131 replies to this topic

[AradoSKy]

Hello,

After an equipment failure, my Orion 6” RC was out of alignment. I had a Cheshire for my Celestron 4” Newtonian. I first tried the Cheshire. I was frustrated because of gravity. The long Cheshire has no support in a Crayford focuser.

I found a YouTube video with this same model. His method took out the tube by judiciously disassembling the OTA.

Then I got a FatPoint Cheshire. This one is very short. Using this device allowed me to get pretty close.

I had a laser collimator for my 4”. I used this next to collimate the RC. Then collimating became easier.

I have the Howi Glater. The HG is 2”. Gravity isn’t a factor.

Lessons learned.
1. Gravity is real. I can’t cantilever any device in a Crayford.
2. The tube must be removed to properly collimate.
3. Alignment begins with the primary primary mirror with the FP Cheshire.
4. The secondary mirror is aligned with the laser.

Note. The laser can be the FP or HG. Both are not cantilevered and droop because of gravity. Gravity cannot be collimated out of OTA.

[Corsica]

I'll keep this short since I'm tired of explaining everything again, but I'm suffering the common problem of collinating a 16 inch Ritchey Chretien.  Need I say more?

 

I've been researching other ideas for this. 

- Takahashi collimating scope

- Autocollimating eyepiece

 

Scope:

GSO 16 inch Ritchey Chretien - our club has had this over a year, I've probably spent more than 20 hours struggling with it, and getting nowhere.  Still haven't taken any images that I'm happy with.

 

Have :

- Howie Glatter 2 inch with the Grid pattern (can't find any info on this type, but it's easier to find than the round type.  Where are the instructions on using this?)

- Cheshire eyepiece with the reflective surface with the hole.

 

Time wasted on this:

- 20+

 

Frustration:

- 95% (I haven't given up yet, but insanity is getting closer).

 

Question:

- What should I try next?  Takahashi collimating scope?  Autocollimating eyepiece (I've just discovered this from another discussion and reading more about this idea).

- If I can get something that will let me collimate this in 2 hours rather than 20 and making zero progress, I'd be thrilled!

 

Help!

 

Tom

RCTs have hyperbolic mirrors each with its unique optical axis making RCT collimation much more difficult than scopes with a spherical secondary mirrors, such as SCT (any radii of sphere is an optical axis).

The challenge is to align the two RCT mirror optical axes such they are coincidental. In order to fully align (collimate) an RCT we need to consider 5 degrees of freedom, two offsets (X and Y), tilt/tip and spacing between the mirrors. There is also the alignment of the focuser/imaging train with the scope.

Most amateur RCTs only provide tilt/tip correction but no offset therefore we can only collimate the scope up to some level assuming that the imaging optical path is concentric and squared enough with the primary mirror. Depending of the brand the focuser/imaging train may be directly attached to the primary mirror itself, this is very common, we can only hope that the mechanic is centered and squared indeed.

The basic procedure goes like this (there are variants of this approach, see your scope user's manual). I'll assume that you have only tilt/tip corrections available too:
 

First you align the secondary mirror with the imaging train using some tools like a laser. Most secondary have either a black dot or donuts which materializes the top of the mirror from where its optical axis is emerging.
Let's assume that we use a laser collimator for this job, be sure that you have a well made tool centered with the imaging axis.
Rotation of the laser by 360 degrees should result to a minimum, if any, wobbling of its spot on the secondary, if no get a better one!
By tilting the secondary mirror, and some time the imaging train and/or primary one (depending whether you have independent controls of the imaging axis and primary tilts) you bring the laser spot on top of the secondary mirror dot, or centered inside the secondary donuts mark. Then you fine tune by making sure that the return laser beam falls to the center of the laser collimator too (both, outgoing and incoming beams are coincidental). At this stage you have mechanically aligned the secondary mirror with your imaging reference axis.

Now you can tilt/tip the primary mirror using its baffles as an indicator for checking its alignment with the secondary mirror, for instance with the Takahashi collimation scope. This is still a mechanical collimation which assumes that everything is squared and centered mechanically at factory. I would only consider it a coarse adjustment, do not put too much faith on the quality of the baffle assembly and secondary spiders centering though.

In the best case scenario you have an independent control of the primary tilt/tip (no linked to the imaging train axis), if not you will have to iterate between tilt/tip adjustments of the primary and secondary to find the best mechanical collimation compromise.

The next step is optical collimation using an actual star on the sky, the real deal.

Take a bright enough star near the zenith and defocus it such you can clearly seen the secondary central obstruction shadow and some concentric rings. Bring the star on axis.

One can use a eyepiece but I would suggest to take an 10 to 20 seconds exposure, depending of your mount tracking capability, and use a red filter to average out the seeing, be sure not to clip the sensor.
Since the the primary mirror optical axis is likely not yet optically coincidental with the secondary you would adjust it by tilt/tip corrections, just a bit, in order to center the secondary central obstruction shadow as well as making the diffraction rings as concentric as possible. If the mechanical collimation was good enough you should only need minimum tilt/tip corrections to reach the goal.

