79 replies to this topic

[pterodyne]

Ugh, so here it goes. For the record also, I am self taught, and terms like HFD/FWHM and techniques of collimation are all learned by reading threads and forums.  Please go easy on me.

 

I recently bought a used AT14RCT off of astromart for a very good price.  The seller didn't want to ship so I got a good deal by picking up the scope in TX and driving it to the observatory in CO.  I was very excited to get it installed, but unfortunately the pier was too tall.  I had done the measurements but didn't account for the fact that the pier is centered.  (I live in Baja California Sur in Mexico), so the planning phase was mostly just sketching to see if it would fit.  Anyhow, I tore out the cement pier and got a Ioptron tri-pier 360, and voila, the 14 just barely fits in the 8' exploradome.

 

Yay! crisis averted!  Not so fast.  Early star tests showed some problems that I could only assume were collimation.  So I borrowed a howie glatter 2" laser with the concentric rings attachment.  The original owner stated that he had only adjusted the secondary (using the same laser) slightly.  I was getting no lower than HFD of 8 or so, but was still generating V-curves, so it was reaching focus, but just very poorly.  Initial thoughts were the CA fires from this past summer were making for very bad seeing.  After time went on, it was clear that the problem was in the optics.  The observatory is in the very dark (bortle 3) dry high basin of Park County Colorado at 9000 feet.  Ive had great success with the prior scope (AT10RC).

 

Specs:

Telescope: AT14RC
Focuser: Moonlight CSL with Stepper and controller
Camera: SBIG STF8300 with FW5/OAG
Guider: Lodestar X2
Mount: Losmandy Titan with steppers/EQDrive controller and EQMOD.

 

The collimation seemed to go ok, and we were able to adjust the concentric rings so there was no shadowing or out of concentricity. 
We used the following technique:

 

Adjust the base ring (focuser ring tilt tip) so the laser is centered in the secondary  (not the moonlight tilt tip, this has never been adjusted)
Adjust the secondary so the conecntrinc rings are concentric
Adjust the primary (not needed)

Still poor HFD.  Keep in mind this scope lives in an unheated obsevatory so it tracks pretty well with the ambient temperture but none the less sufficient time was given to allow the mirror to cool.  The AT10RC could get below 6 HFD even on the very worst of nights in this location.

 

The first thing I noticed is that this 2850MM telescope was showing 2820MM after astrometric reduction in MaximDL6.  This is roughly 30mm out of spec.  Admitedly I don't know how bad this is but I can say from experience the AT10RC was never more than +- 2 mm based on temperature, and that was the steel tube version.  I imagine that this carbon fiber truss telescope should be similar or better.  Having read just about every thread out there on this topic I decided that a focal length reset was in order.  Most people are saying not to adjust the secondary's center screw, so we slowly adjusted the primary by loosening the retention bolts and increasing the focal length by adjusting the larger bolts the exact same number of turns.  Then we redid the astrometic reduction.  After several iterations, we are now within 4mm.  Obviously it could be adjusted more, but I can say this has not improved the ability to achieve good focus.  Again the V-curve has a definite bottom, this is not a back focus issue.

We have also reset the focuser base ring collimation so that it is in the factory position. The laser is reading in the center spot of the secondary.  I even went as far as to buy a newer Moonlight CSL which has larger bearings.  I was worried that the drawtube was out of collimation on my original moonlight as the laser seemed to move in the secondary center spot when racking in and out (or the bearings were worn out)

 

So the main problem seems to be astigmatism.  Please see the video at:

https://www.youtube....h?v=K9Zk1NJke10

Note the astigmatism changes direction inside and outside focus by roughtly 90 degrees.  At the moment really out of focus stars seem to be fairly concentric.

 

Im at a loss as where to go from here.

 

Ive considered reseting the whole system using this technique:
https://www.cloudyni...e-reset-method/

but that would require adjusting the center screw of the secondary, but the effect would be the same, get the focal length correct (secondary this time), reset all the collimation points to the unadjusted position. 

 

Maybe my secondary is not centered? or the center spot on the secondary is not centered (Astronomics suggested this might be the case)

 

Im also concerned that no amount of tightening of the moonlight load screws will keep it from flexing the laser off the center spot of the secondary if I pull gently on the laser.  Right now I have it tightend to the point just before where the moonlight stepper will stall, which seems too tight to me.

 

With so many possibilities for what could be wrong, I have really lost my way.  I never had to collimate the AT10RC and the tolerances on the focuser never seemed to present a problem in the images. 

