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Help please! Collimating an RC with a Howie Glatter


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Hi Kookoo,

yes, that could be the case. The reflection of the camera lens is very sensitive to camera and mirror misalignment. That was exactly why I started using that reflection in the process, it magnifies those misalignments significantly.

Nicolàs

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Last night I received the confirmation that the adjustment of the RC8 was successful. The owner sent me below image of Vega. The spider's diffraction spikes are razor-sharp. The photo was taken without flattener, which gives the opportunity to see whether the collimation is well balanced. Coma is visible in the corners and they all point towards the centre of the image, which is a good sign.
Prior to this final adjustment, the focal length was determined by astrometry to be 1646mm, while it was designed to be 1624mm. Now I had read that adjusting the secondary mirror would have a tenfold effect on the focal length. So we moved the secondary mirror outward by approximately 2.25mm (three turns with the M5 bolts). Using a caliper I checked this displacement and came to a real difference of 2.6mm. Astrometry on the image of Vega taught us that the new focal length has become 1617mm, a difference of 29mm, which brings the factor by which the focal length changes to approximately 11 times the inter-mirror distance (increasing the distance lowers the OTA's focal length).

Nicolàs

2028488594_vega4102020sbin2a.thumb.jpg.90ea1d8f11c67d944c1d28db8e6c3c3f.jpg

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  • 2 months later...

Apologies for adding to this thread but having recently purchased an 8 inch RC I am battling with the collimation. I have spent hours reading about various methods that can be used from cutting out circles of paper and placing in front of the primary to check if the holographic circular projections are correct to complete strip downs and removal of the primary and secondary mirrors.

The method that comes up regularly is using the Howie Glatter laser and is the one I am trying because it is well documented and there are several YouTube videos on it. I have the following tools available;

  • Collimating tilting plate attached between the focuser and primary. There are two extension tubes between the primary and tilt plate as they would be in my imaging set up so I thought it best to include them in the collimation exercise.
  • Cheshire eyepiece.
  • Howie Glatter 2 inch laser with holographic and laser dot attachment.

So far I have attached the Howie Glatter and laser dot attachment and centred it on the reflected secondary donut ring appearing on the primary mirror. I have achieved this by adjusting the collimating tilt plate. (pic below) 


IMG_4169.thumb.jpeg.29a424d66170cd90cdd3d36a4924265c.jpeg

 

Next I then use the holographic attachment on the Howie Glatter to reflect the concentric circles off of the secondary mirror and on to the primary mirror. I understand that by adjusting the focuser you can position a ring on the outer edge of the primary and use this as a guide when adjusting the secondary mirror screws to make the circles more concentric.

 

IMG_4160.thumb.jpeg.d8ff63ab1a7cf16b3b96b6067b7d7b8d.jpeg

 

As an alternative to the concentric rings on the primary mirror, I have read that you can also use the laser dot attachment and adjust the secondary mirror so that the dot reflects back to the laser emitter (see below).

 

IMG_4170.thumb.jpeg.8a4b9549841e6c465fcad04eb4a01404.jpeg

 

The final step before restarting this all over again is to use the holographic attachment on the Howie Glatter and view the concentric rings projected on to a wall.

 

IMG_4155.thumb.jpeg.46b1a23a59a87ce5e1e8242062fbd3f5.jpeg

 

So, you can see from the above photos where I currently am with all of this. Basically, this is as good as I have got it so far but before i ‘ask’ I just want to make some observations.

First, I can’t really get my head around the reflected circles on to the primary bit. I have made adjustments to the secondary to position the outer ring as best I can on the primary but I am struggling to see whether this is the best fit. I have made adjustments then checked with the Cheshire and the dot is way outside the secondary donut ring.

However, when I use the alternative and adjust the secondary to reflect the dot laser back to the emitter a sanity check with the Cheshire shows the dot well placed within the donut ring. 

As for projecting the rings on to the wall, clearly I can see the shadow of the secondary is not central and is extended towards the 9 o’clock position. This is obscuring the left hand side of the inner ring and also the right hand side of the outer ring is missing. I would also point out that the focus draw tube is positioned at the focus point for my camera and where I would be imaging from. 