RCT are free of third order (primary) coma across field, to achieve that they trade (third order) coma for astigmatism therefore RCT have off axis astigmatism (and sever field curvature) but there should not be any astigmatism on axis when collimated.
If the on axis defocus star pattern exhibits a centered secondary mirror obstruction shadow you may expect there is no coma (see below for some cautions on this regards though). You can now go to the four corners and look for astigmatism (elongated stars), if your imaging axis is centered the astigmatism should be symmetrical in the field, hence the stars elongations should be radiating from the center of the frame to each comers of the chip with the same direction and magnitude.

 

Now there are few reasons for facing some challenges and frustrations.

First the scope is (primary) coma free across the field only if the spacing between both mirrors is correct. Normally this has been taken care at factory, if not you can get an hint of a possible wrong spacing by looking at defocus star patterns at the same intra and extra focal disances (defocused by the same amount on each sides of the focal plane). If the is any significant spacing error the concentric rings will look very different (dimmer in one side and brighter on the other side, there sizes may be different too) between both images (intra and extra focal).
Often the central obstruction size will be different too, this is a sign of spherical aberration related to mirror spacing. In the case of spherical aberration you would need to fix the spacing issue first before doing further in the fine optical collimation. Be careful about using the scope focal length, measured using a plate solve, to infer mirror spacing. This is only a coarse predictor since there are inevitable tolerances in the mirror figures leading to few percents of focal length differences between otherwise identical scopes, this is just normal and to be expected.

Another, maybe best lest known but very common on amateur RCT, issue is an offset of the secondary central obstruction. It could be related the mirror or/and its mount and cover. Few millimeters of offset is common, I have see this very often. Offset of the secondary mirror leads to aberrations, especially for small apertures, mainly coma but most of it can be corrected by tilting the secondary bringing the scope SR well above the 80% level, therefore it is not a major concern when designing/making such scopes for the amateur market. This is probably while amateur level RCTs do not have offset corrections.
But there is a catch here, a de-centered secondary obstruction leads to a de-centered secondary shadow in the defocused star pattern while the cope is correctly collimated, therefore when using the central obstruction shadow as a primary indicator and feedback, as we all do, for manual qualitative collimation one may be misled.

 

As an example below the image of a defocused star under seeing limited conditions taken by a collimated (SR = 99.43%) RCT having an offset secondary central obstruction, an usual occurrence:

 

One could come to the conclusion that scope is badly collimated, when in fact it is not. By tilting the mirror one can, most likely, eventually center the secondary shadow leading to the defocused star image below which seems better, but now the scope SR is down to 58%, all this work for nothing...



There is about 0.12 wave rms of vertical coma. Here is its resulting PSF (the green circle marked the core of the Airy disk):



The best way to collimate a scope is by the numbers using a wavefront analyzer, such as a Shack-Hartmann (SH) sensor. Those provide quantitative information through the wavefront and related aberrations. For instance we can look at only the spherical aberration when adjusting the spacing of the mirrors, there maybe some another aberrations but the beauty of the wavefront and its usual (orthogonal) decomposition using the Zernike polynomials is the separation of the various aberrations such we can focus on the one we want, like spherical, coma, or astigmatisms alone or in combination.
Using a SH analyzer makes a huge difference in results and time spent in the process.
SH sensors may be quite expensive tools for most of us, but today there is a new technology, to be released soon on the market, which provides the same results and benefices than a SH using software only.
I have posted an announcement on CN about it few weeks ago, it may help you, here a link to the related thread:

 

https://www.cloudyni.../#entry10902580

P.S: You could join our beta testers if you like, just let me know.

Edited by Corsica, 16 March 2021 - 10:09 AM.

[Tom Gwilym]

Thanks for taking the time to do this write up.  Since I posted the original message, I've probably put in another 50 hours over a year trying to collimate this thing.   I did replace the focuser recently with  a very stable feathertouch which is working very well so far - at least I think I've gotten rid of the sag/flop from the weight of the cameras. 

I thought I had it close and took some images (see above) that looked good. Tried again the next night and found that the stars were still ugly when looking closer.

 

I'm pretty sure the focuser mount is separate from the primary.  It's attached to the rear plate on the scope.  I have centered the laser on the circle on the secondary as best as I can.  Could the tilt of the secondary affect the centering of the laser?  Also, how can you tell if it's bouncing directly back the source?  I'd assume that is when the laser is brightest.  The laser is a Glatter also.

I center up everything with the Tak scope then move to the primary after dark.

Recently, I did try releasing the 'push' bolts on the mirror and lightened it down as much as I could, then unwound them each by 2 turns. That got my FL from 3286 to 3252 (specs say 3250mm), so that was an improvement.  I will admit that I need to use a dimmer star and longer exposure to settle out the atmosphere probably, next time I'll try that.

When I see the secondary of center, it seem to have problems re-centering it with the primary mirror. Seems that when things look better on one side, something else gets messed up....or looks good, then the next night I realize it wasn't.

I've been playing with telescopes for years, Dobs, SCT, refractors, I totally restored a couple of Criterion RV6 scopes, but will admit that this Ritchey Chretien has nearly got me beat.  It's been horribly frustrating to the point that I sometimes just hate the thing and threaten to swap it back to the old C14 Celestron (which I haven't used yet, but with a F/6.3 reducer on that, I'm confident I could do much better than the 16!).