 

Thanks for looking.

Edited by pterodyne, 10 January 2019 - 12:43 PM.

[WadeH237]

To my eye, the stars look roughly triangular.  I would suggest checking to see if either the primary or secondary mirror is pinched.  I don't know how the primary is mounted, but I wonder if getting the secondary collimation screws too tight could be an issue.

[pterodyne]

Wouldn't in that case the direction of the aberration stay the same as focus was racked in and out?

[Terry White]

I have a AT-16RCT myself and I've been digging around for collimation information as well. I think the best technique for collimation is the DSI technique at: http://www.deepskyin...ure_Ver_1.0.pdf

The advantage of this method is that it doesn't rely on secondary center-spotting being dead on the optical axis of the secondary. DSI goes into some detail explaining why the center spot may not be on the optical axis. So if it was me, I would plate solve for exactly 2850mm and then try the DSI method. It is well-known that GSO scopes are designed for best optical performance when they are at their design focal length. I agree with WadeH237 that some pinching may be discenable in your video. It is more likely in the primary mirror floatation cell shown here for a AT-16RCT, but the AT-14RCT should be quite similar: https://www.cloudyni...eering-drawing/

Unfortunately you will need to remove the backplate to get to it, which is major surgery. If you're up to it, you can try something similar to the German fellow here: https://www.youtube....h?v=h9KICeAYurY

A distortion changing direction on either side of focus is characteristic of astigmatism, so you may have more work to do on setting your focal length.

Edited by Terry White, 10 January 2019 - 02:47 PM.

[WadeH237]

I don't think that it's just astigmatism that flips when you rack through focus.  I know that tube currents will do the same thing.

 

If the stars were ovals where they flipped between the major axis oriented towards the center of the field and the minor axis being oriented towards the center of the field, then I would agree with astigmatism (also off-axis astigmatism is expected for an RC scope, and you will notice this when you defocus even a properly set up RC).

 

It's also possible that you have multiple aberrations going on.  If it were me, I would tackle the most obvious one first.  That said, I would recommend that you get a few more opinions - and understand exactly what's needed to make corrections - before disassembling the scope.

 

Just my two cents,

-Wade

[pterodyne]

Terry, thanks for the engineering drawing, I had looked for one and hadn't found it yet. And Wade for your responses, thanks.  I have a friend who is helping me in Colorado who is a fellow astro guy.  We will likely have to try the DSI method.  Disassembling the scope is clearly not for the faint of heart, especially after watching that video.

 

We had a time where the collimation retention bolts were loose after returning.  Likely a thermal differential between when they were set and the day we were there.  The exploradome base building is insulated, but the whole building is in a very barren windy place, and there can be a pretty large difference between day and night, even this time of year.  Maybe the opposite is happing with the pinched optics.

[ecjoyner]

I never thought about the possibility of pinched optics due to thermal differences, but it does seem logical if the tolerances are too close on the mirrors and in the real cold temps they contract too much.

[Terry White]

Terry, thanks for the engineering drawing, I had looked for one and hadn't found it yet. And Wade for your responses, thanks. I have a friend who is helping me in Colorado who is a fellow astro guy. We will likely have to try the DSI method. Disassembling the scope is clearly not for the faint of heart, especially after watching that video.

We had a time where the collimation retention bolts were loose after returning. Likely a thermal differential between when they were set and the day we were there. The exploradome base building is insulated, but the whole building is in a very barren windy place, and there can be a pretty large difference between day and night, even this time of year. Maybe the opposite is happing with the pinched optics.

My pleasure! You may not need to go all the way with a complete disassembly as shown in the video. The three clamps at the edge of the mirror should have a soft material between the mirror surface and the hard metal clamp. These are the first places to look for pinching. I have seen pictures of the secondary mirror assembly. The secondary is sandwiched between some threaded rings and it seems unlikely that this smaller mirror is responsible for any pinched optics.

The RC design has a strong optical axis for both the primary and secondary. Collimation requires both axes coincide at the designed focal length. It goes without saying that the OTA should be in complete thermal equilibrium before any evaluation or adjustments of the collimation should be made. This requires adjustment of tilt in both mirrors and radial offset between mirrors. It appears that GSO uses the machining tolerances to center the primary and secondary mirrors in the OTA because GSO doesn't provide any ability to adjust the relative offset between the two mirrors in the plane perpendicular to the optical axis. All GSO has are tilt adjustments for both mirrors, the focuser adapter tube and an axial spacing adjustment on the secondary mirror. They built the OTA using CNC (Computer Numerically Controlled) machining. Evidently the CNC tolerances must be small enough to guarantee centering of the two mirrors in the OTA. Why am I belaboring this point? Well, be very careful that you mark the position of each truss rod and truss bracket on the backplate so that you put it back together exactly the same way it was before you started.

