davies07

See my post on the Catseye collimator here If it's of any help, I've also spent hours playing with the Ocal collimator (not on a Newtonian) but with little success. The key difficulty for me was in establishing a reference pointing direction (offset) that gave a collimated telescope. David

I would say that I never got my 10” F/4 Newtonian properly collimated until I bought a Catseye collimator. I bought the two-port auto-collimator and the 2” sight tube with a set of radiation marker centre spots. You would need to replace the centre spot on the primary with a radiation marker, but the effort was well worth it. The advantage of the Autocollimator is that it enables what I would call the ‘roll’ axis, or twist, of the secondary to be set correctly, looking from the focuser. I believe no other system (laser, mirror outline shapes through a camera or sight tube) has the accuracy and sensitivity of the Catseye system. It was expensive, but I recommend it. David

https://www.dropbox.com/s/dbhncjtzfursg6b/Collimating GSO Ritchey with a plastic disc V2.pdf?dl=0 Try this link for the document on collimation. https://www.dropbox.com/s/i68gibodxszoga6/Testing with Ronchi eyepiece.pdf?dl=0 and this one for the document on checking the focal length. I will PM you Es’s details. D

Hi, You are not alone to be struggling with an RC8. There are many threads concerning collimating and adjusting the focal length of these scopes. They can be tricky to get right but produce really nice images when correct. From the information you give I would think the collimation is correct but the mirrors are too close together and the scope is undercorrected (I think that’s the right description). I owned one of these for for five years and it gave excellent service once adjusted. Interestingly, the focal length of mine was 1660mm. At this focal length a Ronchi test showed no spherical aberration and the scope was ‘correct’. So your correct focal length might not be the quoted 1624mm. You’ll need to do a Ronchi test to find the correct mirror spacing. Have a look at posts in this thread https://stargazerslounge.com/messenger/331416/?tab=comments#comment-730724 I’ve posted some links to documents I’ve stored on Dropbox. You should be able to download these and read up on collimation and doing a Ronchi test. Different people have their own favourite approach to doing collimation of these scopes but I think the hole in card technique works. I’ve moved up to a 10” RC but am still using the hole in card method even though the mechanical design of the 10” is better than the 8”. The hole in card method was developed by Es Reid by the way. I’m away from home at present so will not be able to give quick responses to questions until the weekend. Your scope can be checked and corrected. If you don’t think you could do it yourself, you could always send it to Es to check (Cambridge). David

Hi Adam, See my response to this post

Hi, I'm very late to this discussion (as usual), but I thought to add my two cents worth. I've used a Canon 450D DSLR for a couple of years, and it produced some nice pictures but only after a lot of data gathering on an EQ mount. Uncooled DSLRs are noisy and require at least 55mm of back focus. I could not get mine to focus on a standard SW 200P scope (many years ago) and ended up replacing the focuser with a shallower version which enabled me to focus the camera but was horrible to use in other aspects. I would wholeheartedly recommend a modern CMOS camera and a cooled one if you can afford it. The ASI 662 mentioned at the beginning of this thread would be a good choice, I think. The pixel count matches the resolution of modern monitors, making the camera ideal for live viewing (EAA). The only issue is that the pixels are rather small at 2.9um. For deep sky viewing with a scope around 750mm to 1000mm focal length, I would go for a camera with pixels in the 4um - 5um range. The ASI 482, for example, is 1920 x 1080 pixels at 5.8um and would be ideal for deep sky EAA. One of the advantages of a purpose-built astronomy camera is that the sensor is at the front of the device, giving a much shorter back focus requirement. A modern CMOS camera has a variable gain enabling you to turn up the sensitivity to view fainter objects at the expense of saturating the brighter stars, losing their colour. I don't understand the need to go for cameras with large sensors and high pixel counts unless you want to explore wide field-of-view images such as large nebulae. Most images are viewed on monitors, and most cameras will produce images that match their size and are suitable for exploring the vast majority of deep sky objects, which are rather small. The important aspect is getting the right image scale of around one arc sec per pixel, essentially the field of view of a single pixel. I would recommend Sharpcap as your software. It works very well with modern cameras, and its live stacking feature is remarkable. Even on an Alt-Az mount, it is possible to take a sequence of very short exposures and let Sharpcap align and stack them. After a few minutes, the object will magically appear on the screen, with the background noise correspondingly reducing. Good luck. David

Pretty much. Once you have cut the new hole, measure the distances from the top edge of the tube to the top of the hole and the bottom of the hole. The average of these two values gives the distance of the centre from the top edge of the tube. Stick some sticky-backed paper around the top edge of the tube. Use a set square to find where the centre line through the hole intercepts the top edge and mark it. Now cut a band of paper with a length equal to the tube's circumference and fold it in half. Mark the half way point. Now lay the band of paper around the top edge of the tube with the join aligned with the centre line of the hole and mark on the sticky-backed paper where the half way mark comes. Stick a piece of card to the inside of the tube opposite the hole. Mark a line parallel to the top of the tube at a distance equal to the distance of the hole centre from the top. Transfer the position of the halfway point to the card and draw a line down the tube using a set square (small carpenter's square). 🙂 P.S I record almost everything I do with photos. They sometimes come in useful for talks.

