The histogram got completely squished...

On another thread, somebody mentioned AutoHistogram:

The usage is fairly straight forward: select the image that you want to use as a reference image (preferably the image with the largest range) and click on "Set As Active Image". This reads out the median pixel value of the selected image to be used for the adjustment.

Then you just apply this process to all the other images:

Doing this to all images and combining them with GradientMergeMosaic results in:
Yei! That looks much better! I had to play with the Feather Radius to avoid a pinched star, but apart from that it was now straight forward.