I too have an 8' insulated ExploraDome observatory in progress.

I plan to install an ductless split mini HVAC system to cool the observatory down to the anticipated outdoor temperature at sunset. After sunset I plan run an exhaust fan with the dome shutters open to thermalize the dome interior to the outdoor temperature. I put the dome on an elevated base to reduce the ground thermal contamination much like Sierra Remote Observatories has done. I am also going to have to make some modification to the dome and OTA to get the AT-16RCT to fit, so I appreciate your problems. Please keep us posted on your progress!

Edited by Terry White, 10 January 2019 - 06:32 PM.

[pterodyne]

Nice build! 

 

Any active cooling or heating is out for me.  I am fully off-grid, with internet provided via rural WIMAX.

 

Not to hijack my own thread, but how are you going to get the 16 in there?  My 14 is less than an 1" from the dome in many places! Another issue for me has been the dome slit diameter.  It works but the 14 leaves little room for error.  What type of dome encoder are you using?

 

Thanks for the reply, lots of good info in there.  Im hoping the DSI method helps my problem. 

 

Thanks,

 

Bryan.

[Terry White]

At 87" in diameter, my dome's mounting ring is 1" larger in diameter than the ring in the standard 8' ExploraDome kit, so I have some extra room there. The two main dome interferences for me are the shutter battery and the two actuator brackets for the lower shutter, and I plan to modify both so they protrude less into the OTA real estate. I'm also making a custom low-profile carbon fiber plate that directly mounts the OTA to the mount. This will replace the thicker aluminum dovetail and dovetail saddle plate. This will move the OTA a little closer to the mount and reduce the OTA moment arm. As far as the shutter opening ( ~ 26") goes, I measured it at the worst-case OTA positions and it looks OK. My stock Fosters Automation encoder is on the East side and does not interfere. I'm hopeful that the latest software and firmware updates will fix the dome tracking issues that people are having. It looks promising based on the user feedback so far. In any event, I'm prepared to buy another more reliable automation kit if the Fosters Automation kit proves unreliable.

Edited by Terry White, 11 January 2019 - 05:01 PM.

[pterodyne]

At the risk of further hijacking my own thread,. I've had three different done controllers, one diy, the foster, and maxdome II. and three different encoders. one was an enclosed optical. No e were particularly reliable. Currently I'm just using the "counting the ring holes" method. Seems to work as well as anything.

Back to the RC scope

anyone have thoughts on doing the DSI method using an artificial star? I realize I'd have to move the scope or the star to get the corners for off axis. But would alleviate tracking/seeing issues. This topic is probably as controversial as the original thread.

[akulapanam]

Yeah that doesn’t look like a collimation issue to me unfortunately. Looks like a more serious mirror issue. Best case maybe just pinched optics.

[Timo I]

From here (Finland and a previous 10" RCT owner) that looks like a clear collimation problem, where your RCT scope is grossly out of collimation there. But please don't despair with it, because you can get it quite easily  into 95% working order with this collimation guide alone: http://www.deepskyin...ure_Ver_1.0.pdf

Forget the collimation lasers, because those are not even nearly accurate enough for this task. The laser attachment to the focuser tube itself causes more errors to collimation views, than you can get with tiny movements from the collimation screws. But that DSI collimation guide will be very accurate method for that collimation need there.

 

What you should get there by adjusting primary collimation screws only, is something like a out of focussed star visible in this photo:

https://astrokuva.ga...y_mirror_ok.jpg

Please read that DSI guide and you will know how to get there by adjusting those primary mirror collimation screws.

 

When you have primary mirror aligned so that your scope gives such slightly our of focus concentric stars in the center of image, then you can adjust your scope's secondary mirror.

Please read that DSI guide again for those screw adjustments and you should get out of focus star view from your scope a bit like this image:

https://astrokuva.ga...y_mirror_ok.jpg

 

Please believe me, I have been there (grossly out of collimation) before with my 10" GSO RCT scope more often than you would believe.

