davies07

Hi Alan, Under the framing tab, Coordinates topic, there is a button to the right of the word Coordinates, which when pressed will transfer the coordinates of the target from your planetarium software (CdC). You can then move down to 'Replace as a Sequence' and tap 'Slew' to get you into the right vicinity. Turn on 'Centre target' on the Sequence page and NINA will then plate solve and re-centre the image before the first exposure. David

I have to agree with KooKoo_gr. I think you'll be pushing it. Even well balanced, you'll have a lot of inertia on the mount and pressure on the quite small worm gears. The motors won't be able to deliver the torque to move the mount smoothly and in the limit will stall and miss steps. Once upon a time, I had my RC8 on an EQ6 with a 100 mm refractor. Most of the time it seems to work fine but then I started losing guide stars and could see that the whole field of view was stepped a small amount to the east in RA. My RA motor had stalled for a step. Removing the refractor fixed the problem.

I agree. I think the scope is essentially correct. Do I detect a bit of tapering top to bottom; this could be a bit of coma. I would check on a slightly out of focus star image. For the record, I adjusted my secondary mirror inwards by around 0.3 mm. Here is the resultant Ronchi image inside focus: So the tendency to barrel distortion (over correction) has gone and the lines look straight. The extrafocal image does not look so good: There is a suggestion of tapering which I interpret as a bit of coma. Note that this image is closer to the focus point but I can't move the focuser any further out. I shall check this when I get on to a star - the sky clouded as soon as I took that last image. For the moment, I'm calling this done. I'll now use the scope and see how the images look. I did manage a single frame of M13 but with guiding errors due to cloud. It did enable me to check the scope focal length in Pixinsight and it came out as 1658 mm which is very long for this design of scope (should be 1624 mm). I suspect I have a rather edge of spec set of mirrors. The coma I'm seeing on the last image could be due to the very extended image train. I'm using all the extension tubes and the focuser is almost at the end of its travel, out, and I'm sure there must be a bit of flexing which might vary with where in the sky I'm pointing. By the way, I've edited my images in Photoshop to bright the white point down to closer to the image data. This has made the images brighter.

Yes, indeed. I've just adjusted my secondary mirror to push the secondary towards the primary by around 0.3 mm. I will retest once the clouds clear and let you see the results. It would be good if the result shows me to be slightly undercorrected this time. At least I would then know that the correct position is between the previous setting and the current one.

Mmm! Thanks, Merlin66. This is using Es's 25 l/mm Ronchi grating with his artificial star. Es says the grating is very sensitive but give low contrast graphs. I thought I'd update you all on progress. I got out at the weekend and manged to get these graphs off my own RC8 scope using the Gerd Neuman 10 l/mm grating. I've used the setup with my Fuji X-T2 camera, 35mm lens and a 43mm - 52mm step-up ring as shown in the post above. I also added a 50mm extension tube to bring the Ronchi grating to the focal plane of the telescope. I started by slewing to a bright star and centring it with my imaging camera. I then replaced the imaging camera with the Ronchi grating but without the camera. Looking into the grating by eye, I could see the lines and was able to adjust the focus to bring the Ronchi grating to focus. I then added the Fuji camera and lens and took this shot: What you can see here is one grating bar across the centre and we can see some of the imperfections in the mirror surface. Racking the focuser in towards the telescope, the grating lines appear. You get more bars the further from the focal plane the grating moves. So this is an 'intrafocal' graph. And a bit further in, we get: The second picture of each pair is taken with the camera at 90 deg to to the other. I think the lines are almost straight and evenly spaced indicating the scope is operating close to its correct focal length and is more or less collimated. There is a slight suspicion of barrel distortion on the first pair of graphs which would indicate slight over-correction - mirrors too far apart. Racking the focuser to put the grating outside of the focal plane, I get this view: This is the 'extrafocal' view. Now there is a slight indication of pincushion effect, again indicating slight over-correction. So I deem my scope to be nearly there. I'll try to get it to a fully corrected state by unscrewing the secondary mirror centre screw out by, say a quarter turn. That should move the focus out by around 5 mm and I'll test it again. Just a reminder, I've used a plastic disk with a hole in it to collimate the scope as I've described earlier in this thread. I've improved the disk by adding a black cross, intersecting at the central hole. This makes it much easier to see where the reflection of the hole is when you adjust the secondary mirror. All I have done here is to adjust the position of the secondary, recollimate the secondary using the plastic disc and then retest; I haven't touched the primary.

