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lukebl

Help please! Collimating an RC with a Howie Glatter

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Can someone help me here?

Now, I know there are endless threads on the web about collimation, but I can't get my head around this problem. Please bear with me while I explain.

I am now the proud owner of a Howie Glatter collimator, complete with the Concentric Circle attachment. It's a gorgeous piece of equipment. Shiny, solid, heavy and beautiful. One of the best, I'm led to believe. I thought that it would be the solution to collimating my 200mm Ritchey-Chretian.

So, I have gone through the two main stages of collimating it:
1: Align the focuser with the secondary mirror, so that the red dot aligns with the little secondary centre circle
2: Adjust the secondary so that the circular pattern projects evenly over the primary mirror.

I've done these stages and it all looks good as shown below. Nice concentric circles on the primary and when projected onto the wall. So I shouldn't need to collimate the primary.

49676611956_8bf8df5c4d_b.jpg

HOWEVER, when I set it up and aim on a star, it doesn't look right and the out of focus image looks like the view on the left in this image (this isn't my image, but it shows the general effect). It's not camera sag, as I've tried it in all positions.

So I then re-adjust the secondary collimation so that the out-of-focus star looks like the version on the right.

49676934332_36494e3ed3_z.jpg

Although the focus and collimation look good by eye, when I re-insert the Howie Glatter I get the projection show below. Which suggest that the collimation is out, when my eye says that it's not.

49676833192_d03cd6b331_c.jpg

So what is going on here? Is the Howie Glatter faulty? I thought they were the best. Or what else is going on?

Edited by lukebl
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Following - I had exactly the same issue with the same scope and same collimator.

 

 

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Howie used to be quite active and responsive on his site but sadly he is no longer with us, think Starlight Instruments are producing his collimators now but don't know if they answer queries.

Dave

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2 hours ago, johninderby said:

Have a read of this old thread.

 

Thanks. Unfortunately, it doesn't shed any light on the problem. IF I've done the first stages correctly, AND the projected concentric circles on the wall are concentric, then the thing should technically be perfectly collimated.

But it isn't. 

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I am in the middle of precisely the same situation, not with an RC but with a 6" Intes Mak which I completely dis-assembled, and am in the process of recollimating. It's the same situation as yours because with this Mak, both primary and secondary are adjustable, just like an RC.

I took 3 steps:

1. lined up the focuser axis to the primary axis, a la kitchen table collimation,

2. pointed the primary directly at the centre of the secondary (I also used a Glatter)

3. adjusted the secondary to "symmetricize" the reflection of the laser onto the primary, in my case getting the reflected dot back onto the laser source

Out looking at Polaris, I got EXACTLY the squashed diffraction star-test shape you showed, and I re-collimated the secondary until I got nice concentric rings, just like you. At that point, the actual view of the stars through an eyepiece was lovely, refractor-like.

But when I got inside, and put the laser back in to the focuser, it was clearly NOT pointing directly at the centre of the secondary, the reflected spot from Glatter laser to secondary was hitting the face-plate of the laser a good few millimetres out. But the actual star views in this configuration were nice. Clearly though they could be better.

This is where I am at the moment. My working theory is this:

step 2 above is quite crude, and very likely quite a bit out as you're only judging the primary centering to the centre spot at a very short distance.

What I think is happening is the primary is out, and after adjusting the secindary to the symmetrical star test, the primary is being very beautifuly compensated by a particular orientation of the secondary.

this is precisely where I am too.

My solution (which I heven't done yet but this is my plan):

Inside, put the Glatter in to the focuser, and note the orientation of its misdirected reflection back on to the facing-plate of the laser. Adjust the primary to bring it back, not all the way, but maybe 2/3 the way (because the secondary is "higher power"). Adjust the secondary to complete the centering.

Go back out into the field and get the symmetrical star-test back using just the secondary.

Repeat the last 2 steps until you've completely homed in to perfection.

Maybe we can both do this and compare notes...

If you're patient, read my account here, the latest post on the 2nd page shows where I am with this, thinking at the time of writing I'd cracked it, but not quite...

Cheers, Magnus

P.S. my Glatter is dead on, I tested it over 26 metres...

Edited by Captain Magenta

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12 hours ago, Captain Magenta said:

I am in the middle of precisely the same situation.......

