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  #201  
Old 24-12-2018, 08:45 AM
Stefan Buda
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Engineering without OCD approach is just agricultural machinery level.
That is why people spend a whole year taming their new astrographs. Not all brands, of course.
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  #202  
Old 27-12-2018, 10:58 PM
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You're a legend Stefan and yes your attention to detail is significantly better that on my AG12. Your anodising is done correctly for one thing. Not mention I doubt my focuser is actually square to the optical line. I really enjoy watching you put a scope together, knowing you have made the components. Truly excellent work, I'd love to own one of your scopes.
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  #203  
Old 28-12-2018, 12:54 PM
Stefan Buda
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Thanks Paul,
Although the corrected Newtonian is as good optically as any RC or CDK, mechanically suffers from the fact that there is no easy way of aligning the mechanical axis of the focuser with the optical axis during construction.

Meanwhile I took advantage of the heatwave and printed the heater elements. Also I tested for secondary flexure by using a dummy secondary mirror made of scrap metal onto which I attached a small flat mirror. I used a crosshair target in front of the alignment telescope and focused on its reflection in the dummy secondary. There was no significant flexure going horizon to horizon.
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  #204  
Old 31-12-2018, 04:59 PM
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Last day of the year and I'm ready for first light. Sort of.

Actually I did hit a problem with the focuser: The image shift during direction reversal is a bit worse than what I was hoping for. I hooked up a MyfocuserPro2 board to the stepper motor and used a test program to drive it back and fort while watching the crosshair target reflection with the Taylor Hobson. I was getting an image shift of about 0.25mm and I could not fix it, so I'm taking the front end back to the drawing board. I will make a new mechanism with 4 guide pins instead of 3, and if that still comes short on performance, then I'll move onto using miniature linear ball bearings.

Does anyone of you have any experience with commercial secondary focusers? I'm curious whether they produce any image shift on direction reversal. Zero shift seems like a difficult thing to achieve without making the mechanism much bulkier. I'd like to avoid that.
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Last edited by Stefan Buda; 31-12-2018 at 06:52 PM.
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  #205  
Old 02-01-2019, 09:29 AM
Stefan Buda
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I achieved visual first light just hours before the end of the year.
The position of the focal plane is spot on, collimation is dead easy with the two thumb screws and the optics show no aberrations with a 10mm eyepiece.


Because I anticipated problems with the focusing mechanism, I did not glue in the front end assembly and that will allow me to make a new redesigned one. Meanwhile I'm converting the existing one to a 4 guide pin version to see if I can make it work well.
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  #206  
Old 02-01-2019, 12:32 PM
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It's looking good Stefan. So when are you going to stick a camera in there?
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  #207  
Old 02-01-2019, 02:37 PM
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So when are you going to stick a camera in there?
Where the eyepiece is in the last picture. Almost too much back focal length without a classic focuser. If I fail at getting the secondary focuser working well, then I can always fall back onto using some sort of Crayford type. I would hate to do that, because of all the potential alignment problems. Just because a Crayford type focuser works well, it does not mean that it has no co-axiality/co-linearity and orthogonality problems. Those are difficult errors to deal with in a Crayford..
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  #208  
Old 02-01-2019, 04:05 PM
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Quote:
Originally Posted by Stefan Buda View Post
Where the eyepiece is in the last picture. Almost too much back focal length without a classic focuser. If I fail at getting the secondary focuser working well, then I can always fall back onto using some sort of Crayford type. I would hate to do that, because of all the potential alignment problems. Just because a Crayford type focuser works well, it does not mean that it has no co-axiality/co-linearity and orthogonality problems. Those are difficult errors to deal with in a Crayford..
Ha yeah I had figured that one out. I was wondering when. Can't wait to see the fov and image scale on this thing.
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  #209  
Old 02-01-2019, 10:16 PM
Stefan Buda
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Ha yeah I had figured that one out. I was wondering when. Can't wait to see the fov and image scale on this thing.
Yes, my eyesight must be deteriorating. Looks like it may be a while until I stick a camera in it. I reworked the focuser to a 4 pin version and although it works better in some ways, it did not eliminate image shift.
Back to the drawing board for a while.
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  #210  
Old 03-01-2019, 04:03 PM
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Lovely piece of work Stefan. I've enjoyed following this.
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  #211  
Old 07-01-2019, 09:24 AM
Stefan Buda
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Thanks Joshua.