As I am new to this it would be helpful to get an expert opinion and an interpretation of what is happening here. My next step is to adjust the primary mirror. It may be helpful to know that the primary adjustment screws are located at the 12, 4 and 8 o’clock positions on the wall reflection picture above. 

I am determined to get this done and would really welcome some constructive analysis on the next adjustment and why so i can better understand the mechanics here.

Thank you in advance.

John

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Hi John,

based on all the excellent info given in this thread I made a step-by-step plan, which I tested on a RC8 and published as an article on a Dutch forum (if you open it in Chrome it should translate reasonably well).

Main importance is to realise that the tilt-adapter between the OTA and focuser should not be involved in the collimation procedure of the primary mirror. Instead the adjustment of the tilt-adapter should be done once the primary is in collimation. We even removed the tilt adapter during the collimation of the primary, but it can be done with it in place as long as the temporary peep-sight in the visual back is near the mirror and not at the end of the focuser. This part of the process is also explained in above mentioned article.

You may need to make yourself a few tools as described in this thread and in my article, but once you have those, collimating the RC8 is easy.

Nicolàs

 

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12 hours ago, inFINNity Deck said:

Hi John,

based on all the excellent info given in this thread I made a step-by-step plan, which I tested on a RC8 and published as an article on a Dutch forum (if you open it in Chrome it should translate reasonably well).

Main importance is to realise that the tilt-adapter between the OTA and focuser should not be involved in the collimation procedure of the primary mirror. Instead the adjustment of the tilt-adapter should be done once the primary is in collimation. We even removed the tilt adapter during the collimation of the primary, but it can be done with it in place as long as the temporary peep-sight in the visual back is near the mirror and not at the end of the focuser. This part of the process is also explained in above mentioned article.

You may need to make yourself a few tools as described in this thread and in my article, but once you have those, collimating the RC8 is easy.

Nicolàs

 

Hi John,

I agree with Nicolas, using the HG laser is not the way to go to collimate an RC8 due to the mechanical design of the primary mirror support and focuser tube. I would suggest that you read my posts of 22 and 31 March on page 1 of this thread; they explain the fundamental problem and a simple approach to achieve good collimation. Many contributors to this thread have now achieved good collimation their RC8s and RC6s using this method.

Nicholas has produced an excellent extension of the method which makes the method more accurate and simpler to do, but it does require the construction of a couple of tools. All you need to undertake the basic method is a piece of good quality card or polystyrene sheet. 

David

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Thank you for the replies so far. I will have a read of the suggested messages and see what develops.

An interesting update. I decided to dispense with lasers and straight eyeball this to see what result I get. I recently ‘upgraded’ the stock focuser to a Baader Diamond Steeltrack. A lot of users recommend this as a good move given the better quality and weight the focuser can carry.

Inserting the Cheshire into the Baader  reveals that the cross hair is up and to the right. That’s with the primary mirror reset. I found that odd and rotating the Cheshire keeps the cross hairs in the same place.

Replacing the Baader with the stock focuser and then inserting the Cheshire reveals good alignment between the cross hairs and spot/donut ring in the secondary.

I know that the Baader Diamond Steeltrack allows the user to adjust the draw tube for tilt, however, the instructions also say that the focuser is centred before it leaves the factory and playing with the settings is not recommended.

Previously I have used a collimating tilt ring but even removing this it looks like I am having to also content with the cantering of the focus tube as well 😩

 

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Hi John,

there are indeed a lot of parts that can mess up the collimation. They all need to be aligned, which is not too difficult as long as you keep the right order:

- collimate primary mirror (without using a focuser and focuser tilt adapter, and preferably with them being completely removed)
- adjust the focuser tilt-adapter
- coarse collimate the secondary mirror
- verify the inter-mirror distance
- fine-collimate the secondary mirror

If you then insert the HG-laser you should have nice concentric rings.

