Tak First Light (Almost)

[MarkS]

I took the Tak Epsilon 180ED to the muddy DSC and did my first experiments there - not the best place to use it for the first time!

A few problems I found:

1) My DIY Peltier mod to the Canon 350D has resulted in a CCD that is no longer orthogonal to the image axis so I can't focus the whole chip at once.  So I need to adjust that somehow (or remove the mod).

2) Using my un-peltiered Canon 300D instead, I found the focusing at F2.8 to be absolutely critical and very difficult to achieve (though I've now added a dial and scale to help me there)

3) I think the optics are either slightly miscollimated or maybe pinched or both.  It's true that the Tak's secondary is offset which means a defocused star will not produce concentric circles (the manual does not recommend star testing for collimation).  But I wasn't expecting what I saw.  Below is the intra-focus, focus (almost) and extra-focus.  What do you think?  Part of the star "lumpiness" could be due to guiding, but what could cause the astigmatic like elongation first in one direction then the other?  The elongations are consistent because I took a number of images with various amounts of de-focus.

Mark

[RobertM]

Hi Mark,

I thought focusing might be interesting at f/2.8, it certainly is on the canon lens I have!

It looks to me like collimation is a bit out though I would have expected fairly similar patterns either side of focus.  The CC will exacerbate any issues and could well be distorting the image enough to cause those problems.  If both primary and secondary are spotted then you could use a cheshire (or something else to get your sight central to the drawtube) to check that both are perfectly in line.  I would also check the Captains wheel has locked the focuser correctly in position and there isn't any missalignment.

Might be worth investing in the 10:1 microfocuser or a motorised focuser unit...

Robert

[Mike]

I said at the field that a motorised focuser would be needed. Focus will be critical at F2.8

Any focuser that could be controlled by FocusMax would be ideal.

[Fay]

Mark, it was the  worst of times to try something new out. It is very interesting to see the images & your assessment. I am sure, knowing your track record, that all will be sorted out.

[MarkS]

Have you seen this?
http://antwrp.gsfc.nasa.gov/apod/ap100220.html

Quite by acident I managed to catch a geostationary whilst doing those Tak focus tests - it passed just above the Running Man and took a few minutes to do so.  I didn't realise what it was until I saw that APOD.

[Fay]

Well, if anyone was going to get it, it was you Mark.......

[MarkS]

Just found this on a site selling the Tak e180 (my scope):

The primary mirror cell is equipped with 3 "double-action" collimation screws (pushing/drawing). Moreover, you have 3 screws with counter-nut to adjust the pressure under the primary mirror in order to minimize the stresses due to strong seasonal temperature changes

I wonder if -7C counts as a strong seasonal temperature?  
If so, could those screws be distorting the mirror I wonder?
The screws exist but, annoyingly, the Tak e180 manual has absolutely no mention of them nor the procedure to adjust them.

[MarkS]

Took the bull by the horns today - collimated the Tak!  Since I was going to re-collimate anyway I decided to remove the mirror cell entirely to clean the mirror as well (see pic below).  Very nice design - undo three big screws (after marking the orientation of the backplate) and the complete mirror cell drops out - in theory it goes back on without requiring re-collimation.

Anyway, I completely collimated the thing and then tested it on my artificial star at the bottom of the garden - the diffraction rings looked good.  I also confirmed that the 3 extra screws that adjust the pressure on the mirror do indeed cause distortion if tightened beyond a gentle tightening.  Whether that was the casue of the elongated stars at DSC, or not, I don't know.  But everything looks good now.

The only thing that remains is to star test against an actual star at "infinity" instead of the bottom of the garden.  But for that I'm dependent on the weather.

Mark

[MarkS]

The only thing that remains is to star test against an actual star at "infinity" instead of the bottom of the garden.  But for that I'm dependent on the weather.

Wow - suddenly there's a chance tonight (Sunday) and tomorrow night!!

Mark

[RobertM]

Well I think you can thank me for that; I'm off to Frankfurt in the morning for a few days and it's nearly always clear skies here when I go...

I hope everything is sorted out now and you get some decent light in the bucket 

[Tony G]

Mark,

I've found a guide scope for the 'Banana'





Tony G

[Fay]

Good luck Mark!

[MarkS]

Blackcurrent?

[MarkS]

I did some more experiments with the Tak last night under clear skies. 

My weekend efforts at collimation (with the Tak supplied Cheshire) and tweaking the pressure adjustments on the mirror have improved the star shapes considerably but it's still not quite perfect - the collimation is still slightly out.  I think I'll have a go with a laser collimator - see if it makes any difference.

I did some focus tests with the DSLR and was pleasantly surprised by how easy it was to achieve focus manually - the precision of the focus knob adjustment was far less critical than my early calculations suggested.  However, I now realise why: my original calcs assumed that the Tak had a single 500mm focal length mirror (with a flat secondary and a corrector lens).  However, my measurements at the weekend showed that the mirror has an approx 650mm focal length and that the corrector lens is actually a 0.75x reducer/corrector.  The key point is that this corrector is built into the drawtube and moves with the focusing.  Calculations show that this makes the focus knob movement a factor of 5 or 6x less critical than I originally thought.

My usual trick of attaching a wire pointer to the focus knob actually works really well!

I was just on the point of taking a proper first light astro-image with my trusty old Canon 300D when the battery ran out.  I couldn't use the 350D beacuse I need to remove the Peltier mod - the heavy copper strip in contact with the CCD is throwing it slightly off orthogonal. Unfortunately at F2.8 this is a critical problem. I'll probably need to re-calibrate the CCD to orthogonal.

It is certainly a very demanding and unforgiving scope but I think I will soon have it mastered!