Images like this from that RCT scope have been familiar to me, when that scope was grossly out of collimation. And then those in focus poor FWHM oval stars too were quite frustrating for me, when that specific 10" RCT scope was totally out of collimation. But after a very careful (DSI) collimation routine done for that same RCT scope, then suddenly the exact same RCT scope could deliver an optical performance like this: https://astrokuva.ga...ionMeasured.jpg

What you would need to do there, is how to learn the correct collimation processes for those hyperbolic RCT optics! (And that DSI collimation guide will be a perfect place to start your journey with that steep learning curve.)

 

I have been disassembling and reassembling my RCT several times during years 2013-2018, because I wanted to learn how to collimate a cheap Chinese scope into a similar condition than those really expensive RCT scopes wit perfect mechanical structures. (And to be frank, it was my own fault in the first place, why that RCT scope got out of collimation ).

You can find my testing photos from that collimation trip here: https://astrokuva.ga...eistot/GSORC10/

It has been a many years long learning curve for me in the past, but I learned how to collimate such a cheap RCT scope properly so that its' performance with AP CCDT67 reducer would please me. (Then it was time to sell that scope to another hobbyist here in Finland, but that's another story ).I'm sorry for the Finnish language in some images there, just be advised that those images have been discussed in detail in this Finnish astronomy forum thread. But like they say, one photo can tell you more than thousand words... (I can tell more about those photos here, if that's needed).

 

Basically, what you need to understand here with those RCT scopes, is that they need very very accurate collimation to be able to perform with their promised quality levels. The optics even from from such cheap GSO/Synta factory have never been a limiting factor for my 10" RCT scope, but the mechanical and optical collimation for those RCT components (=mirrors, focuser) have always been the limit. Please notice, the tolerances for a perfect (99%...100%) RCT collimation accuracy are very, very narrow. With cheap Chinese mechanics that kind of collimation level might even be un-obtainable (for example secondary or primary mirrors might be slightly off-axis mechanically with each other). But despite of that missing mechanical quality, those cheap Chinese RCT scopes could give you very satisfactory results for your imaging. I would even say as a fact that the optical performance from these scopes will always depend on the collimation accuracy for these RCT optics. Something like this or this will be available, if a RCT user is patient enough to learn the secrets of those optics (you can notice the slight error in the star field in the lower right corner of that star field; that could depend on those mechanical alignments of optical parts related to focuser/camera combination).

[pterodyne]

I really appreciate you taking the time.  Yes, the DSI method is for sure the next step.  My old RC10 must have been fairly well collimated because I never event tried to adjust it, and I had good images.  And that is including the focuser sag and other imperfections. I have to believe this will be possible. 

 

Thanks again Timo, and for everyone who has responded to my thread.

 

Probably it's easiest to just do it under the sky rather than try to do an artificial star.  I just liked the idea of removing seeing/tracking from the equation, but seems like the returns would be diminished by the fact that it would be collimated horizontal, and not to mention removing this scope from it's home is no small task.  There would simply be no place to put the artificial star that would be far enough away that I could aim at in the observatory without a bucket truck to hang it.  The bitter cold is a problem however of doing the DSI method in the observatory.

 

Thanks again.  Ill post some out of focus images for fun when weather permits.  It's going to be a while before I can get there to collimate.

[Timo I]

Telescope: AT14RC

Adjust the base ring (focuser ring tilt tip) so the laser is centered in the secondary  (not the moonlight tilt tip, this has never been adjusted)

---clip---

The first thing I noticed is that this 2850MM telescope was showing 2820MM after astrometric reduction in MaximDL6.  This is roughly 30mm out of spec.  Admitedly I don't know how bad this is but I can say from experience the AT10RC was never more than +- 2 mm based on temperature, and that was the steel tube version.  I imagine that this carbon fiber truss telescope should be similar or better.  Having read just about every thread out there on this topic I decided that a focal length reset was in order.  Most people are saying not to adjust the secondary's center screw, so we slowly adjusted the primary by loosening the retention bolts and increasing the focal length by adjusting the larger bolts the exact same number of turns.  Then we redid the astrometic reduction.  After several iterations, we are now within 4mm.

---clip---

Ive considered reseting the whole system using this technique:

https://www.cloudyni...e-reset-method/

but that would require adjusting the center screw of the secondary, but the effect would be the same, get the focal length correct (secondary this time), reset all the collimation points to the unadjusted position. 

---clip---

Maybe my secondary is not centered? or the center spot on the secondary is not centered (Astronomics suggested this might be the case)

I quoted some bits and pieces from your original message here for some commenting.