Hi Magnus. Thanks for that detailed response. 

I'm really not sure if I'm brave enough to start adjusting the primary, as I've heard that can lead to all sorts of problems! In any case, I thought that the whole point of the concentric circles projected on to the wall was to check if the primary was properly collimated.  I'm impressed that you dismantled your entire scope. Mine was bought new last year, so I hope I don't have to go that far.

Well, as we've got a few months of lockdown I should have time on my hands to get it right. I really thought that the Howie Glatter was the answer to all my collimation problems!

 

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incidentally, my "hall of mirrors" from about 2m in front of the scope all looks very concentric, except literally just one "disc" out of the 10 or so, towards the back, that's sticking out a bit. Which tells me that all's not perfect, but doesn't really tell me what it is that's out.

The situation is probably more serious for you as the RC has hyperboloid mirrors as opposed to the spherical (I think) ones for my Mak - spherical much more forgiving.

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Hi

from experience when collimating my  GSO 14 inch RC (Truss) design 

I’ve used the 

Tak collimator  / Howie collimator (green) and the new R.E.E.E.G.O collimator All providing slightly different results... question is which one do you follow!!!  
 

one reason was the focuser end, I have three grub screws with a copper compression ring, 

Good way to test the howie is, insert the laser into the focuser, slightly tighten making sure it’s plush then rotate the howie, whilst observing the laser dot, if the laser dot moves lot ( up down etc ) then it’s not collimated Correctly 

best advice is to look at a real star at night and work from that.

tools will only give you rough error 

 

 

 

 

 

Edited by jaspalchadha
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Hi guys,

 

I'm a bit late to this discussion but I thought some of my experiences might help.

The fundemental problem with collimating a GSO RC scope is due to the mechanical design. The focuser and optical train is attached to the mirror holder, not to the backplate of the scope. I've owned an 8" RC since the Spring of 2016. I spent the first year reading everything I could about collimation, bought all the lasers and cheshire sight-tubes and spent hours trying to get it right. This is a simplified sketch of what we have:

 

1053151891_RC8scopesketch.thumb.jpg.7595628146e7a07664baeca86bb414e6.jpg

In more expensive RC scopes, the focuser tube is attached to the backplate and provides a fixed refernce for collimation. GSO have changed their design for 10" and larger truss tube versions and these are available from Teleskop Service, for example, but they are expensive. The problem with the design we have here is that any adjustment of the primary mirror, for example when collimating on a star, also moves the focuser, undoing any alignment of the focuser with the secondary you might have done in a first step. This is what you end up with:

1728698767_SquintedCollimation.thumb.jpg.a9fbffa5048595e6596f194b6417df06.jpg 

The primary and secondary mirrrors are parallel and a 'hall-of'mirrors' test looks OK but the optical axes of the two mirrors are not co-linear. Images might be OK but will not be optimum. I call this a 'squinted' collimation.

The fact that the focuser is attached to the primary mirror and moves with it effectively prevents any collimation technique based on sighting from the focuser, or using a laser in the focuser being sucessful. Sorry, but that's what I believe. A further problem is that in my scope the focusser tube was not perfectly perpendicular to the mirror mouting plate. You really do not know at what angle the focuer tube is to the primary mirror.

A couple of years ago, Es Reid and I were trying out the Hotech Cassegrain collimator

 

1001852309_HotechCollimator.jpg.a0a9c68af51c2cc6dfa9d0849fc98cd0.jpg

 

This is an expensive device and took several hours of very careful work to set up, but it works. It works by shining lasers into the front of the scope and, in fact, mimic a star. The focuser is not involved in aligning the primary and secondary mirrors and after a bit of work I ended up with this:

 

_DSF7284_after_tilt_plate.thumb.jpg.0016bd30de43d6cad36b38e512c6d9f5.jpg

Success!

As a final step, you can use the Hotech to check where the focus point of the star is in the focal plane. This is what I had:

 

_DSF7282_offset_at_eyepiece.thumb.jpg.c04156896ab93f4ca3c1030a7705af9a.jpg

The artificial star is off-centre.  Now it is at this point one can add a tilt-plate to bring the artificial star to the centre. I did this and got quite good results.

