Collimation, corrector spacing, or tilt?

[codemonkey]

... or all three?

Just got my first newt - it's about F4.3 and I'm learning how to collimate it, but given all the variables involved I'm not sure how to interpret my results.

I think this is about as close as I can get the primary collimation with the cheshire. I'm waiting on a threaded extension tube for the Moonlite so that I can get more outward travel and thus use the auto collimator.

Two images attached, right one is without the coma corrector, left one is with it. What do you think? Bad collimation, bad corrector spacing, tilt, or a combination of the above?

[Slawomir]

From what I understand, a f/4.3 Newtonian can be a very capable imaging instrument :thumbsup:

My guess is that there is an orthogonalty issue resulting in uneven spacing and thus different correction of stars (over, good, under correction) in four corners.

Hopefully you will get it resolved quickly.

[Shiraz]

ahh- isn't it fun

very much FWIW - a few ideas that might possibly help.
1. get a laser collimator and align it precisely - as Alexander showed http://www.iceinspace.com.au/forum/showthread.php?t=75601
2. carefully check the central spot ring on the primary and get it exactly right.
3. do a laser collimation through the coma corrector
4. check that collimation holds reasonably well by moving the scope around the sky with the laser running - if possible tighten anything that allows movement of the collimation spot. pay particular attention to the focuser and make sure that the shaft is tight enough that you do not get any rocking of the draw tube
5. if you still have corner-to-corner variability, then you have tilt, either in the camera or in the CC. make a cardboard spacer shim so that you can screw on the camera at a different rotation angle on the CC. If the tilt has rotated with the different camera position, it is in the CC, if it does not change, it is in the camera. if the corners all look the roughly the same, but are not fully corrected, check the CC spacing.

also FWIW, I had to add strengthening to the CF OTA around the focuser to stop flex and had to put new side shims on the primary to stop it from sliding too far sideways. I also added two nylon locking screws on the side of the focuser draw tube to ensure that the CC tube is locked solidly to the draw tube (it could wobble slightly when only locked at the top end). Also shimmed the camera to minimise very slight tilt in both the CC and the camera - found the rotation angle at which the two slight tilts came closest to cancelling each other. The optics still tends to drift a bit and recollimation is needed maybe every month or two - not a big price to pay for a fast scope.

cheers Ray

[glend]

Or you could buy a nice refractor. Sorry i could not resist. As Shiraz said, many things to check. Good luck.;)

[atalas]

Hi Lee

Ok,It can be confusing so you need to take things one at a time dude.

1:center focuser (use a high quality laser)

2:center secondary to focuser

3:center main mirror to secondary

Ok basics over, now,looking at the second pic looks to me like the spacing of the coma correcter is good.....center right side of the image tells me this.

Top left and top right tell me you have tilt in the focuser.....this should be the first thing to take care of....center focuser before collimation of mirrors otherwise everything else is a compromise.

I see in the bottom right that you have field rotation.....PA dude otherwise It will confuse you.

[codemonkey]

Thanks Suavi :-) I thought there was some tilt, but I've never experienced a newt before, so wasn't sure I was interpreting the images correctly.



Thanks for the detailed response, Ray :-) Of course I had a laser collimator that I used once to identify / correct some tilt on the Esprit...... and sold it.

The center spot is as close as I could get it - I removed the stock one and replaced it with a catseye hot spot.

The tube is a ~5mm foam core CF tube and it's not a cheap scope so hopefully will hold collimation across the sky--I'll be pretty unhappy if it doesn't, but I've also not verified it yet... needed to get it collimated first!

I'm not 100% sure on the spacing of the corrector. I tried a few different positions and this seemed to be the best, though it was about 3.5mm off the apparent spacing for this corrector on a scope of this focal length, so I dunno.



You mean like the Esprit? ;-)



Cheers mate. I did 2 & 3 to the best of my abilities with the tools I had available to me. Not so sure on #1.

When you say the second pic looks like the spacing of the coma corrector is good, did you mean the first pic? The second pic has no coma corrector in place.

As for field rotation... this was taken on a near new, pier mounted AP Mach 1, polar aligned to within a couple of minutes.... periodic correction was enabled and these are both 15 second unguided exposures -- that's not field rotation caused by PA.

[Slawomir]

Congratulations Lee and welcome to the AP club :thumbsup:

[atalas]

(Quote)

Cheers mate. I did 2 & 3 to the best of my abilities with the tools I had available to me. Not so sure on #1.