 

The 1st quoted chapter is very important, because that (focuser tilt) will define your baseline for the DSI collimation method too. You will be later adjusting your RCT mirrors based on that baseline you have created there between your focuser (center point) and center of your secondary mirror. Try to make absolutely sure, that your laser in the focuser has absolutely no tilt (not internally inside the laser itself and not when you attach the laser to the focuser). Adapters like Howie Glatter Parallizer or Starlight Instruments 2" Parallizer are here well worth their cost (those links go to EU web shop, but you can buy those cheaper from US retailers).

After that you can also do a preliminary adjustment for your secondary mirror, when that laser beam is pointing in the dead center (central ring center) of that secondary mirror.

Make sure that the laser beam goes back in the center of the laser hole itself by adjusting your secondary screws. (You can use polarizer filter for watching the laser beam returning there and improve its' visibility by rotating the polarizer.) When you think you have nailed down the alignment of your focuser and secondary mirror tilt, then you can use that Howie Glatter's 2" laser with the concentric rings attachment. Now you should adjust only primary mirror screws. It would be optimal, if the laser would not be removed from the focuser between those stages, but it will be really dangerous to look inside your scope, when those concentric laser ring beams come out of the tube (like they are supposed to come). So please be careful there!

 

I have used a narrow hole adapter in front of my Howie Glatter 1.25" laser, while adjusting that focuser levelling so that the beam goes dead center of the secondary (and returns back to laser's hole by adjusting secondary screws). Then I have changed that concentric ring adapter to my Glatter laser and done the primary mirror's preliminary adjustments before doing that DSI collimation run with the stars.

The 2nd quote about focal length resetting and adjustments makes sense too, because those hyperbolic RCT mirror distance between each other has a significant meaning for the Strehl numbers.

I have used CCD Inspector from CCDWare here as my collimation help. It gives great 3D visualizations from the aberrations of these RCT scopes, which are really helpful too. When your mirror distance is less than optimal, then your scope will have more "curvature" in the 3D view of that CCDI measurement. Example of that kind of 3D curvature is here: https://astrokuva.ga...91_measured.jpg (Star field of the same image is here.)

 

This image (https://astrokuva.ga..._comparison.jpg) shows how this "curvature" changes when I have changed the distance between my 10" RCT mirrors. The curvature 3D view actually inverts, when the distance goes over the "tipping point" of the optimal distance between those mirrors!

What I'm actually saying here, is that you would also benefit greatly from that CCD Inspector software with your RCT scope (highly recommended purchase for all RCT owners interested from their RCT collimation ).

 

Couple of links I have found useful for this area of RCT collimation:

http://interferomete...retien-gso.html

http://www.hnsky.org...collimation.htm

 

The 3rd and 4th chapters about secondary mirror's position in the tube... These are quite dangerous thoughts!

In these RCT scopes the centering or secondary mirror is absolutely essential to the system's total optical performance. And I say this first above all: please do not try to adjust RCT scope's secondary mirror position sideways in any way, because it might cause catastrophic results for your collimation attempts. I tried that, simply by widening that center hole of that attachment screw (=huge mistake), so that I could move my secondary very slightly sideways. NEVER do the same thing, because it took several nights from me to get back into the correct secondary position, dead center of the RCT tube. Not even that otherwise excellent DSI collimation method helped there and while attempting to re-center my secondary in the tube (it should be originally located by GSO or other manufacturers very accurately there). Here's some CCDI curvature measurements from that odyssey: https://astrokuva.ga..._collection.jpg

It's no fun getting such measurements from your scope in the early morning hours

 

Then something about the secondary center screw itself. It's sort of self centering one (conical base) and if you loosen and retighten that screw back, you are supposed to end back where you begin (secondary sideway position-wise, I mean). But loosing that screw totally is always a minor risk, so I'd avoid doing that for the reasons stated above. All in all, that secondary mirror should be centered there by the manufacturer within reasonable tolerances and there's nothing, that average RCT user could do for it better.

 

I guess that's enough for today, but there's quite many other ways of doing RCT collimation day-time and I'd like to tell a word or two on those too.

Maybe later, because here in Finland winters are usually very cloudy and that daytime mirror reflection collimation method is very useful and accurate way of collimating RCTs too. 

Anyway, good luck with your attempts on that 14" RCT collimation, because it will be a great imaging scope after you have managed its' collimation closely enough for it to get sub-four HFD values in focus.

(I had sub-3 HFD values in a good seeing conditions from my 10" RCT after all that pain with years of collimating it before I sold it.) That collimatuion process was part of my hobby over then, I guess

Edited by Timo I, 17 January 2019 - 02:23 PM.