However, I could not envisage using the Hotech collimator long-term. You need a lot of patience to set it up and the results are very sensitive to, for example, slight bending of the surface it is on. i could not imaging using it on a regular basis for a quick check. 

So Es and I went to the pub and had a think. We came up with this:

 

1095873212_Cardwithhole.jpg.4892293db10f2db7684556f0a9529045.jpg 

 

It's a styrene card with a 1mm hole in the centre cut to fit exactly in the hole in the centre of the primary mirror. The hole marks the centre of the primary mirror.

I've run out of allowable images to upload so I'll describe how you use the card in a second post but you can go here, on Cloudy Nights where I discussed this general collimation problem and how to use this card.

https://www.cloudynights.com/topic/646374-not-another-gso-rc-collimation-thread/page-2#entry9223203

See you in a while,

 

David Davies


 

 

 

 

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Hi

I have just refitted my GSO RC10 back on the mount. As has been said the problem with the tube type GSO RC`s is that the image train is attached to the rear of the main mirror cell. This introduces tilt.

My RC10 is fitted with a Moonlite Tilt/Flange adapter the disconnects the image train from the main mirror and allows tilt adjustment. This appears to no longer available. Shame, a great product.

Telescop Services have tilt adapters available for 6", 8", and 10" tube RC`s. This tilt adapter does now disconnect the image train from the rear of the main mirror cell. I believe all GSO truss RC`s now come with a tilt adjustment plate fitted as standard. The Truss scopes do not have the image train is attached to the rear of the main mirror cell.

TS-Optics M90 Tilting Adapter Flange

https://www.teleskop-express.de/shop/product_info.php/info/p4272_TS-Optics-M90-Tilting-Adapter-Flange-for-astrophotography.html

I mange to collimate my scope with a Laser Collimator to adjust the tilt and a Cheshire Collimator modified by removing the wire cross hair`s for alignment. The one thing that has helped me get good collimation is a Howie Glatter Parallizer

https://www.firstlightoptics.com/adapters/howie-glatter-parallizer.html

This is a briliant piece of kit. It can be screwed to the scope using a M48 thread. This ensures that the Laser and Cheshire are rigidly and repeatable held in the centre of the focuser. Normally lasers are clamped into the focuser and this introduces flexure and variability into your collimation.

You can spend a lot of money on kit to collimate RC`s. With a bit of practise to "get your eye" the Cheshire gives good results. After collimation I have used the DSI method, but normally I use a star test. Defocused star magnified 50 time and checked with the help of AlsCollimationAid

AlsCollimationAid

The only place I could find it is

https://www.astronote.fr/telechargements/

I think the GSO RC`s are great scopes for the money but they do need a little work and persiverance to get them to work. That being said I did buy a Esprit 100 Refractor this year🙂

Regards and stay safe

Graham

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Thanks, Graham, I enjoyed reading your post. I thought, like you, I should explain some background. 

After I had owned my RC8 for 12 months, I became convinced that something was not right with my scope. I had, and still own. a TS tilt adjuster,  a HG laser, a Hotech laser, two Cheshires and yet I could not get satisfactory images. I could not get any two collimation methods to produce the same result and I could not get my star images much below 7 arcsec in HWFH measurement - I had fat stars. Some would say that is a feature of an RC scope but I wasn't happy.

After a lot of thought and reading, I dismantled my RC8. I first set up the primary mirror on a worktop, put the laser in the focuser and shone it at the wall.

This is the setup:

DSCF2969_optical_bench.thumb.jpg.555ab207453b1fb506891cf0b3baad2a.jpg

 

This is what I saw:

Spots.jpg.b25832dd3fc6ab97d5753f8cf4f42897.jpg 

 

The bright spot is directly from the laser, the dimmer spot is the reflection of the bright spot from the primary mirror. Clearly, the laser and the primary mirror are not pointing in the same direction.

So I dismantled the mirror cell and this is what I found:

Oring_web.jpg.2cfadcd1b3822239493456be4ad8e919.jpg  

 

Here you can see the 'top-hat' plastic device that is inserted into the primary mirror centre hole. The 'fingers' of the top hat device engage with a rubber o-ring that holds the mirror in place. My o-ring was perished and broke as I tried to remove it. I fitted a new neoprene o-ring (60mm x 2mm) and after some other work, including smoothing the back of the plastic holder with some wet and dry paper taped to a sheet of glass, I reassembled the mirror cell. When I repeated the laser spot test, I got this result.

