10 replies to this topic

[starrybird]

Recently I received a new 12 inch RC scope from CFF. I could get a good collimation via Takahashi Collimation Scope (TCS), just like the following pic:

 

 

 

However, when I did star test, I noticed that if I moved my CCD outside or inside of the focus, doughnuts were elongated and its major axis were in the same direction. I also found that major axis were perpendicular to each other when CCD was inside and outside of the focus. Polar alignment was OK and I double checked that. Field corrector was removed. Primary mirror and secondary mirror separation was adjusted so that the total system focal length was 2400mm (300mm aperture).

 

 

 

 

 

I have tried several ways to deal with this problem:

 

1. Adjust the primary mirror and the secondary mirror separation.
2. Adjust the secondary mirror shift.
3. Adjust focuser plate tilt.
4. Follow DSI's guide to tilt secondary. (I thought that my on axis star was not OK, which not met the requirement to tilt the secondary.)

 

None of them solved the problem.

 

My stars shown like the following pics:

 

 

 

 

Did someone know what was wrong with my telescope and collimation ?

Edited by starrybird, 03 February 2020 - 12:19 AM.

[sg6]

Cannot help but RC's are fairly notorious for collimation and set up. The hyperbolic mirrors are very critical of and in every dimension they can be moved.

 

Know someone that took a year to get theirs set to what they wanted. Very good now but to me not worth the effort. And that took 2 of them, both optics engineers, with test equipment.

[Terry White]

None of your pictures you posted are showing up for me, so it's a little difficult to make any conclusions yet. You didn't state what kind of RC you have. However, If I assume you have the AT 12-RCT (a GSO RC made for the Astro-Tech store brand), then you need to have a focal length of exactly 2432 mm, not 2400 mm, according to Astro-Tech specification. These scopes are designed to work optimally only at their designed focal length. Use the three primary screws turned the same amount to increase the mirror spacing, then plate solve to determine your focal length. There are many threads here on CN that can help you, but one of the better ones can be found at: https://www.cloudyni...imation thread. I don't recommend any of the various collimation scopes or laser collimators because they assume that the center mark on the RC secondary is on the optical axis of the secondary. If the mark is on the geometric center of the secondary and not on the optical axis, then these tools may de-collimate your RC and make it worse. Remember the geometric axis and the optical axis may not necessarily be the same thing for a hyperboloidal secondary, especially if the center mark was made using a rotating lathe. These collimation tools work quite well for SCT's and DK's because they have spherical secondaries, unlike the RC's. You mentioned the DSI method. The best generally accepted method for collimating RC's is to use the Deep Sky instruments method found at http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf So, in summary, set the proper focal length and then use the DSI method to remove aberrations from the image. Good Luck! 

Edited by Terry White, 02 February 2020 - 03:38 PM.

[starrybird]

None of your pictures you posted are showing up for me, so it's a little difficult to make any conclusions yet. You didn't state what kind of RC you have. However, If I assume you have the AT 12-RCT (a GSO RC made for the Astro-Tech store brand), then you need to have a focal length of exactly 2432 mm, not 2400 mm, according to Astro-Tech specification. These scopes are designed to work optimally only at their designed focal length. Use the three primary screws turned the same amount to increase the mirror spacing, then plate solve to determine your focal length. There are many threads here on CN that can help you, but one of the better ones can be found at: https://www.cloudyni...imation thread. I don't recommend any of the various collimation scopes or laser collimators because they assume that the center mark on the RC secondary is on the optical axis of the secondary. If the mark is on the geometric center of the secondary and not on the optical axis, then these tools may de-collimate your RC and make it worse. Remember the geometric axis and the optical axis may not necessarily be the same thing for a hyperboloidal secondary, especially if the center mark was made using a rotating lathe. These collimation tools work quite well for SCT's and DK's because they have spherical secondaries, unlike the RC's. You mentioned the DSI method. The best generally accepted method for collimating RC's is to use the Deep Sky instruments method found at http://www.deepskyinstruments.com/truerc/docs/DSI_Collimation_Procedure_Ver_1.0.pdf So, in summary, set the proper focal length and then use the DSI method to remove aberrations from the image. Good Luck! 

Hello Terry, thanks for your long reply. My scope is CFF 12 inch Ritchey-Chretien, not GSO variant. The clear aperture for the scope is 300mm, system focal length is 2400mm (without corrector). I have updated my image links so it should be OK on mobile now.

 

I have followed DSI's guide to collimate my scope, all I could get was elongated doughnut when I slightly defocused the star. If I moved my focuser to its largest step, I could get a rounded doughnut. As far as I know, doughnut should be rounded and concentric even defocus in a small amount, however, this was not the case for my scope currently.

 

CFF's RC provide several adjustments:

 

1. Primary mirror tip/tilt.

2. Secondary mirror tip/tilt.

3. Focuser flange tip/tilt.

4. Secondary focusing. (This is used to adjust the separation between the primary and the secondary.)

 

I also have done the 4th adjustment so the total system focal length is very close to 2400mm (less than 3mm in difference).

 

Don't know what's happening to my scope or collimation.

[dweller25]

Your photo’s may show astigmatism - suggesting the optics are being held too tightly ??

Edited by dweller25, 03 February 2020 - 02:42 AM.

[Terry White]

Thanks for the info. The fact that the out of focus stars in the image centers are not round (elliptical) and rotate 90° on either side of focus, indicates a gross astigmatism (miscollimation of the secondary). Follow the DSI guide to understand which secondary tilt screw affects which part of your oblong stars axis. See Timo's post #22 in the link I posted. There may be some pinching of the primary too, but the major problem I see is astigmatism, so try reducing the gross astigmatism in the image center first. Have you contacted CFF for help? They may have some tips as well.