I've made some progress and have got my Gerd Neuman grating working on Es Reid's artificial star test bench. This is my camera setup: One thing to be aware of is that the Ronchi grating needs to be close to the focal plane of your telescope. This is the normal location of the sensor on your imaging camera so the Ronchi test camera has to move further back and you will probably need to use an extension tube. To sight the telscope on your test star you can remove the camera and look directly into the grating, rack the focuser in and out with the grating close to the focal plane. The light from the star will fill the aperture of the scope and you should see the Ronchi lines across the aperture. They are quite low contrast so you need care to spot them. Here is an image from a refractor under test at Es's. I now need a clear night to have another go on my RC.

I’ve been chatting with FLO and we’re all confused. Checking on Gerd Neuman’s website, the photographic version is designed to be use d with a DSLR camera complete with a lens on it. Thus the 52mm male thread. That is designed to fit into a 52mm filter holder on the front of the DSLR lens. If you have a lens with a different diameter you can buy a step up or down ring for a few pounds. But here’s the thing. How does it then work? You’ll have a telescope terminated with a T2 male thread that screws into the Ronchi grating and then have a DSLR plus lens attached to that. How do you focus it? FLO are going to contact Gerd for clarification. So don’t buy anything yet D

I think you can get away with just one adapter if you turn the grating around.

Mine arrive today and I concur, the screw threads are really odd. I assume that the 52mm male thread is designed to fit into a 'standard' 52mm female thread on a DSLR camera as if the grating was a filter. That would leave the T2 female filter presented to the telescope side as if it was a astronomy camera such as a ZWO. However, I have no DSLR camera with such a filter size. That leaves the problem of presenting a T2 male thread to the camera side. This is such an adapter from TS Optics https://www.teleskop-express.de/shop/product_info.php/info/p11819_TS-Optics-Adapter-from-SP52-M52x0-75-filter-thread-to-T2-photo-thread.html FLO also have such an adapter but it is for a Tak and costs over twice as much. With the above adapter, the grating will have T2 female on the telescope side and T2 male on the camera side. Now why couldn't it have been made like that!! David

Hi Tony, This is interesting. I confess, I have not looked closely at an RC6. I've been looking throught the photos I took of my RC8 rebuild and here is a picture of my secondary mirror holder, taken in August 2017. The arrangement looks similar to your RC6 but, of course, the cup holding the secondary mirror is bigger on the RC8. What is also evident is that the movable plate and locking ring appear to be the same on the RC6 and RC8. I'm wondering if the original purpose of the twopart construction is to enable the RC6 and RC8 to be made from common parts, or maybe to enable the secondary mirror to be removed without disturbing the secondary collimation. I'd be cautious about unscrrewing the locking screw itself. I wonder how much thread there is? Of other possible interest here is the open page of my notebook with the note that the centre screw needed 9.5 turns to release the centre screw, so there is a lot of thread length on the centre screw to play with. That 9.5 turns gave a focal length of 1607mm, somewhat short. David

Hi Konstantinos, Eccentricity is a measure of the how oval a circular shape is. It is defined as =SQRT(1 - y^2), where y is the minor diameter of the oval and the x diameter is assumed to be unity. Your eccentricity readings are maximum at the tops of the image (~0.6) and indicate that at the top of the image, the minor axis of your stars is around 80% of the major axis. I downloaded your images and detect that the stars are wider in the horizontal direction than the vertical. Stars are at their roundest at the bottom of the images where the value of e falls to around 0.4 to 0.5, indicating the minor axes here are 85% to 90% of the major axis. In a collimated scope I would expect to see the roundest stars (minimum vales of e) in the centre and would hope to achieve e values around 0.3 (minor axis of the star better than 95% of the major axis). I cant make much comment on the FWHM (full width half maximum) measure of star diameter other than I would hope to see the sharpest stars (smallest FWHM) in the centre. Your smallest stars seem to be off to the right. In absence of a Ronchi eyepiece - I'm also still waiting for mine - I would unscrew the secondary centre screw one turn anti-clockwise. This would push the secondary mirror in towards the primary by around 1mm. This in turn should change the focal length to around 1625mm - at least close to the advertised value. Then retighten each of the three collimation screws by around 3/4 turn each, to just snug to the secondary and then recollimate. I would also put the Howie Glatter laser aside and make yourself a card disc with a hole in it as I've described, above, and work through my instructions. I'm willing to bet a glass of ouzo that you will achieve a better collimation result. A couple of weeks ago I put my HG laser on my own scope and it said the scope was out of collimation! I didn't touch anything because I know that it is, in fact, collimated. Best wishes,