New_spot.jpg.71fdad1d3f6cd04d3b07a34289984b5f.jpg

 

Bingo! The direct laser spot and its reflection from the primary mirror are now almost co-incident which means that any pointing error between laser and primary mirror is now small. I could, in fact, detect a very small difference but I was happy with this result.

By the way, here is a shot of the reasembled mirror holder before the centre clamp was screwed in place. You can see the new o-ring. 

1507451782_Newo-ring.jpg.74001ac260e84978e9c0aeefbc81e641.jpg

   

The next step was to align the primary mirror with the secondary holder. I first marked the secondary centre screw with paint marks before unscrewing the centre screw (the one you are not supposed to touch) and removed the secondary mirror:

DSCF3021_secondary_centre_screw-web.jpg.291e05ae20300ca507b798b5b230c0af.jpg

I then used the bottom of a take-away carton to make a screen to enable me the see the laser spot through the centre mounting hole of the secondary support hub. I now adjusted the primary mirror using the pull and push screws so that the laser spot coincided with the pen spot.

DSCF3047_align_primary.thumb.jpg.84b1af576d2d23f94f13fbc5f750421f.jpg

Here is the laser spot sillouetted against the pen mark on the plastic sheet (take-away carton). 

By the way, when remounting the primary mirror holder to the base plate, I started by screwing the pull screws (the larger ones) up fully so that the primary mirror holder was snug to the base plate. This gave me a starting reference. I then slackened the pull screws a couple of turns each and measured what length of the pull screws protruded through the base plate and made sure the lengths were equal. The smaller 'push' screws were then snugged up to touch the mirror holder. Don't do them up too tight, you'll distort the mirror. 

By the  way, you might have realised that the whole of your image train, focusser, camera etc. is hanging off these three pull screws. So I keep the mirror holder close to the base plate to minimise any flexing of the imaging train as the scope tracks the sky.

So I now had the laser pointing directly down the centre axis of the tube (and hopefully, the primary mirror too). The next step was to re-install the secondary mirror, count the correct number of turns of the centre screw and line up the paint marks.  Then I adjusted the secondary so that the laser hit the centre of the secondary mark.

 

DSCF3071_align_secondary-web.jpg.2729d1c53100e8223fb525aa67bd7210.jpg 

 

I thought I now had a collimated telescope from first principles and a check with Cheshire confirmed that. So I took some more images. There were not bad, but not great: the stars were still quite big. So I then took the scope to Es Reid who pronounced it 'quite overcorrected' - the primary and secondary mirrors were too far apart. Using a Ronchi grating, Es then adjusted the secondary mirror (unscrewing the centre screw) to push the secondary closer to the primary and he eventually achieved a good Ronchi image.  On retesting the scope on the sky, I used a PixInsight utility to measure the focal length of the scope which came out as 1660mm, not 1624mm as advertised. In fact, I then made a study of how the focal length of the RC8 varies with the number of turns of the centre screw, and this is what I found.

 

1818676953_Variationoffocallength.jpg.900dc9bd3437bf53f86458fe1bd1b6dc.jpg

 

Clearly, you can't take your scope to bits everytime you want to collimate it but I knew I had, at least, something that was close.

My final colliamtion is always done on a star image in the middle of the field of view. I either use a star image which is very slightly out of focus or I use Metaguide. Metaguide has a collimation function where it adds a marker to the star image indicating the centre of gravity of the star brightness. Here is a good example: 

 

2014426989_Collimation_image_MG_170630_3.jpg.e83e1b1ce2d03863ab0ba803270f9116.jpg

 

The magnified image if the star is in the blue box and Metaguide has added the small red cross to indicate the center of brightness. The seeing has distorted the shape of the star slightly but this is a good result.

 

I need to stop for now. More later.

 

David

 

Edited by davies07
correct typos, add more explanation
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I thought I should give some results from the rebuild outlined above and after Es had corrected the focal length.

Here is an image of a very nice open cluster, NGC 6633, showing the centre and the corners.