[Timo I]

I guess that's enough for today, but there's quite many other ways of doing RCT collimation day-time and I'd like to tell a word or two on those too.

Maybe later, because here in Finland winters are usually very cloudy and that daytime mirror reflection collimation method is very useful and accurate way of collimating RCTs too. 

About those daytime reflections...

"Hall of mirrors" is probably familiar to all RCT owners, but from my experience that gives less than perfect results for collimation. Near there, but not perfect.

Anyway, by looking into tube from the front and then aligning the secondary mirror support vane with its' own (vane) reflection on the mirror you should get something like this view to your scope:

https://astrokuva.ga...mirror_hall.jpg

 

Much better collimation check view will be from the front of the tube some distance away from the tube. Align yourself there so that you can align the secondary spider vanes with their shadows on the mirror reflection. It helps, if you make a piece of cardboard with a small central hole in it and then look into the tube through this sight hole. Now the scope's mirror reflection will be much brighter to be checked carefully. You will soon notice that your eye position there will be extremely sensitive to your movement, but try to be still there. Or use a camera with a live-view instead of your eye, when looking into scope this way. When the spider vane and the shadow of that same spider vane overlap with each other, then you know that you are aligned with the optical axis of your RCT mirrors.

 

Your view to the tube should be something like this:

https://astrokuva.ga...sta_vinossa.jpg

As soon notice, there won't be similar reflection rings there as in my image here. In that case you need to move yourself forward or backward in relation to your scope. When doing this "walking on the line" always keep those spider vanes overlapping with their shadows on the mirror surface. It would be perfect, if you could build a long optical bench, where you would have those peaking holes at different distances from the stationary RCT scope. But even without that bench you will soon learn how to place your eye into the coprrect position, so that the scope's optical axis will actually go through your single eye. If you look at the mirror surface there and watch the reflections (ring shaped), you can quickly tell how near your RCT collimation is from the perfect collimation result.

 

Here's a bit better collimation result from the scope after some mirror adjustments:

https://astrokuva.ga...stus_edesta.jpg

And this is what you are targeting there with your collimation attempts:

https://astrokuva.ga...nt_05032013.jpg

 

I'm not telling how to get there, because you need to learn that by yourself.   Make small adjustment to one secondary mirror screw and watch how the ring reflection behaves there on the mirror surface. Did the reflection ring get closer to the concentric view or further distorted away from the center? By that result you can then move the same collimation screw further or backwards.

 

When you have managed to center those reflection rings viewed from different distances in front of your RCT scope, then you can be quite confident that your scope will give good star test images too. And the best part of this "Staring into your tube collimation" method is that you don't need any expensive gadgets and collimation tools for this. Just your plain eyes and a piece of cardboard + hex keys for adjusting those screws on the secondary or primary mirror

 

And when doing this kind of RCT collimation correctly, then this collimation method will be much more accurate than for example Takahashi collimating scope's results. Tak scope requires that your scope's mechanical axis is perfectly aligned with your scope's optical axis. Then it will give very good results for these RCT scopes. But with these cheap Chinese scopes the mechanics and optics axis do not always match with each other and that explains, why perfect Tak collimation result is sometimes not that perfect, when viewing the star test results. Those in-front-of-scope reflections could be nearly as accurate collimation method as the DSI method with the night stars.

Edited by Timo I, 18 January 2019 - 10:50 AM.

[pterodyne]

Quite a treatise. It is appreciated. Ive read about this method numerous times, but have been a bit skeptical.  But what do I know!  Next to nothing.  It's probably a good, and easy way to start and in the daytime.  And I can't argue because the howie glatter laser has proven useless in my case.  Im glad I didn't buy a takahashi collimation scope.

 

One question, you only mention adjusting the secondary in this hall of mirrors technique.  Is there a process to starting with one or the other?

[Timo I]

One question, you only mention adjusting the secondary in this hall of mirrors technique.  Is there a process to starting with one or the other?

From my experience I'd say that every method of RCT collimation should support each other somehow. What I mean by that?

If you have a perfectly collimated RCT scope by the night time star tests and it produces very good results from the night sky. Would it be reasonable to say, that your laser collimation tools, Tak collimation scope and those pure visual verifications from scope's mirror reflections would all give you similar results from the collimation status? I would say that it's NOT a reasonable assumption with these cheap Chinese RCT scopes with their minor imperfections in the mechanical accuracy (alignment of optical / mechanical parts).