 

Sure, put me on the list as a beta tester for the software!  I'm still willing to try anything with this thing to get it usable.

 

Tom

 

RCTs have hyperbolic mirrors each with its unique optical axis making RCT collimation much more difficult than scopes with a spherical secondary mirrors, such as SCT (any radii of sphere is an optical axis).

The challenge is to align the two RCT mirror optical axes such they are coincidental. In order to fully align (collimate) an RCT we need to consider 5 degrees of freedom, two offsets (X and Y), tilt/tip and spacing between the mirrors. There is also the alignment of the focuser/imaging train with the scope.

Most amateur RCTs only provide tilt/tip correction but no offset therefore we can only collimate the scope up to some level assuming that the imaging optical path is concentric and squared enough with the primary mirror. Depending of the brand the focuser/imaging train may be directly attached to the primary mirror itself, this is very common, we can only hope that the mechanic is centered and squared indeed.

The basic procedure goes like this (there are variants of this approach, see your scope user's manual). I'll assume that you have only tilt/tip corrections available too:
 

First you align the secondary mirror with the imaging train using some tools like a laser. Most secondary have either a black dot or donuts which materializes the top of the mirror from where its optical axis is emerging.
Let's assume that we use a laser collimator for this job, be sure that you have a well made tool centered with the imaging axis.
Rotation of the laser by 360 degrees should result to a minimum, if any, wobbling of its spot on the secondary, if no get a better one!
By tilting the secondary mirror, and some time the imaging train and/or primary one (depending whether you have independent controls of the imaging axis and primary tilts) you bring the laser spot on top of the secondary mirror dot, or centered inside the secondary donuts mark. Then you fine tune by making sure that the return laser beam falls to the center of the laser collimator too (both, outgoing and incoming beams are coincidental). At this stage you have mechanically aligned the secondary mirror with your imaging reference axis.

Now you can tilt/tip the primary mirror using its baffles as an indicator for checking its alignment with the secondary mirror, for instance with the Takahashi collimation scope. This is still a mechanical collimation which assumes that everything is squared and centered mechanically at factory. I would only consider it a coarse adjustment, do not put too much faith on the quality of the baffle assembly and secondary spiders centering though.

In the best case scenario you have an independent control of the primary tilt/tip (no linked to the imaging train axis), if not you will have to iterate between tilt/tip adjustments of the primary and secondary to find the best mechanical collimation compromise.

The next step is optical collimation using an actual star on the sky, the real deal.

Take a bright enough star near the zenith and defocus it such you can clearly seen the secondary central obstruction shadow and some concentric rings. Bring the star on axis.

One can use a eyepiece but I would suggest to take an 10 to 20 seconds exposure, depending of your mount tracking capability, and use a red filter to average out the seeing, be sure not to clip the sensor.
Since the the primary mirror optical axis is likely not yet optically coincidental with the secondary you would adjust it by tilt/tip corrections, just a bit, in order to center the secondary central obstruction shadow as well as making the diffraction rings as concentric as possible. If the mechanical collimation was good enough you should only need minimum tilt/tip corrections to reach the goal.

RCT are free of third order (primary) coma across field, to achieve that they trade (third order) coma for astigmatism therefore RCT have off axis astigmatism (and sever field curvature) but there should not be any astigmatism on axis when collimated.
If the on axis defocus star pattern exhibits a centered secondary mirror obstruction shadow you may expect there is no coma (see below for some cautions on this regards though). You can now go to the four corners and look for astigmatism (elongated stars), if your imaging axis is centered the astigmatism should be symmetrical in the field, hence the stars elongations should be radiating from the center of the frame to each comers of the chip with the same direction and magnitude.

 

Now there are few reasons for facing some challenges and frustrations.

First the scope is (primary) coma free across the field only if the spacing between both mirrors is correct. Normally this has been taken care at factory, if not you can get an hint of a possible wrong spacing by looking at defocus star patterns at the same intra and extra focal disances (defocused by the same amount on each sides of the focal plane). If the is any significant spacing error the concentric rings will look very different (dimmer in one side and brighter on the other side, there sizes may be different too) between both images (intra and extra focal).
Often the central obstruction size will be different too, this is a sign of spherical aberration related to mirror spacing. In the case of spherical aberration you would need to fix the spacing issue first before doing further in the fine optical collimation. Be careful about using the scope focal length, measured using a plate solve, to infer mirror spacing. This is only a coarse predictor since there are inevitable tolerances in the mirror figures leading to few percents of focal length differences between otherwise identical scopes, this is just normal and to be expected.

Another, maybe best lest known but very common on amateur RCT, issue is an offset of the secondary central obstruction. It could be related the mirror or/and its mount and cover. Few millimeters of offset is common, I have see this very often. Offset of the secondary mirror leads to aberrations, especially for small apertures, mainly coma but most of it can be corrected by tilting the secondary bringing the scope SR well above the 80% level, therefore it is not a major concern when designing/making such scopes for the amateur market. This is probably while amateur level RCTs do not have offset corrections.
But there is a catch here, a de-centered secondary obstruction leads to a de-centered secondary shadow in the defocused star pattern while the cope is correctly collimated, therefore when using the central obstruction shadow as a primary indicator and feedback, as we all do, for manual qualitative collimation one may be misled.