I've just ordered the photographic one. We can compare notes later.

I confess, I haven't used the Ronchi eyepiece myself but have worked with someone else who has. I've used Es Reid's Ronchi grating with an optical eyepiece but that was on his optical bench. Yes, there are two versions of the eyepiece, one for a camera and the other optical. Here is the optical one that FLO sell: https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html They also have the photographic one: https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-photographic-eyepiece.html The note on the FLO site says, of the photographic Ronchi: The Photographic Ronchi has the same high-quality grating inside, which is made of evaporated chrome on polished glass. (254 Lines per inch or 10 Lines per millimeter). Contrary to the visual version it has no 1.25" barrel but a T-thread on one side an M52 on the other. With the T-thread you may mount the photographic Ronchi on nearly all instruments. On the other side of the Ronchi you may install your camera (with a normal lens installed!!) at the M52 thread. (You might need a matching Step-Ring for your lens.) You can download the manual here: https://www.teleskop-express.de/shop/Bilder/shop/ts-okulare/ronchi-manual.pdf I imagine the difficulties of using it on the sky are that the quality of the lines you see will depend on the quality of your focus and the seeing. If you need to adjust the secondary to change the focal length, then your scope will be out of collimation for the rest of the exercise. I found using Es's kit that by rocking focus back and fore you could see the pinched and barrel shapes well. At the point of correct focal length, the zooming in and out of straight parallel lines, indicating a correct focal length, was clear.

thank you very much for your reply (i also posted this question on CN so you don't have to answer me there) i think i'll look into a Ronchi eyeiece. Should i go for the Ronchi Okular Photographisch 10L/mm? https://www.gerdneumann.net/english/ronchi-okular-ronchi-eyepiece/photographic-ronchi.html Yes, that is the one my friend bought. It seems to work fine.

Hi, Should you align the focuser? Good question. Your focuser has a facility for alignment, so I think you should check where it is pointing. Assuming that you have aligned the primary and secondary mirrors using my 'disc with a hole' method, then your primary and secondary mirrors should be aligned. I would now fit the extension tubes and focuser and fit a laser to enable you to adjust the focuser to point at the centre of the secondary mirror. Don't touch the primary or secondary collimation screws. However, before that I think you should look at your focal length. Your focal length of 1607mm is close to what mine was (1604mm) when I first started my investigations. At that focal length my scope was very 'over-corrected' - the mirrors were too far apart. Es Reid checked my scope with a Ronchi grating but over the past month I've been helping someone collimate his scope including checking the focal length. You'll need a Ronchi eyepiece; Gerd Neumann makes one. Focus on a bright star and rack the focus in and out; more and more lines appear as you move away from focus. My guess is that as you go inside focus, you will see this pattern: A barrel-shaped Ronchi pattern inside focus = overcorrected scope, too short focal length, mirrors too far apart. Outside focus, I think you will see this: a pinched pattern outside focus, again overcorrected. As a first guess, let us assume your scope will be properly corrected at the specified focal length of 1624mm. So you are 17mm too short. Look at the chart I gave plotting focal length vs. number of turns of the centre screw of the secondary (page 1 of this thread), you'll see that about 3/4 of a turn of the centre screw should bring you to 1624mm. So unscrew each of the three secondary collimation screws one turn, and then unscrew the centre screw 3/4 turn. This pushes the secondary mirror towards the primary by about 3/4mm. Now tighten the three collimation screws by an equal amount and check on the star again with the Ronchi eyepiece. The lines should be parallel inside and outside focus. If so, your scope is working fully corrected. If you do not get parallel lines, you'll need to keep adjusting. The Gerd Neumann eyepiece come with a manual which you can download from the Teleskop Service website. Note that if the patterns are the other way around - pinched pattern inside focus and barrel pattern outside - you've gone too far and the scope is now under-corrected and the mirrors are too close together. (I had to move my secondary by four full turns of the centre screw and adjust the primary mirror up the tube until I got a corrected image. My corrected focal length is 1660mm. I'm not far off not being able to focus without another extension tube.) I use Pixinsight to tell me the focal length of the scope. Take a single image of a star field, say an open cluster, and use Script/Image analysis/Image solver. Once you are at the correct focal length you will need to recollimated the scope, hopefully by only adjusting the secondary. David