On a new scope the inter-mirror-distance should be fine and could be skipped (although it never hurts to test it). Once or if correct, changing the inter-mirror distance can be avoided when only using two of the three adjustment screws of the primary and by not changing the orientation of the central bolt of the secondary mirror. So first mark the screw of the primary that you are not going to use (e.g. with a dot of paint), so that the next time you know which one not to touch again. Likewise mark the central screw of the secondary (with a dot of paint on the edge of the screw and on the edge of the hole, see below image), so that you can see next time that it still has and maintains the right orientation.

Of course you will need to remove the secondary for the first step in the collimation process, so either count the number of turns that were required to remove the central bolt of the secondary mirror or (and I think that is preferred), measure the distance from the secondary mirror to the spider using a caliper on four locations like this:

DSCN3816.thumb.JPG.bb98d7e3944904df544794c5a6421cd8.JPG

When only counting the turns of the central screw it is essential to mark the orientation of the secondary mirror as well (although it is good practise to measure it always as a verification). In above image two pieces of masking tape can be seen above the caliper with blue marks on them indicating its orientation.

Nicolàs

Edited by inFINNity Deck
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  • 2 weeks later...

sorry for bumping this. but it hasn't been inactive very long. i didn't get any replies on my post so hoping this will at least get some eyes that have some rc experience based on this thread. 

can someone please tell me if the outer diffuse ring needs to also be centered around the ota shadow (i think that's what it is?)? mine is thinner on the top but nothing i do changes it. changes only affect the inner secondary shadow and inner rings. see attached.

as i focus out, the outer ring breaks at the top first and fades down until it's gone when the focuser is all the way out. if everything within is even and concentric is that ok or does it mean something else is out of alignment to start? ie primary mirror in ota? (not tilted but rather shifted up or down?) sorry. this outer ring is the only other thing i see that might point to something still out of alignment. my focused stars are still off but narrow beam tests and other tests seem to show everything is aligned, except this outer ring.

i'll also be looking into the alternative methods posted in this thread too. i need more time processing them but am hoping to find out if my image result is relevant or if it can be ignored as i proceed.

thanks.

 

target4.jpg

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Hi Iwannawon,

You seem to have achieved an excellent alignment of your focuser with the secondary mirror and these are text book rings. However, I suspect your scope is not collimated.

Regarding your query on the appearance of the outer ring, the laser diode in the HG laser is rectangular in shape and produces a ring which varies in thickness around the ring. - narrower in one direction compared with the one at 90 deg - thus the variation in the appearance of the outer ring. 

You mention that despite this apparent excellent alignment,  your stars are still off. This doesn't surprise me as I suspect that although the HG laser rings indicate the scope is collimated, I suspect that it isn't. The key thing is to get both the primary and secondary mirrors parallel and facing each other exactly. Forget about what the focuser is doing because you don't know what angle it is to the primary mirror and I suspect it is not orthogonal. I would urge you to read through the posts on this thread to understand the challenges of collimating this particular design of GSO scope and the methods that can be used to achieve good collimation.

If I put the HG laser on my scope, the ring pattern is absolute rubbish. To see it, you would immediately be reaching for your tools to adjust the mirrors. However, the scope is well collimated and as evidence, I 'll share part of a recent image showing what can be achieved with a well collimated RC8. This is the edge of M31, towards the edge of the frame of my latest image of M110.
 

Here is the complete image:

M110_LRGB-web.thumb.jpg.da526ae050c6eefc6a5016304f56fbe4.jpg
  

And here is the detail:

 

M31_LRGB_detail_crop.jpg.a769d3b6da68bb9129b730d01a2491e9.jpg

I love this scope. We have round stars no aberrations, nice star colours and lovely detail emerging from the dust lanes and HII regions in M31. This is what you can achieve using the methods described in this thread. 

