Binning, Sag and System Synergy With a Solstice Sun 21st June

[marktownley]

Hi All.

We're having a spell of dare I say it summer weather at the moment here in England, which is good as i'm able to grab some opportunities before and after work at times, which is always a nice thing. Tuesday was no exception and with relatively clear skies but poor seeing I decided to go for a trio of full disks, doing a few experiments along the way...

First off the full disk in Ha - and in the words of Led Zeppelin 'There's a feeling I get when I look to the west...' - there certainly is (and Stu alluded to a similar thing) - that feeling is I have etalon sag on the front of my scope. Probably 90% of Ha full disk imaging I do is towards the east, and my scope through tinkerings performs very well straight out of the case with minimal adjustment every time looking eastwards. Looking west and a different story it is not on band and takes lots of fiddling and even then is not on band as I know it should - see unevenness in the below disk:

Ha-FD-DS50-2x-cemax-zwo183mm-2x-binning-evening-bw by Mark Townley, on Flickr

Ha-FD-DS50-2x-cemax-zwo183mm-2x-binning-evening-colour by Mark Townley, on Flickr

This was taken with the double stacked Lunt 50 etalons on the 60mm f6 Tecnosky scope, 2x cemax barlow and the ZWO183mm camera. I've been using 2x2 binning with the camera to get a better sampling match for the system which I thinks works well. The other experiment i've been doing is with 16 bit capture (well 12 bit really as this what the ADC on the camera is)... I've not really had any successes worth pursuing with my PGRs set in 16bit, but I am getting some tangible differences here with the ZWO. Simply put there is oodles and oodles of headroom in the highlights on the histogram, I can really lengthen exposure, lift gain or both before the histogram clips. This is very evident around bright flaring in active regions - something to explore more. The other thing I discovered is the 'hardware binning' check in firecapture. If you tick 2x binning in FC the frame rate doubles in 8 bit as you would expect, if you have 16 bit checked and then 2x bin the frame rate falls low. In the settings there is hardware bin, which when checked lifts the frame rate back up again. You might know this, but was news to me and I will be using it in the future...

Next up a vanilla WL disk, the Skywatcher ED80 at f7.5, Lunt wedge, continuum filter and the zwo183mm - no binning but in 16 bit, I don't feel 16 bit brings anything to the party here. What I did notice was how much less resolution there is when I shoot a WL disk with the 100/800.

WL-FD-80mm-f7-zwo183mm-bw by Mark Townley, on Flickr

WL-FD-80mm-f7-zwo183mm-colour by Mark Townley, on Flickr

And finally is the CaK full disk, seeing was really boiling in the late afternoon sun, and I didn't think it would come out, but it did... As you can with a rear mounted Ha filter, changing the focal ratio it sees will act as a fine tune for the centre wavelength of the filter. I've been experimenting with this behind the scenes, and found for the 183mm camera, and my CaK filter f15 is bang on for best contrast. YMMV. So, using the SW ED80 masked down to 40mm gives me 15 at the filter and the chip. Also, recording in 16 bit (no binning), as with Ha I seem to have loads of histogram to play with before clipping, this really makes the chromospheric network and plage really stand out. Double click the disk below to view full size in flickr, there's a lot going on!

CaK-FD-40mm-f15-zwo183mm-bw by Mark Townley, on Flickr

CaK-FD-40mm-f15-zwo183mm-colour by Mark Townley, on Flickr

The CaK is my favourite from the session, I need to explore this further and also try some f15 masks on some different scopes for different scales, lots to explore!

Hope you like these!

MArk

[DeepSolar64]

Mark,
I wonder what difference east vs west makes? Maybe late day you are looking west during a time of day that fosters poor seeing. But how would that throw you off band?

You still have good resolution on your WL images and the CaK image is my favorite. You do so well in calcium.

Thanks for posting these.

James.

[rigel123]

Nice shots Mark! That HA is just strange, those images look like the ones I see when a leaf or a branch from a tree is getting in the way.

[DeepSolar64]

Clouds maybe?

[JohnW]

Nice set of images Mark - I like the comparison between the different wavelengths. Regards John W.

[Carbon60]

Interesting results from your experiments, Mark. Thanks for confirming the east/west viewing challenges that can occur in Ha with front mounted etalons. With clocking the main bodies and rotating the casing of mine (I use Bob’s compression tuning method on both front etalons) there is quite some ‘play’ in the loose threads and I’m sure this is the issue in my case when gravity pulls in different directions. When I say quite some play, the movement (tilt) in reality is only small, but tiny movements make a big difference with these fine tuned systems, as we know.

It’s interesting to hear your observations in CaK. I’ve been considering stopping down my Tecnosky 125 in CaK for the same purpose of increasing the focal ratio at the filter.

Thanks for sharing your results.

Stu.

[marktownley]

Thanks guys!

Regards the Ha disk you can see how it goes off band at either pole, and there is a darker scallop at the left hand side of the disk, it's not seeing or clouds etc that cause it. It only happens when viewing to the west. The etalons are fixed onto a metal cell which in turn is fixed onto the dew shield on the scope. The dew shield is rotatable which allows me some fine tuning which is a good thing along with the tilt of the etalons. I've created an interference fit between the dew shield and the ota so there is not sag. This works pointing east but not west, as east is the predominant viewing preference I wonder mechanically if I have some differential compression occurring at the interface of the ota and the dew shield? Could be down to how the scope is stored imparting similar compression that has a tendency to a bias? It does irk me, but as with everything like this the way to see a problem is just an unsolved challenge, and I actually don't mind that

I definitely think you will get better results with the 125 stopped down Stu, spherical aberration aside you are only really going to use full aperture when seeing is running at 0.8" and lower, which is probably when you are using the RCT. Try it for one of your full disk mosaics stopped down to say 60mm, you'll be running with a lot less glass than the 72mm and a barlow combination. All good fun experimenting!

Mark

[Dennis]

Very nice fulldisk images Mark,
the unevenness i only realized because you pointed it out.
Ca k images like always very contrasty that combination with the camera seems to rock.

Dennis

[MAURITS]

I like them all Mark, the unevenness is not so visible.

[ffellah]

A beautiful set of images, Mark, and good experimenting on your part. Nice results.

Franco

[arnedanielsen]

Very nice, Mark!

Thanks for sharing,
Arne

[marktownley]

Thanks guys!