When you say the second pic looks like the spacing of the coma corrector is good, did you mean the first pic? The second pic has no coma corrector in place.

As for field rotation... this was taken on a near new, pier mounted AP Mach 1, polar aligned to within a couple of minutes.... periodic correction was enabled and these are both 15 second unguided exposures -- that's not field rotation caused by PA.[/QUOTE]

(End Quote)


Yes sorry the first pic Lee....the right middle side looks well corrected so that would lead me to think the spacing is good. I should add to that you should see the coma well corrected right round the image when you aliminate the tilt I would say.

So,ok then your PA is good and this is a result of the tilt.... just goes to show how things can look confusing when indeed you can get the same result from field rotation(in ref to the bottom right of the corrected image)

Like I mentioned Lee,first off you need to get your focuser centered to the secondary...

[codemonkey]

Thanks Suavi--hopefully I can get this scope sorted out soon and then I can actually get a chance to really use the AP!



All good, thanks Louie, appreciate your advice :-) Weather is looking like it should be good again tonight so I'll see if I can identify where the tilt is coming from tonight.

[Paul Haese]

Lee,

I have just been travelling down this same road. I have learnt that a good many things I took for granted in imaging are very different with an f4 Newtonian. Last night was the first night I had actually obtained round stars in every corner with nice tight focus.

This is what I did and it may help you with what you are going through.

I bought a Moonlite focuser and am using a GPU coma corrector (CC). I installed it thinking that a longer draw tube would be better. This is in fact a mistake. I found that having the draw tube extended too far created greater slop and made star shapes worse. I suggest using a 1.25" of 2" flange which goes on the bottom of the focusor. Doing this reduced star shape deformation remarkably.

Next I stiffened the secondary assembly. The secondary assembly on the GSO truss Newtonian is at present pretty average and not suitable for carry a good focuser and QSI683WSG-8. Spiezy was the first to put extra poles in the cage and I did likewise, though I think it needs diagonal bracing to reduce racking. Being a tube you are less likely to have this problem but you might have some hoop flex and stiffening via more support is a good thing as you have done.

Next I looked at every bolt and fixing on the scope and I mean every one of them. Tighten up everything, check your primary support, check your primary side slop (not an issue with a conical mirror on my scope but worth checking) and pack out if needed. Also check the secondary assembly itself. Make sure it does not have any movement in the stalk.

Now this I think is important and might pertain to your scope. Check the actual secondary fixing to the stalk. Mine was glued down with a very wide narrow smear of silastic. That caused astigmatism and I think I can see some in your image. The way to test this is to take an image intra and one extra from focus. If star shapes near focus on both side have an oval on both sides and each oval is 90 degrees to each other then you have some astigmatism. It's worth a check to eliminate this issue. I reglued mine down with 3 small blobs of aquarium grade silicone and supported the stalk with match sticks to keep them up high enough so as not to smear. This eliminated astigmatism.

As to collimation, I use the Glatter laser to check but essentially use the Cats eye collimation system for collimation. I cannot stress enough how finicky you have to be with this. The hotspot or triangle has to be perfectly inside the black cat white circle. Just a tiny bit out makes a huge difference. Then use the Infinity Cheshire very accurately to adjust the secondary. Then importantly repeat the process until you have both ends perfect. I finally use the laser to check both the centre spot alignment and use the Tublug to check the primary return. If every thing looks aligned you are now highly collimated. I found I had to be particularly fussy to get good collimation and again I think I can see a little collimation error in some of your star shapes.

Now check you CC back focus distance. CC's are particularly fussy about the correct back focus. Most on the market have a 55mm back focus but some vary a little to 50mm. The GPU allows some slack of about 1mm. The Baader one was a little more tolerant and allowed 5-6mm. Make sure you have that correct distance. This is probably one of the major problems you have here.

Screw thread every thing. Don't use compression rings and these can slightly miss align the CC and that can create tilt. Testing as suggested will tell you where the error lies. I bought adapters to screw thread and it made a huge difference.

Adjust your Moonlite for the correct lifting capacity. I did mine up too much and it caused flexing in the tube. Make sure it holds the weight under gravity but only just. That seems to work best. Though I still seem to have a tiny amount of slop in the focuser but I think that is part of how gravity is working against the focuser.