Seasons greetings and best wishes,

 

David

 

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thank you david. that's a wonderful image. i'm looking forward to taking the same someday. i haven't had a chance to try the alternate methods to collimate yet but it's next on my list. i've been a bit hesitant because i'm afraid i won't get it back to where it is and it will even be worse. but i guess it's needed. i'll definitely reread the thread and try again. what confuses me about the outer ring is it doesn't change when i rotate the laser. if it was the beam thickness, i think it would? i see slight differences in thickness in inner rings too and i rotate the collimator to see if it's really misaligned or if it looks better (on average) if i rotate it. but that doesn't seem to affect my outer ring that i labelled. 

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Hi Iwan,

I agree with David, so please forget about the HG-laser and try to follow the methods described in this thread. The HG-laser is a fine tool for a final check, so after you calibrated the RC8, but is better not used for the collimation itself. A well collimated RC8 should provide a proper laser pattern, but a good laser pattern is no proof of a correct collimation.

Nicolàs

 

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just to doublechek... is there a way to align it without fully taking out the secondary mirror? i tried by taking off the whole front secondary holder unit and making a similar template with a hole in it (that you showed on the focuser end) that i attached to the ota. would this be the same as aligning with the secondary's center screw hole?

also when testing the primary mirror alignment by removing the ota and pointing at a wall, how critical is it that the mirror is exactly parallel to the wall? for the reflection to be centered. if i'm a bit (ie 1/32") off and don't know it, will it skew the results?

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2 hours ago, iwannawon said:

just to doublechek... is there a way to align it without fully taking out the secondary mirror? i tried by taking off the whole front secondary holder unit and making a similar template with a hole in it (that you showed on the focuser end) that i attached to the ota. would this be the same as aligning with the secondary's center screw hole?

Hi Iwan,

The idea of taking out the secondary mirror is to use the screw-hole (that has the screw that holds the secondary) to align the visual back with the camera that stands in front of the OTA and at the same time to use this camera to see its own reflection in the primary mirror (in order to get that reflection properly centred). As long as removing the whole front end and replacing it with a template allows to exactly replicate the location of that hole and to still see the primary mirror, that would indeed be the same. The problem lies, I think, in that part "exactly"...

2 hours ago, iwannawon said:

also when testing the primary mirror alignment by removing the ota and pointing at a wall, how critical is it that the mirror is exactly parallel to the wall? for the reflection to be centered. if i'm a bit (ie 1/32") off and don't know it, will it skew the results?

Not sure which part of the procedure you are referring to, so please provide a link.

Nicolàs

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1 hour ago, inFINNity Deck said:

Hi Iwan,

The idea of taking out the secondary mirror is to use the screw-hole (that has the screw that holds the secondary) to align the visual back with the camera that stands in front of the OTA and at the same time to use this camera to see its own reflection in the primary mirror (in order to get that reflection properly centred). As long as removing the whole front end and replacing it with a template allows to exactly replicate the location of that hole and to still see the primary mirror, that would indeed be the same. The problem lies, I think, in that part "exactly"...

Not sure which part of the procedure you are referring to, so please provide a link.

Nicolàs

hi. that part was from page 1 on this thread, davies07 reply on march 30 that has a couple pictures of the primary mirror (without ota attached).

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Hi Iwan,

ok, that post, thanks. That method does about the same as my method with the camera. My method has two advantages over the one in David's initial post:

- it is not affected by misalignment of the laser;
- it can be done without dismantling the OTA (although David's method may have been possible with the spider in place), only the secondary needs to be removed.

In addition I think my method is more sensitive (but that has yet to be proven ;-)).

In David's method the parallelism of the mirror with the wall is not critical at all. What happens, is that the mirror 'sees' the laser light on the wall and projects it back onto it, so even at a steeply inclined wall this should work (although being parallel may work easier).

Nicolàs

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12 hours ago, iwannawon said:

hi. that part was from page 1 on this thread, davies07 reply on march 30 that has a couple pictures of the primary mirror (without ota attached).

Ah. Dismantling the OTA in the way I showed is not part of the collimation procedure.  I was showing what I had discovered when I dismantled my OTA and the sources of the problems I was experiencing in collimating my scope at that time. My description of my collimation procedure begins further down the thread beginning with the photo of the plastic disc. 

