Makes sense... pretty sure my wife will divorce me if I buy an Atlas though, given how much money I've just spent on astro gear, with a new Mach 1 and this ONTC, so not an option for me.
I wonder if we could reduce the slop by adjusting the "C-axis drawtube collimation" screws on the Moonlite... if we do that evenly such that it's still properly collimated, but tighter, it might reduce the slop...
Makes sense... pretty sure my wife will divorce me if I buy an Atlas though, given how much money I've just spent on astro gear, with a new Mach 1 and this ONTC, so not an option for me.
I wonder if we could reduce the slop by adjusting the "C-axis drawtube collimation" screws on the Moonlite... if we do that evenly such that it's still properly collimated, but tighter, it might reduce the slop...
I reckon it is worth contacting Ron about this. I suspect he is not really aware of the problem existing. If I could find a cheaper solution to doing this I would be happy. Buying adapters for the Atlas is not cheap, plus I'll need something to sort collimation on the scope.
I've just sent Ron a lengthy email and specifically asked if this is likely to resolve the slop issue or if there's an alternative adjustment that could be made; I'll let you know what I hear back.
#1 Increase the shaft to drawtube tension. Could be set too low and would allow play in the drawtube. turn the 2 set screws about 1/20th of a turn in a CW direction. that will increase the pressure on the drawtube and make it stiffer.
#2 Adjust the C-axis collimation push rod set screws on the bearings. Maybe one is not set rigid. That would allow the bearing section to bend around. You may have to look to see what bearing block is moving.
If you get stuck, I would be happy to tune the focuser for you free of course, Just send it to me. It should work perfectly.
I did briefly try the two drawtube tension screws already, but I'll wait for my laser to arrive and try the c-axis adjustment and hope it does the trick.
Ron also offered that should the imaging train simply be too much for the focuser, he could offer me a deal on a new Lite version of the NiteCrawler that they're working on for Newts.
I did briefly try the two drawtube tension screws already, but I'll wait for my laser to arrive and try the c-axis adjustment and hope it does the trick.
Ron also offered that should the imaging train simply be too much for the focuser, he could offer me a deal on a new Lite version of the NiteCrawler that they're working on for Newts.
As always, great service from Ron.
Any info on the new "Lite" version of the NiteCrawler ??
I have the Moonlite 2.5" Newt focuser which doesn't have the C-axis adjustment. Must be an older version.
Well, I spent four hours tweaking the focuser's tilt last night -- and only in this one position, so if I rotate it, it's all going to crap. Most of the time was spent oscillating between having terribly elongated stars in the bottom right corner, or terribly out of focus stars in the top left corner. You'll note that top left is still a bit bloated and bottom right still a bit elongated, but this may well be as good as I can get it.
Prior to this I had attached my imaging train to a piece of twine, hung it off the focuser and then recollimated the scope using the AC / cheshire. I'd hoped this would account for the previous issue where the collimation looked good but the scope was clearly not collimated when I started taking images.
This time the collimation was "perfect" but when I put the camera on the scope, the images again told me otherwise. I'd learnt a couple of tricks about collimation that I'd hoped would resolve the issues I was having, but sadly this was not the case. This is extremely frustrating... I'm not sure why but I can't seem to properly collimate it with tools and am inspecting images to get it right which is obviously a very tedious process.
One thing to note is that the before was bin 1, the after is bin 2 so the after is probably not quite as good as it looks.
Basic workflow is:
Capture a 6sec bin 2x2 image
Find file in filesystem, copy from capture computer to another computer that has PI (takes about 6sec to do the transfer + time to navigate to the file)
Open image in other computer
Run FWHMEccentricity script and generate support images, then analyse the contour maps generated
Run aberration inspector script if support maps can't be generated (or even if they can when I'm getting close)
Make some adjustment to the focuser
Are we done yet? if not, take it from the top!
I might try and write some software to interface directly with the camera to generate contour plots / FWHM / eccentricity stats because the above takes forever. When I was using bin 1x1 it would take 20+ secs to copy the file, and I was taking 15sec exposures. If I can make the manual inspection / tweaking as easy and fast as normal collimation then this won't be a problem, but right now... well, I hope it holds collimation well is all I can say.
Last edited by codemonkey; 24-09-2017 at 08:11 AM.
