Monday, April 22, 2013

Improving guiding

Even after hypertuning my mount, guiding did not improve enough. It's definitively smoother (PHD's corrections are much smaller). But after some time, the guiding star slowly drifts out of the crosshair.

On the celestroncgem mailing list, I received the advice to set backlash compensation. Last night I started it, but didn't get far. The RA axis was OK, but I could not work out the DEC axis at all. And at some point, the power to my mount and equipment went off. I decided to go to bed and not to work on this during the night.

Next morning, everything worked again. I measured the current but it was way below 1 Ampere. Not sure what it was. Then I wanted to work on the backlash compensation. First, I worked on the mount itself and tried to remove backlash as much as possible by meshing the spur and worm gear tighter. Next, I spend A LOT of time on balancing the mount. And finally, I worked on the backlash compensation. I reduced the slew rate to 3 and then set the backlash compensation.

... and then clouds moved in, and I couldn't test if guiding is now better :-(


Here are the latest results:

10 min unguided10 min guided
It seems as if guiding works pretty well along one axis. Here is a shot when I moved the DEC axis:

Looks as if guiding is not working on the DEC axis.

... and after close inspection of the PHD settings, I noticed that "Dec guide mode" is Off !!! Aaaahhhh!!!

OK, I enabled and adjusted DEC guiding. Now 10 min exposure looks like this:

Definitively better. But the smaller stars are still elongated. At closer inspection, I see that the PHD graphs looks like this:

For some reasons, the correction in the DEC axis go wilder and wilder.

Reading the backlash compensation instructions again, it says that you should keep DEC backlash compensation in both directions the same - I don't have it the same (slightly higher in positive direction). That could explain why one side constantly corrects more then the other.

After fixing that, this is how the graph looks:
Pretty much the same. But the 10 min exposure:

That looks much better! Looking at the corners:
Upper left:
Lower left:

Lower right:
Clearly some field rotation - which is not surprising, considering that I don't do drift alignment. So, I seem to be able to guide for several minutes. Still, have to figure out why the DEC corrections get larger and larger.

After trying several more times, I found out that I can guide for some time properly, but at some point, the mount gets out of control. Here is an image of a 25 guiding session:

After ~8 mins corrections in both RA and DEC get out of control until PHD completely looses the guide star.

After careful inspection, I found that the Robofocus arm can hit parts of the mount, which would explain this. So, I rotated it upwards.

I also updated my MC firmware to the latest beta. One of the main fixes of this firmware is that it fixes the "DEC cogging" issue - which I have seen before. Furthermore, I received some advice on the CelestronCGEM Yahoo! list about the PHD parameters:
  • Lower aggressiveness to 0.6 (using an OAG makes guiding rather sensitive)
  • Set max. DEC duration to 1000 or even 1500 - especially for not-perfect polar aligned mounts
  • Set exposure to 2-3 seconds to avoid over correction
Now, my guiding graphs looks like this:

Much better - but still somewhat erratic. Next, I changed the exposure time to 5 seconds:

Even better - (almost) all corrections are between -1.5 and 1.5 - and no longer up- or downtrends.

Just to test it, I took an image of M51:

Ignoring the dust donuts, this is actually a pretty good image - and even in the corners is not too much rotation! Furthermore, when I stepped outside to disassemble and move my scope back inside, I realized that it was actually quite foggy. Considering that, this is actually a great result!

Saturday, April 20, 2013

First Hyperstar imaging

So, I could finally start imaging with my Hyperstar lens. I tried the Leo Triplet (M66, M65 and NGC3628). I focused manually with my Bahtinov mask (will figure out the @Focus2 parameters later). I then played with the exposure times. I was surprised how quickly the whole image gets saturated (a 6 minute exposure is completely white, a 1 minute exposure is getting into the 50000 range). So, I took 1 min RGB exposures and 2 min Lum exposures.

Exchanging the filters wasn't too bad with the Hyperstar filter drawer. But I can't do it easily with the dew shield on the scope - so, I had to remove it. I'm a little bit afraid if this will bring in some stray light from the side...

Taking flats turned out to be a major pain in the ... I tried with my flat panel, but it was too bright. And even with the dimmer I could not get them dark enough (I later found out about grey filters from Neumann that you can put over the flat panels to dim them). So, I took flat frames at dusk. It took me two evenings to do that as the hyperstar lens is so sensitive, that by the time I was done with 2 series of flats, it was already too dark and too many stars were coming through.

I then went into CCDStack and processed the images. During processing, I could already see that the flat frames weren't very good (they didn't neutralize the background evenly). And that's why the final picture looks like this:

Yikes! This images has everything that you don't want to have: uneven background, vignetting, field rotation, unsharpness... Well, maybe hyperstar imaging isn't as easy as I thought...