 

NGC_6633_afterMG_mosaic.jpg.2ed869c155456cae082df06b12d47cdf.jpg

 

I used PixInsight to measure the eccentricity of the stars in the image. An eccentricity factor of 0.3 means that the minor axis of the star elipse is 96% of the major axis, i.e. a 4% error. The error rises to about 9% for an eccentricity factor of 0.4.

 

NGC_6633_afterMG_eccentricity.jpg.fbe630d5fa1ddafcdd5b1e2124d8642e.jpg

 

And I also used PixInsight to measure the FWHM of the stars (you might recall, I was getting about 7 arcsec previously). This means my HFR readings in Nebulosity is now around 2 arcsec.

 

NGC6633_after_Es_sec_adj1_FWHM.jpg.b78c10fa643346f3e1a33382df90aa9c.jpg

 

So, yes, I was happy with this result. The results show the optics are symetrical.

Whilst all this was going on, I was still investigating collimation methods that could be used routinely. One thing that puzzled me was the role of the TS tilt-plate. If you attach the tilt-plate to the RC8 as shown on the TS website, then you can no longer adjust the primary mirror because the tilt-plate hides the primary collimation screws. So I phoned TS to ask them. Their response was that the tilt-plate is NOT intended to be used to collimate the telescope, it should only be used AFTER the telescope primary and secondary mirrors have been collimated and then only if you have tilt of the sensor.  Tilt of the sensor would give you oval stars across the field of view and whilst stars might be in focus in the centre, they will tend to be out of focus at opposite edges.

I then discovered this reference:

http://interferometrie.blogspot.com/2013/01/10-rc-gso.html?view=sidebar

This is a review of the construction and collimation of a 10" GSO RC and is well worth a read (English version unless you speak German). Here is a key diagram from it:

 

Snag_a5be262.png.63b88243985f7ad5817a32673adee6af.png 

This is a view into the back end of an RC scope and shows how the secondary mirror and spider support vanes should look. The method outlined in the paper is to stand about 1m behind the scope and line your eye up to get this view. Adjust the secondary mirror to centre the secondary centre mark and then adjust the primary mirror to equalise the refections of the spider vanes. Simples.

I had a go at this using my DSLR camera as shown here:

 

IMG_4471_plus_blur_web.jpg.9a0741ff4ca8b62451f54e55e2f91b9a.jpg

 

Using the wifi remote control for the camera on my iPad I could adjust the scope 'live' and take test shots to record what I had. So this is with the secondary mirror mark in focus:

 

DSCF4715_secondary_mark.thumb.jpg.d058c79e5598d83926beecf0b04bbe46.jpg

 

And this is with the reflection from the primary in focus:

 

DSCF4714_primary.thumb.jpg.a320e6a07ec5951d67e3c9d132fe47ef.jpg

This is a good, quick check of the state of collimation and with practice you can do it with the scope on its mount.

So to round this off. This is how I collimate my RC8 today. No lasers, no tilt-plate.

First remove the imaging train and extension tubes. Now measure the internal diameter of the centre hole in the primary mirror and cut a styrene disc, or piece of thin card (postcard), to fit. Now drill a small hole (1mm) in the disc (say with a Dremel).

_DSF5520_disc_small.jpg.0c21518679beed74efe096705a499645.jpg

Now insert the disc into the hole in the primary mirror. t should be a snug fit. I wrapped a single layer of tape around the edge of the disc to give it some cushioning. Don't push it in too far; it will drop into a gap and you'll have to fish it out. The hole in the centre of the card is now marking the centre of the primary mirror.

 

173273425__DSF5510_inplace_small.jpg.c402b5294a41bc7817f6f16c21bbbce7.jpg

 

Now go the front of the scope and remove the secondary mirror - mark the screw with a paint spot and count the number of turns. You will also need to unscrew the shade tube - do it carefully and leave it sitting on the bottom of the tube. Place a lamp behind the scope.

Look into the centre hole of the secondary mirror support and you should see the light of the lamp shining through the hole in the card.