Hi Cedric I owned a Quattro for about four years and the first thing to go was the focuser; I replaced it with a Moonlite. From memory, almost the first thing I noticed was the poor lateral support. The drawtube could move sideways in the focuser so I imagined that a heavy camera could shift as the scope tracked. I was fortunate in already having a Moonlite focuser from an earlier scope that I could transfer. The Quattro is a bit of a beast to handle but produced lovely images once properly collimated. Be warned, collimation of a Quattro is not trivial and I achieved accetable colliamtion only after purchasing a Catseye Collimation kit. I think their 2" sight tube is unique and the autocollimator is the is the only device I've used that enabled correct orientatrion of the secondary mirror. Also, do consider the site you are imaging from; the Quattro is a sail in anything more than a puff of wind, especaily with a dew shield fitted. David

I was going to follow up on the topic of collimation screws - thus the photos, above. By the way, if anyone is wondering, I record everything I do with photos and notes. That is why I have photos of what I discovered when I dismantled the scope - and I've now done that several times. Yes, it was my misnomer to call the screws push-pull. the 3mm silver-headed screw is the collimation screw. It is tensioned with a spring - see the first picture in the post above. The white circular pad is the impact surface for the 2.5mm, black locking screw. Note that the collimation screws bear the weight of your focuser and imaging train. So I keep my collimation screws well screwed in to minimise flex. You can see the collimation screw protruding through the backplate in the third picture. The second picture shows the standard lock screw. This is a cup-ended screw that seems to be a standard fit for all Chinese orgin adjustment screws I've come across. They are wonderful for digging into and marring the metal (usually aluminium) that they bear on to. I've replaced mine with dome ended screws. The effect is to eliminate the indeterminate stop that you feel when tightening the lock screw. That softness is the screw digging into the metalwork.

Sorry, guys, I made a complete cods of that last post. I drafted a post last night to add a bit more info and lost all the text due to finger trouble but the images have remained and attached themselves to my next post - the one you see now.

I think this is an excellent result. Well done. Not only have you now centred the star but. looking at your earlier images, the contrast in the nebula has improved. David

Hi Luke, I was thinking, today, that your topic has been hijacked somewhat into a general discussion of RC scopes. I hope you don't mind that too much. We are not, after all, addressing your original question. I agree with Andy, once you get the RC collimated properly, it will capture lovely images and the collimation should be held well without further adjustment. Coming back to your original laser questions: I think that if you put a Howie Glatter laser on a well collimated GSO scope of the type we're talkng about, the likelyhood is that the laser will say the scope is out of collimation, due to the (unknown) pointing error of the focuser relative to the primary mirror. I moved to the 8-inch RC from a 10-inch Skywatcher Quattro and whilst the Quattro was super fast and I got lovely images from it, it was a tricky one to collimate and because of general flexure of the beast, it wouldn't hold collimation to my satisfaction. And I hated the messy star halo due to the various defraction elements in the light path. I think the RC gives me star colours which are pretty close to truth. They seem cooler than from my refractor (APM 107 Apo) but closer to truth, I think. The symetrical optical path gives lovely round stars once the collimation is sorted. So I suggest you have a read and see what you can do to improve your scope. Best, David

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

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

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: This is what I saw: 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: 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. 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. 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: 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. 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. 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. 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: 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

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: 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: 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 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: 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: 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: 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