Looking at the flat frames:
Clearly, the blue and green flats are very uneven. So, I chose the 1x1 flat for all images. And this is the result:

By far not perfect, but much better.

Friday, April 19, 2013

Cleaning my corrector plate

When my secondary mirror holder came loose (when I attached the hyperstar lens), I ended up with a few finger prints on my corrector plate :-(

Of course, blowing some air or using a soft brush won't work here, so I worked on the solution from the Arkansas Sky Observatory:
I ended up being lazy and bought everything from Amazon (plus, some of the items were "Add-On" items, i.e. they only get shipped with orders of $25+).

After I received all the ingredients, I mixed the two solutions together. And then followed the instructions carefully - they were super helpful!


Looks pretty good. And breathing on it doesn't show any streaks!

Wednesday, April 10, 2013

@Focus2 with Hyperstar lens

I updated TheSkyX to the latest build and @Focus2 started working again!

So, I tried to set it up. I calculated a range of 452 steps for @Focus2. But when I ran it for the first time, the difference apparently wasn't big enough. So, I extended it to 800 steps, but I also increased the sample number from 10 to 20 to make sure that I get enough samples - especially within the V.

And - voila! Worked at first try. Of course it took some time (20 samples averaged over 5 images!) But focus was great. Tomorrow night, I will play with it more and try to improve these parameters to make it faster.

To my surprise, @Focus2 never asked me for covering the scope because it wants to take darks. I noticed that I had 'None' in the Reduction drop-down selected. I didn't realize that this feld is used for the @Focus2 routine ...

After that I tried autoguiding. It also worked again. I could get it calibrated and working in PHD really fast. But in TheSkyX I couldn't get it to work. But I didn't even try to improve my polar alignment - that could explain it. Will try later again.

One of those nights

I schlepped my scope outside and started with alignment. Everything went really well. First calibration star was close to center, second calibration star was in center!
Next: polar alignment. Also going really well...
... until the VERY last step: pressing <Enter> to confirm that after alignment, the star is in the middle. I fumbled the handset and pressed <Back> - all lost!
At this point, I didn't know if my alignment is still correct, because I changed the scope alignment underneath. So, start from scratch.
2-star alignment, 2 calibration stars. Next polar alignment. Now I was so frustrated that when I had to align the mount I continued to use the handset to move the alignment star into the middle! AAAAAGGHHHH!!!
3rd attempt. All well. Phew!
Next I wanted to work on configuring @Focus2 from TheSkyX for my Hyperstar lens. Have all the parameters prepared. Enter them. But every time I want to start the focusing sequence I get a "Error 206" error message. But all individual components (mount, camera, focuser) can be perfectly controlled by TheSkyX. Play with several options (automatically slew to star on/off, adjust exposure time on/off) - nothing. Reconnect all components - nothing. Reboot computer, reconnect - nothing.
After an hour I gave up.
So, instead I focused with my Bahtinov mask, improved the collimation and then wanted to some imaging of M81/M82. But now, guiding does not work. Neither in TheSkyX, nor in PHD. The scope simply does not respond! Checking the cable connection - nothing. No idea what's broken.
At this point, it's so late that I don't feel doing more analysis. So, I switch off the mount, put my dark filter into the Hyperstar lens, create several imaging series in TheSkyX to take darks and bias frames. Go to bed.
... next morning I when I wake up I check my laptop: it has automatically restarted during the night because of a security update. But at least not before all my images were taken!

Saturday, April 6, 2013

Cleaning my optical train

Last time I took some flats, I could see lots of pollution:

Well, considering that I played a lot with my scope (with / without focal reducer, with / without filter wheel...) it's not surprising that lots of stuff found its way into the light train. But where?

Somebody on the ccd-newastro mailing list forward me a link to the CCD Dust Shadows calculator from Wilmslow Astro (they have several other interesting calculators on that page: eyepiece projection, filter reflections, auto guider rates ...)

For these flats, the values are:
pixel size: 4.54 microns
focal ratio: 6.3
size of donuts: 200 pixels

Which leads to a distance of 5.7mm from the CCD chip. Which means on the camera window. has some excellent instructions on how to clean optical surfaces (not mirrors). I love their first tip: TRY TO AVOID CLEANING AND ONLY DUST !!! So, I'm just doing the basic:
  1. Use a very soft brush (I still have such a brush from cleaning my camera sensor).
  2. First whisk the metal around the surface
  3. Then carefully brush top to bottom (not round or forth-and-back) with the brush
With this, my flats now look like this:

Can't believe how easy that was... :-)