 

1047093210__DSF5509_centrespot2_small.jpg.7e258f3e80e5b242c2e0dc1f15eea869.jpg

 

Now centre your eye to keep the bright spot centred and check if the spider vanes coincide with their reflections in the primary mirror. In the picture, above , you will see that the spider reflections are just alongside the spider itself. Now gently adjust the primary mirror to bring the spider and its reflection into coincidence, whilst keeping the bright spot in the centre of the support hole. Again, you can set up a camera to help. This adjustment isn't absolutely crucial because you will do a final check of the adjustment of the primary using a star on the sky.

Now put the lamp at the front of the scope, shining onto the mark in the centre of the card. You can't see the card if you have the shade tube in place which is why we need to remove it. Now go to the back of the scope and look throught the hole in the centre of the card. You should see a reflection of the hole in the card in the secondary mirror. This is a bit tricky, you need a lot of light and a good eye. 

 

_DSF5518_centred_web.jpg.c82415604cd6de09836ea8e00b72999c.jpg

Now adjust the secondary mirror so that the reflection of the hole in the card is centred in the secondary ring mark. Take your time. This is a crucial adjustment. This adjustment centres the secondary and governs the symmetry of star images over your field of view.

99981581_SecondaryColl20190521usingcard.jpg.6244964a9d8924b3bc18f03c8003c222.jpg

When you're done, replace the shade tube and secondary mirror. 

When the clouds clear, check the collimation on a star in the centre of the field of view. Defocus the star only slightly so you can just see the central hole, a bit like this:

 

 339162950_10-06-201723-03-47outoffocusout-web.jpg.c4646906728ae6f04307ac9bf8ae85fa.jpg

 

Don't use a hugely out of focus image, it is not sensitive enough for collimation.

This is the display from 'Fine Focus' mode in Nebulosity. The star is slightly out of focus 'out'. I discovered that the bright edge was due to a hot plume from my dew heater.

 

 And that's it. You can collimate your RC using just a piece of card, some understanding and patience.  

 

Clear skies and stay safe,

 

David

 

 

 

 

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Hi everyone, reading this with a lot of interest as it is very much to the point:

  • I collimate my primary/secondary with a Howie Glatter first (adjust primary to point the laser to the secondary central circle, adjust the secondary to bounce it back to the center of the emitter)
  • I then re-adjust the secondary on a star

However on bright stars I have an off-set halo that I suspect should be at the center of the star (at least when at the center of the image) (see attachment). Is that because the optical axis of the train (especially when factoring in sagging due to weight on the crappy standard focuser)? My attempt to solve that via primary collimation did not get anywhere.

Is the ghost simply amplifying an error in secondary collimation? (my out-of-focus ring is far from perfect and squished in the opposite direction as the ghost offset)

Any ideas?

collimation.thumb.png.af2d2d412b4612b9af0f81a6f41643ff.png

 

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Hi B4silio,

You have an interesting couple of images. Looking at the first one, I think the secondary shadow is biased toward the 3 o'clock direction and the halo is brighter towards the right than the left. In the image with star in focus, the star is also off centre towards the right. I would read this as a real effect. Going back to the first image, and as you focus on the star, I suspect you would see the right edge of the star image brightening and a coma appearing towards the left (9 o'clock) direction.  

Coma in the centre of the image is controlled by the pointing of the primary mirror, so I suspect your first step of pointing of the primary based to get the laser to hit the center of the secondary is not a good strategy. You could try one of my original approaches, above, of removing the secondary (carefully) and using a translucent screen to adjust the primary to point the laser at the central hole of the secondary spider. That would eliminate any errors introduced by  the initial pointing direction of the secondary.  Then re-attach the seconday and do your step 2. (Remember to mark the position of the secondary with paint marks.)

My final step would then be to check the comma on a central star at focus. I use Mataguide to do this. It's free but you need to use a video-type camera such as a ZWO. Alternately, you could use a high powered eyepiece and look at the slightly out of focus star. If the shadow of the secondary is towards the right and the bright edge of the star is towards the right, then the shadow of the secondary needs to be pulled towards the left. This is done by adjusting the primary mirror. You slightly tighten the left hand pull screw (the larger one) and at the same time slightly slacken the push screw on the left.

I have a saying "pull the secondary shadow", meaning tighten the primary pull screw to pull the secondary shadow towards that pull screw. And you must do things very gently with very small adjustments.