If your Moonlite has a rotator facility with the tilt adjuster, check that the tilt adjustment is actually adjusted correctly. Mine was way out and causing a focuser mis-alightment. To check this I put the laser into the focuser and very lightly clamped that down in the focuser and then rotated the entire focuser. The laser should not travel around in a circle; the laser should stay in the one spot.

Finally, and I think this is the main problem you have here; is tilt. Once you have the correct spacing of the CC and established you have tilt in the camera, then pack out with shims. I used the metal out of a coke can which is thin and it works well. This should go under the camera adapter behind the CC. It takes more than you might think.

I hope this has helped. It has taken me a while and I got some good advice from several people, which I have relayed to you. Best of luck.

[codemonkey]

Thanks very much for your response, Paul. That'll take a while to fully digest!

I'm also using a Moonlite and a GPU coma corrector (TS branded, but it's the GPU). This morning I played around with a few different adapters and now have the spacing at about 55.7mm, according to the calipers, which is hopefully close enough. I doubt I'll get any better unless I go down the precise parts path, which is always expensive... and even then there's measurement error so I couldn't be too sure it'd do more good than harm!

Everything in my imaging train (including the focuser -> coma corrector connection), is threaded, except for the TS OAG -> filter wheel connection, which uses a crappy system with three thumb screws--that may be causing tilt but I did my best to apply pressure and keep it square when tightening it.

When my camera is focused the focuser's 2 inch draw tube is only racked out about 5-10mm. In fact, I may need to move the primary mirror further up the tube as the GPU corrector is inside the focuser's draw tube and actually sticks into the OTA by about an inch when it's at focus.. when looking at out of focus star images you can see a nice flat side on all of the stars which I assume is the corrector.

I hope I don't have any astigmatism, but I did look at how the secondary is connected to the holder this morning and it seems to have a smear of some kind of glue / adherent so I may be in the same position as you there... hopefully not though.

I measured, using calipers, the distance between the coma corrector's adapter and the face of the camera's body and that seemed to be consistent, though I was restricted in where I could measure, which suggests either focuser or sensor induced tilt... I should be able to verify by rotating the OAG -> filter wheel interface and seeing if the tilt shifts with it.

Thanks again, Paul!

[multiweb]

I got caught out by that one when I glued back my secondary after having it recoated. It is incredible the pull silicon can have on even the thickest piece of glass when it cures, even three small dabs. On my C11, too much of this stuff managed to stress the glass between the baffle tube and the bore in the primary and we're talking of about 100mm of thickness at that point in the mirror because it is bell shaped. I had to slice parts of it to release the pull.

[codemonkey]

Made some progress... after correcting the spacing of the coma corrector, the star shaped improved some... still definitely not close to good enough, but better. At that point I could see very obvious tilt -- stars in focus in the center, round donuts in one corner.

I rotated the camera in the OAG and the tilt moved relative to the camera sensor, so the tilt is before the OAG... I'm thinking the focuser now.

I've been waiting on a 2" drawtube extension so that I could get the autocollimator close to the focal plane and verify collimation. That arrived today and I think I have it collimated now. Unfortunately my secondary was spotted when I got it and I didn't remove it, so it's pretty hard to tell when looking through the central pupil because I have a fuzzy donut of the secondary spot overlaying the radioactive symbol that is the primary spot. In the offset pupil I have two circles though (comprised of the two sets of opposing radioactive symbols).

Interestingly, the Moonlite focuser can be rotated relative to its base... if I do that while looking through the offset pupil, the circles unstack, until I rotate it back to the position at which I collimated it. I'm guessing this confirms that the focuser is not square. I guess it means I'll have to recollimate the scope if I rotate the focuser... which I need to do to compose my images.

Fingers crossed the dark sky tonight shows me pinpoint stars, but I have the feeling I'll be seeing more donuts...

[Paul Haese]

Lee, I think you might need to check the tilt adjustment on the Moonlite. I found that mine was way out and when I rotated it the laser I was using would form a rather large circle on the primary. Sorting that will ensure you focuser is square to the optical axis. Then you can collimate from there.

[Merlin66]

Paul,
With all the adjustments you have to make , what did the Ronchi screen show.
I found the Ronchi very helpful with the 12" f5 that and the final star test.

[codemonkey]

Cheers Paul. I think you're right. I tried adjusting it using trial and error tonight and got nowhere. I've just ordered a laser, sounds like that will make it easier if all I have to do is adjust the focuser until rotating it causes the laser to remain in place as opposed to tracing a circle.