[starrybird]

Thanks for the info. The fact that the out of focus stars in the image centers are not round (elliptical) and rotate 90° on either side of focus, indicates a gross astigmatism (miscollimation of the secondary). Follow the DSI guide to understand which secondary tilt screw affects which part of your oblong stars axis. See Timo's post #22 in the link I posted. There may be some pinching of the primary too, but the major problem I see is astigmatism, so try reducing the gross astigmatism in the image center first. Have you contacted CFF for help? They may have some tips as well.

But my on axis defocus star was not round, tilting the secondary not solve the problem, I tried that before. The image balance section in DSI guide deals with the off axis stars.

[Darth_Takahashi]

You system has astigmatism all over the field of view, both on axis and off axis. Why did you adjust the mirror separation? It should have been supplied from the factory collimated. If you haven't already done it, I would undo this adjustment and start from that point with the collimation process outlined by DSI.

 

It is critical not to change the spacing between the two mirror surfaces. This has the potential to introduce both spherical and astigmatism into your telescope. No amount of tilting the primary or secondary will change this if the spacing is now wrong between the two mirror surfaces. You will be able to improve things but only up to a point.

 

Good luck & Regards

 

 

Neil

[andysea]

+1 what Neil said. The spacing between the primary and secondary is critical with RC scopes. That’s why my RC focuses by moving the secondary.
You may try to check the spacing with a ronchi eyepiece.

[Timo I]

Recently I received a new 12 inch RC scope from CFF. I could get a good collimation via Takahashi Collimation Scope (TCS), just like the following pic:

---clip---

However, when I did star test, I noticed that if I moved my CCD outside or inside of the focus, doughnuts were elongated and its major axis were in the same direction. I also found that major axis were perpendicular to each other when CCD was inside and outside of the focus....

---clip---

I have tried several ways to deal with this problem:

1. Adjust the primary mirror and the secondary mirror separation.
2. Adjust the secondary mirror shift.
3. Adjust focuser plate tilt.
4. Follow DSI's guide to tilt secondary. (I thought that my on axis star was not OK, which not met the requirement to tilt the secondary.)

None of them solved the problem.

---clip---

Did someone know what was wrong with my telescope and collimation ?

Hi, I noticed this thread a bit late, but here's my two cents for this discussion...

What happened there is highly probably the fact that you trusted your Takahashi collimating scope too much (exactly the same way I did in the past with my 10" RCT scope).

What you need to know about Takahashi collimator scope's functionality is that Tak relies really heavy on the mechanical alignment of your RCT scope. Your focuser needs to be aligned with very high precision towards the optical center of your secondary mirror. And your secondary mirror (and primary mirror etc.) needs to be dead centered with he scope tube. And so on...

 

If (for instance) your CFF scope's focuser has been only a little bit tilted compared with the optical axis (=imaginary line passing through center of your secondary and primary mirrors, line where light passes between these mirrors), then you have mis-aligned your RCT secondary mirror (compared to primary mirror) with that Takahashi collimating scope. With this Takahashi scope it's very easy to mis-align only some parts from this kind of optical system, because its' base line (=standard reference point for other soon-to-come measurements) will always be aligned with the scope's focuser. And if that focuser's alignment does not meet exactly (with very high precision) your RCT scope's secondary/primary mirror optical axis, then you will manually mis-align your secondary mirror with this excellent & accurate Takahashi scope (which has been installed inside your focuser).

 

As a conclusion this "Tak collimation action" has probably left now your primary mirror into somewhat mis-collimated status, which can be seen in your sample star pics shown above.

How to get back into full RCT collimation? You have read about DSI method (stage 4. there in your steps), so now you need to trust DSI more than this precision tool aka. Takahashi collimating scope.

But before you start checking star fields with this DSI method, it will be quite crucial to check out that your focuser (and imaging camera there) will be aligned exactly to the center of your RCT scope's secondary mirror.

Takahashi collimating scope is high quality precision tool, but its' Achille's heel is its' own alignment in the RCT scope's focuser, which does not tolerate any tilting there.

 

Here's an image showing all adjustment positions from such Takahashi scope, when RCT scope's secondary has been aligned as perfectly as I could adjust with my current Tak scope/focuser alignment:

 

 

In my own 10" GSO RCT scope I have never been able to get a 100% perfect result from Takahashi collimator view(s), while the same RCT scope was showing nearly perfectly collimated starfields. Takahashi is a highly accurate tool for those expensive precision manufactured high quality RCT scopes (not for cheap AT/GSO/etc. branded Chinese ones), so I'm wondering here what on earth has happened there, if your CFF scope had such a little bit tilted focuser alignment (which was not 100% aligned with your scope's optical axis)? I have learned this via the hard way with my GSO RCT and Tak scope...

Anyway, your best bet seems to be to 1.) level your focuser and then 2.) run the DSI collimation routine as precisely as you can. Good luck with it!

[Darth_Takahashi]

Again, at the risk of sounding redundent this is one of the issues collimating these telescopes. People are placing too much faith in the tools without fully understanding what it is they can and cannot do. In the DSI document they clearly state that the optical axis might not be the same as the mechanical axis.

 

That for a collimated system. Just because someone puts a dot / ring on the centre of the secondary doesn't mean thats haow the secondary mirror is behaving! the same for the primary. So mechanical alignment should get you close to collimation but doesn't guaranty it. Further, now your coming at the problem from a known position. In the end only sussessive interations under the stars will bring you the final collimation that you are looking for.

 

Good luck.

 

 

Neil

Edited by Darth_Takahashi, 02 July 2020 - 01:46 AM.