 

As an example below the image of a defocused star under seeing limited conditions taken by a collimated (SR = 99.43%) RCT having an offset secondary central obstruction, an usual occurrence:

RCTOffset.jpg

 

One could come to the conclusion that scope is badly collimated, when in fact it is not. By tilting the mirror one can, most likely, eventually center the secondary shadow leading to the defocused star image below which seems better, but now the scope SR is down to 58%, all this work for nothing...

RCTOffsetTilt.jpg

There is about 0.12 wave rms of vertical coma. Here is its resulting PSF (the green circle marked the core of the Airy disk):

RCTComaPSF.jpg

The best way to collimate a scope is by the numbers using a wavefront analyzer, such as a Shack-Hartmann (SH) sensor. Those provide quantitative information through the wavefront and related aberrations. For instance we can look at only the spherical aberration when adjusting the spacing of the mirrors, there maybe some another aberrations but the beauty of the wavefront and its usual (orthogonal) decomposition using the Zernike polynomials is the separation of the various aberrations such we can focus on the one we want, like spherical, coma, or astigmatisms alone or in combination.
Using a SH analyzer makes a huge difference in results and time spent in the process.
SH sensors may be quite expensive tools for most of us, but today there is a new technology, to be released soon on the market, which provides the same results and benefices than a SH using software only.
I have posted an announcement on CN about it few weeks ago, it may help you, here a link to the related thread:

 

https://www.cloudyni.../#entry10902580

P.S: You could join our beta testers if you like, just let me know.

 

[Tom Gwilym]

Update:  I did just order the book you recommended yesterday!  Should be here next week.

I took a break from the 16 inch RC and have been using my own scope for a few nights.  I'll probably go fight and cuss at the 16 inch again later in the week if the skies are clear as forecasted.

 

Seriously, if you can get up on the ladder, I'll talk you through it assuming you have a speaker loud enough on your computer.

 

You probably don't need to redo the Tak - it's already close enough from an alignment standpoint, it's the pinched optics we need to fix and I can see that through the ASI071's feed.

 

Once that's done, we can start tuning the two mirrors.

 

[Corsica]

Thanks for taking the time to do this write up.  Since I posted the original message, I've probably put in another 50 hours over a year trying to collimate this thing.   I did replace the focuser recently with  a very stable feathertouch which is working very well so far - at least I think I've gotten rid of the sag/flop from the weight of the cameras. 

I thought I had it close and took some images (see above) that looked good. Tried again the next night and found that the stars were still ugly when looking closer.

 

I'm pretty sure the focuser mount is separate from the primary.  It's attached to the rear plate on the scope.  I have centered the laser on the circle on the secondary as best as I can.  Could the tilt of the secondary affect the centering of the laser?  Also, how can you tell if it's bouncing directly back the source?  I'd assume that is when the laser is brightest.  The laser is a Glatter also.

I center up everything with the Tak scope then move to the primary after dark.

Recently, I did try releasing the 'push' bolts on the mirror and lightened it down as much as I could, then unwound them each by 2 turns. That got my FL from 3286 to 3252 (specs say 3250mm), so that was an improvement.  I will admit that I need to use a dimmer star and longer exposure to settle out the atmosphere probably, next time I'll try that.

When I see the secondary of center, it seem to have problems re-centering it with the primary mirror. Seems that when things look better on one side, something else gets messed up....or looks good, then the next night I realize it wasn't.

I've been playing with telescopes for years, Dobs, SCT, refractors, I totally restored a couple of Criterion RV6 scopes, but will admit that this Ritchey Chretien has nearly got me beat.  It's been horribly frustrating to the point that I sometimes just hate the thing and threaten to swap it back to the old C14 Celestron (which I haven't used yet, but with a F/6.3 reducer on that, I'm confident I could do much better than the 16!).

 

Sure, put me on the list as a beta tester for the software!  I'm still willing to try anything with this thing to get it usable.

 

Tom

If your focuser is not attached to the primary mirror this is a good news since you have the freedom to tilt the primary without touching your imaging axis, which acts a your reference during the all collimation process.

 

Your first step, assuming you are not totally out of collimation, is to be sure that both mirror spacing is correct.

RCT are (third order) coma free across the field only if this spacing is right, otherwise you will have off axis coma even perfectly collimated (tilt/tip and offset).
As a general rule, valid not only for RCTs, you should not have any on axis coma when collimated. Coma is only an off axis aberration, if any, when all the optical surfaces are well aligned (assuming of course a good optics, like the GSO one which is very good).

 

As mentioned before setting the FL to the specification one may not be the right thing to do since there are tolerances on mirror figures, which are otherwise aberration free. For instance on professional grade telescopes, like for the RCOS scopes, the actual back working distance (BWD) between the primary mirror and the focal plane (where you have your imager) is actually etched into the back of the primary mirror, by the original master optician who polished the glass mirror. It will be different from one (same) scope to the next and different from the value provided in a CAD drawing and specifications which are just nominal design values (without tolerance). Actual FL, BWD and mirror spacing are specific to your instrument. This is because every mirror made can have a slightly different “mirror pair system” back focus, hence FL. The BWD is measured on an optical test bench at factory. The mirror spacing is set for each scope, using either wavefront, interferometry or simply a Ronchi test, resulting to different FL and BWD values for every scope.