[Paul Haese]

Ken, I have not used a Ronchi eyepiece on this system. Might be worth taking a look, since GSO don't provide test results themselves.

[Paul Haese]

I am going to get adapters made up so I can use my Atlas focuser on the scope and that will narrow down where movement can take place.

[codemonkey]

Interesting... good luck with the Atlas, hope it works out!

I think I might have narrowed down the problem. It may not be *the* problem, but it's certainly a problem.

If I put the autocollimator in the focuser drawtube and have the drawtube parallel to the ground, then apply some downward pressure to the drawtube (simulating load from the camera and other gear), the collimation clearly shifts.... looking through the offset pupil I can see the reflections "unstack" as I apply pressure. Looks like there's significant slop in the drawtube.

I've also noticed that I can "unstack" the images in the offset pupil of the autocollimator by:


Wiggling the AC in the focuser; or
Rotating the AC in the focuser; or
Rotating the focuser itself


It's hard to know if it's collimated given the inconsistencies.

Anyway, last night before I discovered the above (which I noticed this morning), I got everything well collimated according to the offset pupil of the AC and the cheshire. I then attached the camera and captured the first image. You can see very bad tilt, which I think now is due to the weight of the imaging train screwing up the collimation.

Taking 15 second exposures and then analysing them in PixInsight's aberration inspector I was able to achieve the next image by making small adjustments to the focuser's collimation. Clearly still out of focus in one corner and poorly corrected in some areas, but much better than it had been. I assume I could have managed this by adjusting the secondary as well. The field illumination improved during this adjustment also. Sadly, this is still not usable and if I'm correct, it's going to shift out of collimation again as I point to different targets and gravity has varying impact on the drawtube. Not sure how I can resolve this... might send Moonlite an email and see if there's an adjustment I can make to reduce the slop in the drawtube.

[Paul Haese]

This is the very reason I am going to get the Atlas working. I think the problem essentially is that the focusor cannot hold the load and hence introduces tilt and astigmatism. Pity really because Moonlite promises so much with some of its features and finish.

[codemonkey]

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...

https://youtu.be/Mw4hIq_byp4?t=1m8s

[Paul Haese]

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.

[codemonkey]

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.

[codemonkey]

Heard back from Ron:



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.

[DaveNZ]

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.

Im currently looking at upgrade options.

dave

Dave

[codemonkey]

Hey Dave,

It's called the "Litecrawler" and has the following specs:


Format 2.5”
Profile 2.75”
Travel .3”
12 lb. capacity
374920 steps per revolution
.0000105” per step, or .2667 Microns per step
Uses 68mm Threaded Accessories
Has all features of NiteCrawler
Uses same NiteCrawler SW and drivers


If you PM me your email address I can send you a word doc I got from Moonlite.

[codemonkey]

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.

[Paul Haese]

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.

[codemonkey]

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!

[DaveNZ]

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

[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

[codemonkey]

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.



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.

[Slawomir]

Interesting test with artificial star field Lee :thumbsup:

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?

[codemonkey]

Good thinking, Suavi! Thanks :)

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!

[codemonkey]

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!

[peter_4059]

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.

Peter

[codemonkey]

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.

[peter_4059]

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?

[codemonkey]

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!

[billdan]

Hi Lee,

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.

[codemonkey]

Thanks Bill :-) Forgive my ignorance, but would the two blur factors not be additive? That would make sense though, it did occur to me how everyone else imaging with a newtonian got by...

[billdan]

The CFZ is the focus zone where there is no more improvement in the scopes resolution.

The diffraction limited resolution is defined by aperture size, Starizona use the formula of 140arcsec / aperture in mm (though I've seen other websites use 120/aperture).

So 100mm is 1.4arcsec, 200mm is 0.7 and a 400mm scope is 0.35 arcsec resolution.

As CFZ is defined by Focal ratio, and if each of these scopes were F5 then the CFZ for each is approx 66microns.

If the seeing limits our resolution to 2arcsecs, then we just multiply the 66 microns by the difference in resolution.

So the 100mm scope is 66micons x 2arcsec/1.4arcsec = 94micron CFZ
The 200mm scope 66 x 2/0.7 = 188 microns, and the 400mm is 376 microns CFZ.

The 2 arcsec seeing blur is an average over time, so there will be brief moments when we do get diffraction limited resolution.

All this is academic and doesn't really solve your problem off course, but I thought I would explain my understanding of whats going on.