This is essentially where I am at with my scope and I put it down to slop of the focuser. To that end I am waiting on adapters to arrive to fit my Atlas focuser. That will resolve slop for good.
I think that Moonlite focusers, whilst pretty good, cannot hold even a QSI stiff enough to eliminate slop which is creating this sort of tilt. I would suggest you take images on one side of the meridian and then flip and do it on the other. Use the exact same star field. I bet that the star shapes change from one side to the other.
Cheers mate. Yep, I plan to do some testing by various points in the sky... I was going to do it last night but frankly after 4hrs tweaking that ****, and being sick, I just wanted to go to bed... I may have a shot tonight. I'm just hoping you're wrong!
Cheers mate. Yep, I plan to do some testing by various points in the sky... I was going to do it last night but frankly after 4hrs tweaking that ****, and being sick, I just wanted to go to bed... I may have a shot tonight. I'm just hoping you're wrong!
I think Paul is correct. The Moonlight does struggle. I improved mine slightly by using a flange extension rather than a focusing tube extension. If you have a look at the expensive focusers, they rack out very little. Keeping the camera as close as possible to the focuser helps.
This thread is showing the unfortunate inescapable geometry problem with a Newtonian. Basically the focsuser (and OTA upper end I suppose) needs to be a lot stiffer than in a Cassegrain.
Ignoring at this point off axis loads (like ONAG mounted cameras that's another story), in a Cassegrain/refractor the side ways torque applied by the mass on the focuser/OTA, is proportional to the sin(za) where za is the zenith angle for the observation (i.e. za = 0 when looking straight up, za=90 looking at the horizon). For a Newtonian it is proportional to cos(za) because of the 90deg bend in the light path by the secondary mirror.
So the ratio cotan(za) is not at all favourable for the prime observing area far from the horizon. It means maximum deflection for the Newt at the zenith (the ratio being infinity there), >11x more than a Cass ~5 deg from zenith, and still ~1.7x 30deg from zenith. Once you get down to 45 deg elevation/za then things are even, except the focuser is attached to some big'ole back plate in a Cass, where the Newt can still have truss flex.
I think Paul is correct. The Moonlight does struggle. I improved mine slightly by using a flange extension rather than a focusing tube extension. If you have a look at the expensive focusers, they rack out very little. Keeping the camera as close as possible to the focuser helps.
Dave
Cheers mate... for the record, I was using a threaded drawtube extension made by Moonlite, rather than one of those extensions that require thumbscrews and inevitably introduce some tilt into the system. In the original primary position my focuser was also only racked out by about 3mm... in fact it was so far in that the coma corrector was poking into the OTA introducing a 5th diffraction spike.
Quote:
Originally Posted by ericwbenson
This thread is showing the unfortunate inescapable geometry problem with a Newtonian. Basically the focsuser (and OTA upper end I suppose) needs to be a lot stiffer than in a Cassegrain.
Ignoring at this point off axis loads (like ONAG mounted cameras that's another story), in a Cassegrain/refractor the side ways torque applied by the mass on the focuser/OTA, is proportional to the sin(za) where za is the zenith angle for the observation (i.e. za = 0 when looking straight up, za=90 looking at the horizon). For a Newtonian it is proportional to cos(za) because of the 90deg bend in the light path by the secondary mirror.
So the ratio cotan(za) is not at all favourable for the prime observing area far from the horizon. It means maximum deflection for the Newt at the zenith (the ratio being infinity there), >11x more than a Cass ~5 deg from zenith, and still ~1.7x 30deg from zenith. Once you get down to 45 deg elevation/za then things are even, except the focuser is attached to some big'ole back plate in a Cass, where the Newt can still have truss flex.
And the fast Newt doesn't make things any easier!
EB
So mathematically speaking, this is gonna be a pain in the arse?
Today I brought the scope into the house. I decided I may well have some sensor tilt and devised a way that I could (in theory) measure the tilt and then correct it.
So I took a piece of advehsive black velvet stuff I had laying around from other adventures. I stuck it to some corflute and proceeded to poke a crapload of holes through it with a small pin. Basically I created a small artificial star field. I then taped that to one of our glass, sliding doors.
Unfortunately, in order to achieve focus, I had to move the primary further down the OTA, and rack out the focuser much more than usual. In fact, I had to use the threaded drawtube extension I mentioned earlier in addition to racking out the focuser about 3/4 of its possible travel.