I think it is vital to eliminate coma at the centre of the field for good images. It is the primary mirror that controls coma. So it is the last adjustment you make (and on a star). The secondary mirror centralises the 'quality' of the image in the field of view. So if you have distorted stars in one corner, say, then it is the secondary mirror that needs adjusting.

 

I hope this helps,

David

Edited by davies07
correct typos
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10 hours ago, davies07 said:

Hi B4silio,

You have an interesting couple of images. Looking at the first one, I think the secondary shadow is biased toward the 3 o'clock direction and the halo is brighter towards the right than the left. In the image with star in focus, the star is also off centre towards the right. I would read this as a real effect. Going back to the first image, and as you focus on the star, I suspect you would see the right edge of the star image brightening and a coma appearing towards the left (9 o'clock) direction.  

Coma in the centre of the image is controlled by the pointing of the primary mirror, so I suspect your first step of pointing of the primary based to get the laser to hit the center of the secondary is not a good strategy. You could try one of my original approaches, above, of removing the secondary (carefully) and using a translucent screen to adjust the primary to point the laser at the central hole of the secondary spider. That would eliminate any errors introduced by  the initial pointing direction of the secondary.  Then re-attach the seconday and do your step 2. (Remember to mark the position of the secondary with paint marks.)

My final step would then be to check the comma on a central star at focus. I use Mataguide to do this. It's free but you need to use a video-type camera such as a ZWO. Alternately, you could use a high powered eyepiece and look at the slightly out of focus star. If the shadow of the secondary is towards the right and the bright edge of the star is towards the right, then the shadow of the secondary needs to be pulled towards the left. This is done by adjusting the primary mirror. You slightly tighten the left hand pull screw (the larger one) and at the same time slightly slacken the push screw on the left.

I have a saying "pull the secondary shadow", meaning tighten the primary pull screw to pull the secondary shadow towards that pull screw. And you must do things very gently with very small adjustments.

I think it is vital to eliminate coma at the centre of the field for good images. It is the primary mirror that controls coma. So it is the last adjustment you make (and on a star). The secondary mirror centralises the 'quality' of the image in the field of view. So if you have distorted stars in one corner, say, then it is the secondary mirror that needs adjusting.

 

I hope this helps,

David

David 

You should start a collimation service - just a thought. 

I've recently bought a 2nd hand RC8 & rather than do what some people do - buy one - struggle - then sell on, I'm going to persevere with it & want to get it as good as it can be. 

Unfortunately I'm not as technically competent (or brave enough lol) as i need to be to follow your steps myself. 

Great post & great resource for reference

Cheers

Andy

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Hey David!

Thanks a lot for the comments, very helpful and I definitely think it's helping! After a bit of tinkering with the primary as you suggested (haven't tried MetaGuide yet but I've been trying to align the laser to the wall) the current result I have is much better, looks perfect on one axis and slightly off on the other one, but the improvement in the couple of test I was able to do before getting to bed is very visible already. I feel that with a bit more time I should be able to get that properly centred.

Incidentally that would mean I have a "single star" method for getting both Primary and Secondary collimated.

895691657_collimation-secondtry.png.86e9b995fc1564186ffa2daa083ce753.png

I'm left with a question though: Where is the flare coming from? I suspect it's likely my filters? The other weaker reflections seem to come from microlensing of the camera, but the initial big ghost ring I'm not entirely sure I know how to understand it (and potentially get rid of?)

The other question I have is about the collimation screws of the Primary (this might be relevant only to the 8" version of the GSO/TS of the RC): Depending on the source they're either described as "Pull-Push" or "Lock-Collimate". Here are 3 sources contradicting each other. I've been working under the assumptions of the leftmost image (from the Orion manual for the RC😞

  • I unscrew the black screws
  • I use the silver ones to adjust the primary
  • I tighten back the black screws when done

Am I missing something? Is that really a Push-Pull system where I should be adjusting both?

1001153074_CollimationScrews.thumb.png.336ca88730f24ca48bce57a3a96ebe17.png 

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18 minutes ago, B4silio said:

Am I missing something? Is that really a Push-Pull system where I should be adjusting both?

I think that push pull is just misnomer - there is no such thing as push pull system (as far as I know - I would like to know how one can use two screws - one to push and one to pull without turning both at the same time to adjust collimation) - it's usually just collimation / lock.