 

In a very expensive RCT scope you actually pay for the perfect alignment of those RCT mirrors, where all optical parts have been aligned extremely accurately mechanically with each other. BUT with GSO and other similar mass manufacturers, there you are bound to accept that even though you do a perfect DSI collimation routine under the starry sky, then you won't see perfectly aligned mirrors in Tak collimating scope. It's those miniature mechanical inaccuracies (for example with primary mirror attachment to its' mirror holder cup with silicon or secondary mirror centering), which will cause some kind of deviation between the results from A.) the methods using optical parts alignment only (=DSI method and visual reflections daytime collimation) and B.)  the methods using scope's mechanical structure as their reference points (Tak scope, lasers etc.).

 

For that reason I try always to be as much certain as possible, that my RCT scope's focuser is aligned 100% correctly, my secondary is accurately in the center of the scope etc. etc.

Then I use the Glatter laser for preliminary alignment of my secondary mirror's tilt like I told before. I also make the primary mirror's initial adjustment with the Glatter's holograhic laser attachment. I don't believe that the collimation accuracy is any better that 85% from the perfect RCT collimation, but that basic adjustment will be a good enough starting point for doing more accurate collimation attempts relying on the optical parts adjustments only.

 

I can then start with the "Hall of mirrors" method for secondary adjustment, because its should give me similar (at least 85%) level of collimation result from my scope. I might see that there's some tilting in secondary mirror, which I'd like to correct by minor adjustments, but normally I don't trust "Hall of mirrors" too much, because I haven't been able to align myself looking there correctly from all four corners. (That is a very accurate method for secondary alignment too, if you can interpret it's views accurately. I could not.) So I might... or might not make adjustments to that secondary at that stage. That's all I can say about it.

 

Something about the Tak collimation scope, I own it too 

It is a very accurate tool in its' own way for these RCTs. Here's an image series, which shows secondary mirror tilt adjusted with the axis of scope's focuser. In a perfectly mechanically accurate RCT scope the Tak scope's view tube should give you views/results almost equal to real star test results (just like when the DSI method is used).  By the way, those views from Tak scope look a bit like examining star's Airy disk in a good seeing. Just push and pull that sight tube and look how the rings grow or get smaller with the sight tube flowing inward/outward. Image for that is here:

https://astrokuva.ga...focus_range.jpg

Do you think that secondary is now aligned with the focuser perfectly?

 

Well, in my RCT scope I could not trust that view fully, because the DSI method (under the stars) gave a bit different collimation results from my scope. Or the visual inspection of those reflections told me that my collimation was a little bit off in that kind of situation   But I learned gradually that I could use my different collimation tools and methods in parallel with each other simultaneously. Each of those gave me a little bit different results from my RCT scope. In the end I learned that the best collimation accuracy for the final image came out of all those tools together, verified with each others in differnet stages of that collimation process.

I can also say that I value most these optical collimation methods, because these will give the best results for my imaging purposes.

 

Tak scope has a good feature for aligning the primary mirror too. So it gave me a good enough starting point for example doing collimation by those daytime reflections. After those (usually quite minor) adjustments with reflections I could say that the RCT scope's collimation accuracy had got over 90% of the perfect collimation result. But at the same time I need to say that you really need patience to decipher those collimation results and make small adjustments for either primary or secondary mirrors. If you think you need to make any bigger changes in your reflection collimation method, then I can tell you have gone wrong somewhere before that need. With DSI and visual reflection adjustments the screw rotations should not be any bigger that 1/16... 1/32 full turns. I think you should get your basic RCT collimation level up to 80% or better (from perfect 100%) before starting with those minor adjustments done by the visual inspections from mirror reflections. DSI method under the stars can be used for more un-collimated RCT scope than those daytime collimation attempts, where you would need other collimation tools to get you started.

 

Disclaimer: I do not take any responsibilty for any nearly collimated RCT scopes, which get totally out of collimation by attempting those daytime reflection adjustments. Overall view to RCT collimation is such, that you can compare it to walking on a wire above 10-15 meters higher from the earth level. When you make one small movement wrong, then you are likely to drop from that wire quite suddenly. And it will need some effort to get your scope back to the nearly collimated status again. But being there on that kind of collimation journey can be quite rewarding experience too

 

Here's just some different star shapes from my 10" RCT scope from worst to best: https://astrokuva.ga...star_shapes.jpg

Only difference being the collimation accuracy of that scope. Which stars will be yours?

[pterodyne]

for what its worth, still to cold to attempt a star collimation.  Also I'm thousands of kilometers away, so there is that.  Here is a manual focus video.  200 steps per image, from outside focus to well inside.  This is plate solving at 2847.2mm on the best focused image.