I've been experimenting with more modifications of the secondary focuser. I changed the spring-loaded nut mechanism to a floating nut solution that decoupled it from the mirror carrier plate. It works beautifully but the tilt hysteresis remained the same, unfortunately. But it occurred to me that there is a software solution to this problem. Any loaded motion system suffers from backlash and hysteresis but a proper anti-backlash routine can deal with both problems. So on the short run I will leave the front end as it is and I will start testing the optics on the stars. For that I will have to 3D print a temporary box for the full version of the myFocuserPro2 controller. Eventually I intend to make my own controller that also deals with thermal management.

Initially I won't be using autofocus software so I made a Bahtinov mask for manual focusing.
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  #212  
Old 11-01-2019, 09:11 PM
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Just about ready for testing on the sky - with a temporary myFocuserPro2 controller.
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  #213  
Old 13-01-2019, 12:53 AM
sharpiel (Les)
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"SN: 000"

Love it!
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  #214  
Old 13-01-2019, 09:46 AM
Stefan Buda
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Thanks Les.

I did some testing last night despite the not very good conditions.

Collimation took only a few minutes, with my ZWO ASI120 watching the defocused doughnut of Canopus and adjusting the two thumb screws. The long baseline between the adjusting screws and the 0.5mm pitch of the screws makes the tiniest adjustments very easy.
The attached image is from a 1min AVI using 100ms exposures, stacked with Autostakkert and stretched in AstraImage. I'm quite happy with the optics and tonight I will try to use a bigger sensor.
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Last edited by Stefan Buda; 13-01-2019 at 02:30 PM.
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  #215  
Old 13-01-2019, 11:18 AM
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Very nice trap! It's alive!
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  #216  
Old 13-01-2019, 02:18 PM
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Thanks Mark.

I sent Bratislav some Canopus doughnuts to be put through his Roddier analysis software and he has just sent me the results. The optical train came it at over 0.92 Strehl and I'm happy with that. There is a small amount of residual astigmatism that we haven't been able trace. It could be thermals related or a small amount of mirror pinch. Rotating all the individual optics, did not change the orientation so it has been turned off in the attached report.
Damn good this test. Clearly shows the low zone near the edge of the primary, that I knew about (not significant), and the slightly turned up edge.
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  #217  
Old 14-01-2019, 11:55 AM
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Quote:
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Damn good this test. Clearly shows the low zone near the edge of the primary, that I knew about (not significant), and the slightly turned up edge.
Yup, it also easily picked up the "by design" residual high order spherical at mere 13nm! (undercorrected primary plus two lens corrector leave some minor residual aberrations uncorrected - this is unavoidable consequence of all CDK's).
It always surprises me to see that people still go to inordinary lengths using all sorts of lyrical adjectives and convoluted descriptions to describe how good or how poor their optics are. Roddier spits out all the gory details, in numbers, not bullspeak. And it takes just about the same time to perform as a decent (eyepiece) star test would.
Yes, it does take a bit of a learning curve as well as decent preparation, but it still amazes me to see this test not being more widely embraced by amateurs.
Perhaps it leaves too little to wax lyrical (well, none) - it sounds much more boring to talk about secondary quadfoil in nanometers rather than of "diamond specks of dust on a black velvet" and "needle pinpricks of light".
Oh well.
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  #218  
Old 14-01-2019, 07:40 PM
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Thanks Bratislav for helping me with this. One day I will have to tackle that Roddier learning curve.

Last night I tried to produce an image with a larger sensor, to asses the off axis field correction. I had a few problems that were not OTA related. I had installed a guide scope, but forgot to add extra counter weight to the mount. So the mount was being driven down hill and it was quite windy too, therefore my tracking was quite bad. Not to mention that polar alignment was not very good either. In the end I managed to capture a bunch of 10sec frames of IC2391 with a few calibration frames. My old QHY8 camera is not responding to gain and offset changes since I started using a different laptop - another issue to sort out.
Anyway, the wind died down completely by the time I packed up, and the data collected was good enough to convince me that I a have nice evenly corrected field corner to corner.
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  #219  
Old 15-01-2019, 01:03 PM
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Yep! Looks excellent!
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  #220  
Old 15-01-2019, 01:31 PM
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It’s fantastic to see this coming along so well Stefan!

If you do want to test out a larger sensor than the QHY8 I have an ASI094 you could borrow; Full Frame 36.3mp sensor.
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