I've realised that this thread has wandered here and there and it is not clear what a newcomer should do. Therefore, I have prepared a write up of the whole process and you can download it from here:

https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey with a plastic disc V2.pdf?dl=0

Let me know if this works for you, or if something needs to be clarified and I'll do my best to update it. That invite applies to all; do have a look.

David

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  • 1 month later...

Apologies for resurrecting an old thread – but it is an extremely useful thread - many thanks to David for his detailed explanations of his method. I’ve downloaded and am working through the PDF.

I have a 6 inch RC. I bought it used, it was well collimated by the previous owner and held collimation well. However after 3 years heavy use, the primary had been getting more and more dusty to the point where I eventually had to clean it. Turned out to be sticky pollen – water and detergent wouldn’t get it all off, eventually needed pure IPA. Nice and clean now, no scratches.

I measure collimation using Metaguide and CCD Inspector. I took some metrics before starting so I have a baseline.  When I reassembled the scope after cleaning I checked collimation before touching anything and it was well out. 1st iteration of the disc method and it halved all the errors, but I still have double the error before I touched it, so more work to do. Haven’t done a star test yet – just measuring star fields with CCDI.

I have a few problems –

1.       I’ve checked the native FL by plate solving and it is 1366mm, spec 1370. However I want to check mirror spacing using a Ronchi eyepiece, which I have. Can someone (David if you see this?) please confirm the meaning of barrel distortion inside focus? David’s post of 12/06/20 on this thread says “A barrel-shaped Ronchi pattern inside focus = overcorrected scope, too short focal length, mirrors too far apart.” On this old Cloudy Nights thread https://www.cloudynights.com/topic/423163-precise-rc-mirror-spacing/ post 24 says “a barrel pattern inside focus means mirrors too close together”. I’m now officially confused. Doesn’t take much….:-)

2.       Because the gaps between the spider vanes of the RC6 are so narrow, I can’t get my hands in to reach the primary baffle tube or hold the secondary when I loosen the centre screw. This means that to use the “hole in disc” method I have to take the complete secondary end cap off the scope after I have collimated it, thus disturbing the fine adjustments I have just made. David, again if you read this - you mention calibrating RC6s using your method. How did you get round this?

3.       Finally, once I have collimated the scope using the hole in disc method, I need to star test it. Once I attach extension tubes, a focuser and an eyepiece or camera, I am at risk of the scope optical axis being misaligned with the focuser / camera / eyepiece axis, so is there a risk of chasing my tail here, how do I mitigate that? I have a Cheshire if it helps.

Any and all suggestions gratefully received….

Paul

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Hi Paul,

may I first suggest to read my article as well (if you have not already done so)?: http://www.starry-night.nl/stap-voor-stap-collimatie-van-een-rc/

It is in Dutch, but should translate reasonably well in a browser like Chrome.

Ad 1): I too have seen this discrepancy in barrel distortion at various sites and checked and double-checked it using the C8 I collimated. For what I have seen, figure 8 in above article is correct. I do need to add that the Ronchi test is not very sensitive, so the focal length of the RC8 was tested using plate solving and then adjusted again. We found a ratio of approximately 1:11 for this adjustment as explained in my article (so 1mm displacement of the secondary changes the focal length by 11 millimetres).

Ad 2): I will leave this one to David to comment upon.

Ad 3): this too is explained in my article. First get the primary mirror properly aligned with the OTA not using a focuser and extensions, but using a disc as close as possible to the primary mirror surface. Then add the focuser and extensions and mount a disc in that to see if all is still aligned and remains aligned when operating the focuser over its whole travelling distance. These steps (aligning the primary and checking the focuser) are checked on a bench with an external camera at the 'sky-end' of the telescope. I found this method to be quite sensitive, especially regarding the alignment of the primary mirror where the slightest adjustment directly shows.

In the end a star-test is required to do the final collimation of the secondary. I used a 12" Newton for that (again in Dutch, but Chrome will deal with that): http://www.starry-night.nl/collimeren-met-een-collimator/

You can use any scope for star-testing your RC6 in that way as long as it has a larger aperture than 6" and a smaller diameter secondary mirror than that of the RC6 (but make sure their optical axis are properly aligned).