I would no recommend changing the mirror spacing even if the actual FL seems out of specifications unless there is a huge difference. A few percents difference between the actual and the specification (nominal value without any manufacturing tolerances) is quite normal. For a 3,250mm you could easily have few inches of FL difference when mirror spacing is optimal.

As mentioned before if the mirror spacing is not correct this results, for an RCT, to coma off axis which negates one of the main features of the RC design. There will be spherical aberrations too making the PSF and in turn stars wider. Spherical aberrations are symmetrical aberrations hard to spot and quantify during a star test unless you compare intra versus extra focal defocused star images taken under similar seeing and exposure. When the scope is focused it is easy to take spherical aberrations for the seeing effect since the stars are perfectly round (if there is no other aberrations). Spherical can very rapidity degrade your scope sharpness and related MTF. This could be a serious limitation when doing planetary imaging.

 

Our SkyWave (SKW) wavefront based collimation tool reports spherical aberrations (among many) including the sign, like any wavefront sensors do, for proper mirror spacing adjustments. Short of this kind of quantitative data you can use intra versus extra focal defocused star images to spot spherical aberrations and to adjust the mirror spacing to minimize those differences, below an example of intra/extra focal images with an 8th of a wave PV primary spherical (credit telescope-optics.net). The related SR>80%, therefore it is quite easy to spot even a small amount of primary spherical when using extra/intra focal images (the defocus distance must be the same for both intra and extra focal images in order to be able to compare them though):

 

 

When done don't be surprise if the BWD and FL are not identical to the specification, this is to be expected, what does matter is the result of the optical collimation. This is also true for the geometrical look of the defocused star pattern, as mentioned in my previous post, it is not uncommon for the central obstruction (and baffles) to be offset when your scope is well optically collimated. Mechanically aligning landmarks, such as baffles, and central obstruction is a good and necessary first step to do but it remains a coarse adjustment and totally dependent of the quality and accuracy of the telescope mechanics. At the end of the day the optical collimation using a star (artificial or actual) is the key and final goal, every else are proxies. For instance the Takahashi collimation scope is a very nice tool, I won one. I have no doubt that if you follow the user's guide procedure you will get your Takahashi Mewlon telescope (for instance) well aligned because the scope mechanics is well made to being with (squared and centered) I am not sure that we can say the same for some other brands.

In your case, since there is no adjustment for offset, you should tilt the secondary to center the laser spot on the black reference mirror mark. If the secondary is well centered indeed the return beam should be centered too, for monitoring this you can use laser collimator tool with a return target, such as (just an example):

 

https://www.hotechus...or-p/sca-2c.htm

If the return beam is not centered (while the spot lands on the mark) than you can tilt the focuser/imaging axis if you have such option. The goal is to eventually make the imaging axis coincidental with the scope optical axis such a your frame is centered in the scope FOV.
If not, a star in the middle of the sensor will not be on axis.
Now assuming your RCT mirror spacing is correct this is not a too big issue, at least for collimation, since RCT are free of third order coma across the all field. You will just center a defocused star in the sensor while adjusting the secondary mirror tilt/tip during collimation. When there is no more coma (usually the secondary mirror shadow is centered when doing qualitative collimation, but as discussed in my previous post be aware that may not be the best indicator of coma) you are done. You may sill have some astigmatism on axis if your imaging axis is not aligned though.

 

One drawback of having the imaging axis not fully aligned with the scope optical axis could be a left over field curvature, and maybe other aberrations like coma, when using a flattner attached to the focuser. RCTs exhibit significant field curvature.

Aligning  both axes may require an external tilt/tip compensation device, or even some way to adjust offset. Since most amateur RCTs have limited collimation adjustments you will be somehow limited but you should be able to reach a good compromise and performances under seeing limited conditions with such scope. I own a 10" AstoTech (GSO's optics) RCT and when collimated it provides superb images.
 

A wavefront analyzer measures the actual scope optical performance and therefore it is handy for collimation.

For beta testing of SkyWave just email me directly (gaston@innovationsforesight.com) we'll talk more about the details and I'll send you the SKW installation package. We could organize a remote session for SKW training then.

Edited by Corsica, 18 March 2021 - 05:43 AM.

[Tom Gwilym]

Ok, I'm building my motivation again after a few clear nights away from this thing.  I may go try tweaking it some more early evening when I still have some daylight (easy background with dome open).  I think I may try chasing down the pinched optics.  I'm wondering if I may have one of the secondary screws too tight?  I know one of them seems tighter than the others when I adjusted it, so I may try backing them off a bit and trying again.  I may end up in the same place since I don't want to touch the center screw, but try more of loosening the opposite side rather than tightening.  

 

The fun just keeps going with these RC things.   :-P

 

Again, thanks for all the advice and ideas passed along, I'm still reading through it.

 

Tom

[Corsica]

Ok, I'm building my motivation again after a few clear nights away from this thing.  I may go try tweaking it some more early evening when I still have some daylight (easy background with dome open).  I think I may try chasing down the pinched optics.  I'm wondering if I may have one of the secondary screws too tight?  I know one of them seems tighter than the others when I adjusted it, so I may try backing them off a bit and trying again.  I may end up in the same place since I don't want to touch the center screw, but try more of loosening the opposite side rather than tightening.  