I recollimated the scope, using only my new Hotech SCA laser. I attached my imaging train and then captured some images. I could not detect any tilt, or none of significance... certainly not what I'd been seeing under the sky.
I rotated the OTA to two additional positions in order to try and introduce flex from the focuser, recollimating each time. In none of the orientations could I detect any significant tilt.
One thing I question... does the severity of the impact of sensor tilt vary with the proximity of the sensor to the secondary? Seems like it would in my mind and may explain why I couldn't detect any in this experiment.
I'm thinking about buying a Litecrawler and have started selling off some bits and pieces to raise funds, but I'd still prefer to avoid it if I can... cool though it is, they're not cheap.
I was thinking if in your setup at home placing a flat mirror where you had your telescope and positioning the telescope next to the star field (glass window) and facing the mirror would allow to increase (double) the distance between stars and telescope thus helping with reaching the focus without the extension?
I bought CCD Inspector this afternoon and did some analysis on the images. The images captured inside with the artificial star field with only very quick attempt at collimation showed little tilt and pretty good collimation (about 7").
I had done an altitude test recently and I ran that through CCD Inspector and found that near zenith the scope was about 16" off perfect collimation, and towards the horizon it went as bad as about 45". Definite shift in collimation as the scope is moved, and I think it's related to the focuser. Happy days!
Update on this. I've now placed an order for a Lite Crawler rotator / focuser. Not sure on ETA, but hopefully not long... Ron assures me there's no flex with 12lb payloads and mine would be considerably less than that. I'll be posting back the CRL2.5 to him which will be reduce the price of the Lite Crawler. He's also making me a custom flange to fit onto the existing lower flange I have so that I don't have to drill more holes into my tube... can't ask for more than that!
Looking forward to hearing how it goes Lee. I'm contemplating getting one also. Have you thought about how you will set the focuser up so it is at the correct position in terms of draw tube travel? Are you planning to use spacer rings to get the correct height or pack between the ota and the focuser body? I guess if Ron is making a custom flange you could optimise the thickness of that? The litecrawler only has 1/3" of draw tube travel.
Yeah, I'd worked it all out based on the flange extenders available for the Nite Crawler but Ron advised that they don't make them for the Lite Crawler (since the whole point of them is to be lower profile).
With my primary in its current position, I have about 4.5" from the face of the lower flange on the OTA, to the focal plane. With the custom flange Ron's making it'll take up about 3" of that. The Lite Crawler will come with a set of 0.25", 0.5" and 1" drawtube extension rings (and I already have a 2"). These are threaded adapters, not dodgy thumbscrew jobs. I can also easily move my primary down 30mm or 60mm due to the design of the ONTC tube.
Looks like the tilt isn't as easily adjusted with the Lite Crawler as it is on the standard Moonlites. I just bought a Gerd Neumann 68mm camera tilting unit which will take up ~17mm of the available distance as well.
Last edited by codemonkey; 19-11-2017 at 03:47 PM.
I just bought a Gerd Neumann 68mm camera tilting unit which will take up ~17mm of the available distance as well.
Sounds like you have plenty of flexibility to move the image plane. I want to avoid screw on rings on the end of the draw tube as it moves the weight of the camera further away from the draw tube bearings. Will the Gerd Neumann tilt adjustment still work if you rotate the draw tube?
Possibly not, I'm not entirely sure to be honest. My working theory is that whenever I collimate the scope I may need to adjust the tilt; this may be misguided but it's my theory anyway which is based on....
Whenever you collimate the scope you're essentially adjusting the focal plane. Tilt is fundamentally a disagreement between the focal plane and the sensor / eyepiece. When it comes to imaging, the critical focus zone at F5 is about 66 microns depending on wavelength so unless you're able to collimate the primary to that level of precision, I'd anticipate having to do "tilt adjustments" regularly. I'm thinking this is really just part of collimation and imaging with a newtonian. I also know SFA about optics and newtonians so you should probably disregard everything I said above ;-)
I also think that if I'm able to adjust the tilt such that the focal plane and the sensor are in agreement, I should be able to rotate it without affecting that, unless there's any kind of shift or flex in the system.
So that's my theory and why I bought the adjuster... hopefully it's sound!
That CFZ of 66 microns at F5 is for diffraction limited optics, as we are seeing limited the CFZ is more like 200 microns. That will give you a bit more breathing space.