 

https://www.youtube....eature=youtu.be

[Timo I]

Thanks for the video.

It confirms nicely that your 14" RCT has serious collimation problems and at this stage you should concentrate on your RCT's primary mirror adjustments only!

Your video's time 0:04 shows that your RCT image has clear On-Axis Coma.

Compare those unfocussed stars with this properly collimated RCT image's star format (the center image there).

https://astrokuva.ga...llimation_1.jpg

 

The disk brightness should go around the unfocused star disk evenly (there should not be any brighter sections in any side of that disk, but your seeing conditions might cause random brightenings become visible for all around the disk and those should average out nicely with longer exposures). Collimation error brightening (On-Axis Coma) does not disappear by exposing longer.

 

When you will be at your RCT scope, then let camera expose countinously similar unfocused images and cover scope's front with some object (your hand is just fine) and find out where that object darkening resides relative to primary mirror adjustment screws. Find out the collimation screws on such side, where the star disk has the brightest area visible (in your case it points to 7 o'clock direction in your image, so you should select collimation screw either 7 or 1 o'clock direction = the opposite one). Loose locking screw on that side and then make a small adjustment to the actual collimation screw (it's the smaller one of those two) and lock down the setting with that (bigger) locking screw.

 

Then check if your unfocused star shapes got better of worse in your image and continue adjustments accordingly until your defocused stars become illuminated evenly.

Remember to check this from a star in the center of your image. In that video there's such a huge collimation error, that it shows all around the image so clearly. It's no wonder, if your star shapes in the focus resemble triangles and have poor FWHM/HFD measurements. (You should end up into star shapes like seen in this image here, but that end result might need several iterations between primary and secondary mirror adjustments, so please don't despair in between. It will take some time to learn this kind of collimation skill ).

 

Please post either image or video from the primary mirror adjustment result, when done. This DSI collimation method is described in this link: http://www.deepskyin...ure_Ver_1.0.pdf

(You can refer to Example of Removing Coma by Adjusting the Primary Mirror paragraph there.)

 

[pterodyne]

Thanks again Timo for a detailed reply. I agree that is what it appears to be. However, if it's on axis coma of the primary, why would the direction of the coma change inside and outside focus? Just trying to understand.

[Timo I]

That is because you have now a mixture of both (primary and secondary mirror) collimation errors there in your image.

Look for the DSI guide once again (pg. 17 and Adjusting the Secondary Mirror to Remove Off‐Axis Astigmatism this time). Your image is now so much out of collimation for secondary adjustment too, that the image has no center of collimation point visible there in your imaging area. What you see there is just those "pointy stars" (oval, astigmatic ones) all over your image. Astigmatism transforms your stars into one direction ovals on one side of the focus, then the direction changes 90 degrees, when you focus your scope to the other side of focus point. That astigmatism disappears, when you nail down your secondary mirror collimation.

 

In order to get to the 90% (and better) collimation accuracy you would need to get in a situation, where you actually can see the "center of collimation view" on your imaging area of the sky. Here's such an image for some unfocused, but well collimated RCT star field: https://astrokuva.ga...y_mirror_ok.jpg

You can see how stars are there well balanced as they say in that DSI guide.

 

Right now you can see only those "pointy stars" in your image (refer to pg. 15 on that DSI guide for definitions on Round, Oblong, Pointy and Flat Stars). In your image the major axis of your stars is pointing towards the center of the collimation point way outside your imaging area, which camera sees now. What you need to do there, is adjust also the secondary mirror tilt so that this "center of the collimation point" is moved into your camera view. This area of nearly spot on collimation is very narrow and it might be difficult to get that adjustment done successfully with stars under night sky. That's the reason, why also told you about day time collimation methods, using "hall-of-mirrors" method, laser collimators, Tak collimation scope usage etc. above. It would be helpful, if you could get your secondary aligned properly enough, so that you would get nearly the "balanced image" to begin with (pg 13. in that DSI guide). But now your scope is deep in woods now in maybe 75% collimation accuracy only and you need to find your way out of those "bushes".

You can start that travel by aligning your primary (pg 11 in that DSI guide for removing on‐axis coma).

Edited by Timo I, 10 February 2019 - 06:29 AM.

[pterodyne]

Ok, I get it now.  Thanks.

[Terry White]

Good luck, Bryan.

[Erik30]

It is post like this that really make CloudyNights.com shine.  In depth help with scope issues to get someone up and running again..    

 

I hope you get it collimated soon.. When you do post up some images..