 

The fun just keeps going with these RC things.   :-P

 

Again, thanks for all the advice and ideas passed along, I'm still reading through it.

 

Tom

Pinched optics lead to trefoil (quadrafoil, ...) kind of aberrations, this is quite easy to spot on a defocused star, it may look like this:

 

 

Here the star pattern has a triangular shape, which is typical of a trefoil aberration found in pinched optics. Higher order aberrations may show a more complex polygonal effect.

 

Is your RCT a truss OTA, if so is there a shroud around the truss structure?

What is your imaging camera pixel size?

Any idea of the scope central obstruction, in % of its diameter (D=406mm)?

 

P.S: I replied to your email.
 

Edited by Corsica, 19 March 2021 - 01:35 PM.

[Tom Gwilym]

It's a 16 inch truss.

Imaging camera is an ASI071MC Pro (yes, the pixels are probably a bit small for this FL, but still happy with what I get - when it's close!).

Not sure of the obstruction.  I'll have to look closer at the specs -- https://www.highpoin...telescope-16rct

Here is one of the recent images. Sure does look kind of pinched.

I see your email, I'll check that out!

Tom

Pinched optics lead to trefoil (quadrafoil, ...) kind of aberrations, this is quite easy to spot on a defocused star, it may look like this:

 

Trefoil.jpg

 

Here the star pattern has a triangular shape, which is typical of a trefoil aberration found in pinched optics. Higher order aberrations may show a more complex polygonal effect.

 

Is your RCT a truss OTA, if so is there a shroud around the truss structure?

What is your imaging camera pixel size?

Any idea of the scope central obstruction, in % of its diameter (D=406mm)?

 

P.S: I replied to your email.
 

 

[Corsica]

It's a 16 inch truss.

Imaging camera is an ASI071MC Pro (yes, the pixels are probably a bit small for this FL, but still happy with what I get - when it's close!).

Not sure of the obstruction.  I'll have to look closer at the specs -- https://www.highpoin...telescope-16rct

Here is one of the recent images. Sure does look kind of pinched.

I see your email, I'll check that out!

Tom

Well it looks ugly for sure

 

Do you use any shroud over the OTA?
I have seen cases where the shroud, which is an elastic type of fabric usually, can cut part of the beam creating such polygonal defocus star patterns depending of the scope design and brands.

Maybe there is a problem with the baffles alignment, I think it is unlike to be a pinched optics, at least that would not be my first guess, if pinched it is very badly!

I have a suggestion, could you take a defocused star image, on axis, exposed say 10 to 20 seconds, if you tracking is good enough, near the Zenith?
Try to reach a signal level around a half to 2/3 of the ADU full scale, but be sure not to clip, do not use any shroud, remove it completely, if any.
If doable I would suggest to defocus it, from you best focus position (say established using a V curve, a mask, or equivalent method) extra focal (focuser moving outward from the best focus position) by 6,236 microns (or 6.236mm), take the closest value you can get. If there is no enough room extra focal do it intra focal then, just let me know.
I like to see the pattern of an on axis defocused star. Ideally you would use a red filter. Could you send me the raw FIT (16 bits or float format) file without any processing?

You can crop the star from the frame to save space if you want but be sure that the crop is at least 193x193 pixels (4.78µm) with the star pattern centered as much as possible, the crop can be larger too. Finally be sure there is no other star/object inside the copped zone (193x193 pixels).

Edited by Corsica, 19 March 2021 - 02:17 PM.

[xthestreams]

I have a suspicion that in aligning the secondary one of the bolts was not backed off as teh others were tightened.

 

Unlike the primary, which floats on just the three push/pull bolts (and therefor can "walk" forward or backward if you don't leave one of the three adjustment static), the secondary on the GSO is held at the correct distance by a fourth axial bolt.

 

The implication being (I think), that it's possible to tighten two of the bolts (and unless the third is backed off, potentially deform the mirror sufficiently to cause the problem Tom is seeing.

 

As mentioned, I was also able to accidentally deform the primary by messing with the screws in the floatation cell (the ones visible through the fan blades) - mine had come loose, but in fixing the problem, I severely deformed the mirror to the point it would never come to focus - Tom's stars look very similar to what I had then - but it's just a theory, something that can be tested under the stars, ideally with Gaston's software to aid the process.

[Corsica]

I have a suspicion that in aligning the secondary one of the bolts was not backed off as teh others were tightened.

 

Unlike the primary, which floats on just the three push/pull bolts (and therefor can "walk" forward or backward if you don't leave one of the three adjustment static), the secondary on the GSO is held at the correct distance by a fourth axial bolt.

 

The implication being (I think), that it's possible to tighten two of the bolts (and unless the third is backed off, potentially deform the mirror sufficiently to cause the problem Tom is seeing.

 

As mentioned, I was also able to accidentally deform the primary by messing with the screws in the floatation cell (the ones visible through the fan blades) - mine had come loose, but in fixing the problem, I severely deformed the mirror to the point it would never come to focus - Tom's stars look very similar to what I had then - but it's just a theory, something that can be tested under the stars, ideally with Gaston's software to aid the process.