HTH,

Nicolàs

 

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8 hours ago, PaulE54 said:

 

2.       Because the gaps between the spider vanes of the RC6 are so narrow, I can’t get my hands in to reach the primary baffle tube or hold the secondary when I loosen the centre screw. This means that to use the “hole in disc” method I have to take the complete secondary end cap off the scope after I have collimated it, thus disturbing the fine adjustments I have just made. David, again if you read this - you mention calibrating RC6s using your method. How did you get round this?

Paul

Hi Paul,

 

Yes, you are right about the RC6. I've collimated one myself and the gaps between the vanes are too small, even for my small hand. I was able to reach in with a couple of fingers and hold the secondary holder sufficiently to unscrew the centre screw, but then discovered that the secondary holder is so large compared with the diameter of the tube that the secondary, resting in the tube, obscured the centre of the primary mirror. So there was nothing for it but to remove the secondary spider support completely by unscrewing the support ring from the tube. This does mean, of course that once the secondary is removed you have to replace the support ring (minus secondary holder) to align the primary mirror. You then have to remove the support ring, once again, to replace the secondary to align it.

I think I did find that I could get a light to shine from the front of the tube onto the card in the centre of the primary mirror, without the shade tube casting a shadow onto the card. If this is the case for you, then you do not need to remove the shade tube to adjust the secondary.

Hope this helps,

David 

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Thanks David, that's very helpful. Photographing the primary collimation (as you did in your PDF doc) is useful, - I found choice of lens to be critical. With my cropped sensor DSLR, my weapon of choice was a 100mm f2.8 macro at 3 feet distance and F25 for sufficient depth of field. Tripod mandatory of course.  I'm waiting for clear skies to do a star test, so I'll report back in a few months...

Paul

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Well I said I'd report back. This is a CCD Inspector plot for my RC6 - I've just collimated with David's method, I've not done anything else yet. 

FWHMs are a bit high, but I think that's just focus being a bit off.  Basically these figures are better than it was before I took it apart to clean the primary. 

I have an artificial star, so I'm going to check mirror spacing tomorrow, and then recollimate if it needs adjustment.  

Many thanks to David (and Nicholas) for your advice, really pleased with progress so far. 

Paul

CCDI.jpg

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Paul, I think this is a cracking result. I think the readings look good. If anything, there is a slight asymmetry from top left to bottom right which could be corrected with the slightest tweak of the secondary. But it is so close I would hesitate to touch it. There is next to no tilt. The curvature is normal for an RC, I think.

Check the star shapes in the corners. They might look slightly soft and oval if you're not using a flattener but should be equally so. If you see more oval star shapes in one corner compared with another then I think that would indicate that the secondary is not quite correct. Central stars should show no coma.

I think you should get some good results with this.

Your FWHM readings are comparable to mine. Remember that the presence of the secondary obstruction is going to spread the light energy from the Airey disc into the first diffraction ring, so stars will look a bit fatter. On the other hand, it might be that the mirrors are not quite the 'correct' distance apart; a Ronchi test would check that. By the way I found that a headtorch with the lens removed made a reasonably small and bright source to do a Ronchi test from 30m.

Good job.

David

 

Edited by davies07
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  • 9 months later...

I am new to RC after buying a second hand Altair truss RC 10" f8 that was in need of attention; read and followed to the best of my abilities David & Nicolas instructions and believe to have achieve a fairly decent collimation using a cheshire, a HG and diffused star to fine tune. However, the RC does not get into sharp focus. The best focus is at 264-265mm, but the drawings say is at 239mm. I also got a GN rochi eyepiece to see if the root cause of the problem are aberrations in the mirrors, but unable to interpret the images. Please refer to the attached images. Many thanks in advance for your help.

Difuse Star image 1c.jpg

Difuse Star image 2c.jpg

Difuse Star image 3c.jpg

Difuse Star image 4c.jpg

Pic11c.jpg

tsrc10-optikset.kl.jpg

Imaging system distances.pdf

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