Collimation can be spring loaded, or threaded version.

With spring loaded - collimation screw provides distance and spring makes sure mirror is held against that screw. In threaded version - it just behaves as worm or focus system on SCT/MCT type scope.

I remember collimation being lock / collimate type on my RC, but I've forgot - which ones are lock screws and which ones are collimation screws.

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8 hours ago, Andy274 said:

David 

You should start a collimation service - just a thought. 

I've recently bought a 2nd hand RC8 & rather than do what some people do - buy one - struggle - then sell on, I'm going to persevere with it & want to get it as good as it can be. 

Unfortunately I'm not as technically competent (or brave enough lol) as i need to be to follow your steps myself. 

Great post & great resource for reference

Cheers

Andy

Andy I agree with you!!

And yes, I definitely think it's worth the effort of making it work even if it takes a bit of pain and time to understand how things work.

I've just finished stacking a test from yesterday night, and compared it with the night before yesterday. The coma in the image before re-collimating was very pronounced (as David predicted), the new one is not perfect but already a step forward

 Before-After-collimation.thumb.png.274160828246de036afa1c0c00bc6e2e.png

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2 minutes ago, vlaiv said:

I think that push pull is just misnomer - there is no such thing as push pull system (as far as I know - I would like to know how one can use two screws - one to push and one to pull without turning both at the same time to adjust collimation) - it's usually just collimation / lock.

Collimation can be spring loaded, or threaded version.

With spring loaded - collimation screw provides distance and spring makes sure mirror is held against that screw. In threaded version - it just behaves as worm or focus system on SCT/MCT type scope.

I remember collimation being lock / collimate type on my RC, but I've forgot - which ones are lock screws and which ones are collimation screws.

Thanks a lot Vlaiv, that makes a lot of sense!

From what I see happening I'd say that the black ones are indeed simply locking things in place (i.e. turning them doesn't really have an impact unless I tighten too much), while the larger silver ones actually turn the mirror, so unless I'm doing something bad I'll just stick to that assumption!

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With my 8” Classical Cassegrain which uses the same backplate as the RC (but different mirrors) when collimating you loosen the lock screws then colimate using the colimation scews then when finished tighten the locking screws. 

Edited by johninderby
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As the originator of this thread, I've only just caught up with the replies and many thanks for the responses.

After reading all this, all I can say is that I doubt anyone will EVER, EVER buy a Ritchey Chretien EVER AGAIN!

Edited by lukebl
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I'm following this thread too.  I just posted my own frustrations with my club's 16 inch RC that I've spent hours on.  I wish I could get a round star like your example showed.  If the star was round and focused I'd be tempted to leave it alone.  I've got the focuser, primary and secondary all fiddled with now and just not getting anywhere.   :(

 

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A very quick trick I found for collimating the secondary mirror and get a very good result ridiculously fast (2-3 minutes).

I'll assume two things:

  • You have a laptop/tablet/electronic viewfinder or anything that you can turn around and see when you're in front of the scope
  • You can see a bright star

Now do the following:

  • Point to a bright star, get out of focus and put the ring at the center of the image, it should be deformed
  • Get close to the screen so you can see well and slew around so that the ring changes shape
  • If you slew in the direction of the THIN side of the ring, the thin side should get fatter and the ring should become more even
  • When you have a perfectly even ring leave the star where it is (probably nowhere near the center)
  • Now turn the screen toward the scope so you can see it when in front of it
  • Adjust the screws on the secondary to move the star to the center of the screen
  • Your secondary is now perfectly collimated

The main idea is: By slewing around until you have a perfect ring, the image of the star is being projected in the correct way, and you adjust the mirror afterward to point that "correct image" right back at the camera

The whole thing takes ~3 minutes and the only issue is whether I can find a star that is bright enough to see well how even is the ring.

The Primary, however, is still a pain in the bum!

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1 hour ago, lukebl said:

As the originator of this thread, I've only just caught up with the replies and many thanks for the responses.

After reading all this, all I can say is that I doubt anyone will EVER, EVER buy a Ritchey Chretien EVER AGAIN!

I know what you're saying Luke, 

They're a mare to collimate, but if & when you persevere &get it working - its a proper weapon 

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