You may be right, it could be pinched optics too, I am just surprise how sharp the edges of the polygonal shape are. I think we need to look at all the possibilities, a misaligned good optics (like GSO) dos not exhibit defocused star shapes like this. Here we face either pinched optics or some vignetting or a combinations of both, Mr. Murphy is very creative...

If I can analyze an image taken as suggest on my previous post, without any shroud, if any, I could tell more.

 

[xthestreams]

:-) indeed! Gaston, I am very much NOT an expert, so can only rely on my limited experience - very happy to learn from one of the best in the business!

 

I don't know if you read the earlier parts of the thread, but Tom shared some "in focus" and outside focus images midway through this thread, in one of them you can make out the shadow of the secondary quite well - to my eye is looks very much like it's way off axis and not a very round shape. I don't know if there's any useful diagnostic information in there whilst we await the dat you requested (which I am also keen to see).

 

Tom again, my offer stands, I am happy to get on a Zoom call and be your eyes on the ground whilst you're up on the steps twiddling the knobs. Hopefully we can get to the bottom on this.

 

BTW - when I mentioned/recommended getting an ONAG to replace your OAG, Dr Gaston is the guy who makes them - glad he joined the thread!

[Tom Gwilym]

Another nearly 3 hours on this thing and it's worse than ever.   I first checked the focuser with the laser and aligned it a little bit.  Since i had one screw that seemed too tight (pinching the optics?) I backed off on all the secondary screws (not the center, just the 3 adjusters). I then spent another 2 hours trying to get it back again. I tried to line up with the Takahashi, but it was so far off I couldn't see the center mark.  I then removed it and tried to "eyeball" it back to center then go back to the Tak.  No matter what I tried I couldn't get it back.  Probably 50 times up and down the latter between the secondary and the end of the scope. 

I finally got annoyed enough that I gave up and went home.

 

I can't get this thing to work.   If it takes this much fine-tuning and precision it's just not fun anymore.  

I'm not giving up just yet, but I think I'll email our board of directors and suggest we put the old Celestron C14 back on the mount and try to work on the RC on the bench where I can look at it closer and maybe line it up with the wall so I can shine lasers through it.  

 

Sure, we are in a pandemic and it's limited how much on-site help I can get, but I'd think I'd have this functional now.  It's been over a year now and I'm really burned out on this thing.  The C14 is F/11, but with the f/6.3 reducer I think I could really make that thing fly!   

 

Talk me out of this, but right now I've really had it with this thing.  It looks really cool, but if it doesn't work, it's no better than a Tasco from Walmart.

 

Gaaahh!  ...end of rant. 

 

Tom

[xthestreams]

Ignore the Tak, they will just mislead you. Got the Glatter to the point where the laser is hitting the approximate mechanical  Center and reflecting back to its origin and then let’s do a joint session under the stars. 
 

Yiu might be right though, it could be that it’s a dud, but unlikely. 

[Tom Gwilym]

I've got the secondary so messed up now that I probably can't even properly defocus a star now and do anything useful.  How do you know when the laser is pointing exactly back at the source?  I assume its when the dot is centered and at it's brightest.  Or do I try my Dobsonian collimating laser that has the 45 degree circular aiming pattern?  

I'm not sure what is a dud, the telescope or the insane guy spending a year trying to make this thing work?!

 

 

Ignore the Tak, they will just mislead you. Got the Glatter to the point where the laser is hitting the approximate mechanical  Center and reflecting back to its origin and then let’s do a joint session under the stars. 
 

Yiu might be right though, it could be that it’s a dud, but unlikely. 

 

[xthestreams]

We’ll get there. Grab your Glatter and point it so that it’s hitting the secondary bang on the little donut (use the Tilt/tip feature of your RC 117mm collar to do that).

 

once it’s hitting that circle, adjust the secondary to point the laser back at itself taking care to relieve the strain on the bolts that aren’t being tightened, small movements beat big turns. if your 45’ collimator is accurate enough that will also work to do both tasks - just make sure it’s square. 
 

if you can’t  get a roughly defocused  star after doing that. then I do think the trip to see AP is a good idea. I’d,advise to put away the Tak, I found they are an incredibly misleading device on a mechanically imperfect scope - the Glatter should get you 99% there (the circular holographic adapter can be ordered from Feathertouch if you want to get a little more bench collimation done).

 

once you’re there, let’s talk. You’ve effectively got a “known” optical axis from which we start the process of adjusting the primary to get as close to a coma free on axis star as we can - provided you only touch two of the three primary adjusters, we can even make big bold adjustments  to begin with until you’ve got a star close to the axis that starts to resemble what we’re chasing.

 

From here we stay on the primary until we’re confident that the coma is as good as it gets, remembering to note the relationship between the push/pulls and what they do to the image (where does the hot spot on the coma move to as you adjust - knowing this will help a lot later)

 

then, and only then, do we progress to the secondary and start to smooth out the ovals shaped starts at the periphery. That will probably result is some on axis coma that we’ll need to Remove back at the primary (tiny tiny moves - using the notes you made earlier) before we finally adjust the secondary and take a well earned slug of Angels Envy

[xthestreams]

Progress?

[xthestreams]

Hey Tom, any luck?

[Tom Gwilym]

Hey Tom, any luck?

Sorry for the slow replies.  Cloudy skies lately and off on other projects.

We did make some good progress using the very basic "eyeball" method of just peeking through and centering your pupil on the secondary mirror center dot.  Then carefully line things up symmetrical.

The non-focused star is slightly off center, but better than it's been in a long time. Then the other photo does show some nice symmetrical diffraction spikes now too.

 

I think we did mess it up a bit after this trying to center the mirror some more, but the newer method is easy to line up again.   It just took me a few minutes to line up the secondary pretty well, then after that just do a star test and fine tune it....but it requires the secondary mirror, so as always it's tricky, but I think progress in the right direction is being made.

 

Tom

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[xthestreams]

Glad to hear/see it!

 

you’re getting close, keep working on the primary until your on-axis star is close to centered/coma free before touching the secondary off axis and then gently fine tune

 

looking good

[Tom Gwilym]

Getting closer for sure.  

This was all done with no cheshire, laser or Takahashi scope.  Although, I did check with the Tak and found that to focuser tube was off a bit so I re-centered that and double checked the secondary and primary. 

I did get some help this time so my friend was at the screen, and I would run up and down the ladder tweaking the secondary and see what happens.  

I do think we should get some of those large primary knobs on the scope for adjustments - once we figure out the tricks to this!

Tom

 

Glad to hear/see it!

 

you’re getting close, keep working on the primary until your on-axis star is close to centered/coma free before touching the secondary off axis and then gently fine tune

 

looking good

 

[xthestreams]

Getting closer for sure.  

This was all done with no cheshire, laser or Takahashi scope.  Although, I did check with the Tak and found that to focuser tube was off a bit so I re-centered that and double checked the secondary and primary. 

I did get some help this time so my friend was at the screen, and I would run up and down the ladder tweaking the secondary and see what happens.  

I do think we should get some of those large primary knobs on the scope for adjustments - once we figure out the tricks to this!

Tom

Great plan.

 

I found that once you've wrapped your head around the DSI method and you are at least reasonably sure the laser is bouncing back to it's point of origin, then you pretty much HAVE to throw away all the alignment tools or they will lead you astray (I LOVED the confidence the Tak inspired in my bench fettling the scope, until I realised it was leading me down a totally wrong path when you point it at the stars - if only there was an Astrobin for perfect bench collimation images...)

 

Two people makes it so much easier! can't wait to see more!

[Tom Gwilym]

I'm still working on mastering the DSI. The "eyeball" method gets things close, but then the secondary part is still a little tricky past that point to the the outlying stars round.  At least I have a friend who is just as Covid isolated that we don't have to worry about each other sharing.  

Last night I did some basic mechanical stuff.  I recently re-balanced the scope, and found a squeal in the RA. So was doing some greasing last night.  I'll do the Dec next time, some dome fixing...etc.   I'll check the alignment again and review the DSI procedures again.  

Here is one of the attempts, top left stars aren't too good, but the next night we did make some improvements with M104, I still need to try processing that one. 

-- Tom

Great plan.

 

I found that once you've wrapped your head around the DSI method and you are at least reasonably sure the laser is bouncing back to it's point of origin, then you pretty much HAVE to throw away all the alignment tools or they will lead you astray (I LOVED the confidence the Tak inspired in my bench fettling the scope, until I realised it was leading me down a totally wrong path when you point it at the stars - if only there was an Astrobin for perfect bench collimation images...)

 

Two people makes it so much easier! can't wait to see more!

 

[xthestreams]

That's a MUCH healthier looking scope, I am guessing the secondary screws being overnight deformed the mirror, but again, I am just a newbie.

 

The coma at the 8:30 position, which you can see in the on and off axis defocused stars above "pull" the mirror back on that side and you're REALLY close (again remember to leave one set of screws untouched, you can do everything you need to with 2 screws - it just takes a little more thought).

 

The trick is to determine how the screws and the mirror movement relate, once you're there, it should get faster.

 

The eyeball method and the DSO method are essentially kissing cousins, assuming that by eyeball you mean through the eyepiece versus the camera.

 

Where the camera WILL help is off-axis, nice long (8-12 second) exposures (turn your camera gain down to 0 if you have to) and away you go.

[Tom Gwilym]

Good idea with the 2 screw method.  I did read about that, or maybe you mentioned it?  I forget where, I've been all over the internet seeking ideas!

I can see that method should old the focal length pretty well also, I now have this thing within 2mm of factory specs.

 

 

 

That's a MUCH healthier looking scope, I am guessing the secondary screws being overnight deformed the mirror, but again, I am just a newbie.

 

The coma at the 8:30 position, which you can see in the on and off axis defocused stars above "pull" the mirror back on that side and you're REALLY close (again remember to leave one set of screws untouched, you can do everything you need to with 2 screws - it just takes a little more thought).

 

The trick is to determine how the screws and the mirror movement relate, once you're there, it should get faster.

 

The eyeball method and the DSO method are essentially kissing cousins, assuming that by eyeball you mean through the eyepiece versus the camera.

 

Where the camera WILL help is off-axis, nice long (8-12 second) exposures (turn your camera gain down to 0 if you have to) and away you go.