I had a 200P on an HEQ5, and it worked fine. Later, I did upgrade the HEQ5 to an EQ6, but I wouldn't say it was essential. EQ6 mounts do come up on Astro Buy Sell, so I would have a look there. I would go for a 200PDS if you are contemplating imaging. The 200P is primarily a visual scope, and the focus point is fairly close to the tube, and the secondary mirror is small, which is all that is needed for visual. The close focus point means that a camera, such as a DSLR, with a deep body, might not be able to reach focus. Modern CMOS cameras with the sensor close to the front face of the camera might work on a 200P but the smaller secondary could give significant vignetting. The 200PDS has a focus point further out from the tube giving space to instal a camera plus filters if needed. The larger secondary mirror gives a wider field of view. David

I'm a bit late to this conversation, but I would add a few words of encouragement. I updated a 10-inch OOUK Newtonian several years ago and replaced the focuser and the secondary holder. Changing the focuser required the drilling of a new fitting hole plus screw holes. As you can see in the photo, I used a Dremel drill and cutting wheel to cut the new mounting hole. The ragged pieces of metal were the remains of the previous owner's attempt to cut a hole for a focuser. Note the use of a marked-up card to provide a template. Here is the new focuser in place. It is a Skywatcher focuser which was surplus to requirements. To get good collimation, you should align the new focuser with the tube. I did this by determining the point on the inside of the tube exactly opposite the centre of the new focuser hole. This point was marked on a card fixed to the inside of the tube, enabling the focuser pointing to be adjusted using a laser. I then flocked the tube with Protostar flocking. The original OO secondary holder was a simple device which gave ugly diffraction spikes. I replaced this with a set of curved spider vanes, which resulted in a telescope giving some beautiful visual views. By the way, I would not recommend curved spider vanes if you plan to use the scope for deep sky imaging, but they are great for visual and planetary. David

FLO should be able to supply a bracket. Just drop them an email.

I agree. Newtonians are good value for money per aperture but need a lot of maintenance, and you will be constantly checking collimation. The secondary mirror obstruction diminishes contrast, and I never liked the diffraction spikes - having used a 200P and a 10" Quattro. A good refractor is a lot less trouble, and you can concentrate on enjoying using it rather than fixing it.

If you're thinking of doing astrophotography, the 200P is not the right scope, it should be the 200PDS. I used a 200P for astrophotography on an HEQ5 mount but eventually upgraded the mount to an HEQ6. I couldn't reach focus with a camera until I replaced the stock focuser with a shallow one.

My understanding is that the adjustment of the primary mirror controls the amount of coma in the centre of the field of view - when properly adjusted you should get no coma in the centre. The secondary mirror controls the distribution of the characteristics in the centre across the rest of the field of view. So a correct secondary mirror will produce symmetrical star characteristics into the corners of the field of view. If you see elongated stars in just one corner, for example, it indicates that the secondary needs adjusting to make all of the corners look the same. The primary mirror has a strong effect on the collimation, the secondary mirror less so. So getting the primary correct is pretty crucial. I guess that is why some advise not touching the primary because messing up the primary can do lots of damage to the image. The advantage of the RC is that, when correctly adjusted, it can eliminate the three key optical aberrations: coma, astigmatism and spherical aberration (SA). The RC doesn't have a strong lens so there is no chromatic aberration (a flattener is a weak lens). Spherical aberration is controlled by the distance apart of the mirrors and you would use a Ronchi test to check it. Astigmatism looks like oval shaped stars, either radial or tangential. You might see astigmatism if you go too far off the centre of the field of view where the mirrors can no longer operate correctly. So oval stars at the edge of the field is astigmatism. Using a very large sensor might lead to seeing astigmatism at the edges. Hope this helps.

I think what you've done looks good to me. Your final tests should should be to test for coma on a centrally placed star. Zoom in on a central star at focus and then slightly defocus it moving the focuser out. Turn off any auto-stretching and defocus only enough to reveal a tiny doughnut. Study the distribution of the light around the central hole. It should be symmetrical but poor seeing makes it tricky to assess. If the annulus of light shows a soft edge on one side and hard edge on the opposite side, you have some residual coma. Also, look for the Young Spot (a tiny dot of light) in the centre of the dark area. It should be in the centre. Make any corrections to the primary, only. You could use Metaguide, for example, for checking the collimation on a central star. I've found it found it to work well. Take a test image on a star field and use Pixinsight to measure the FWHM and eccentricity of the stars (script/Image analysis/FWHMeccentricity). Click the support button to see the graphs. They should show the smallest and most circular stars symmetrical about the centre of the field.

Hi, Yes, I would agree. Your scope is out of collimation. The asymmetric halos around the stars are a dead give away. That is coma. Unfortunately, RC scopes need care to collimate well and the RC8 is particularly tricky to collimate because of its mechanical design. In a normal RC scope, eg the RC10 truss, the focuser is bolted to the back plate of the scope and forms a fixed positional reference for collimating the mirrors, but in the RC8 scope the focuser is attached to the back of the primary mirror. When you move the primary mirror you also move the focuser. So normal collimation methods might not work so well. You can end up in an endless loop of adjustments which can be time consuming and very frustrating. Imagine being out under a lovely sky, collimating your scope and anything you do seems to make matters worse. I've got the T-shirt for that one. I got Es Reid's help to sort mine out. What we developed was a method to collimate the scope on the bench during daylight. Then do a final check on the stars. Es's method does not involve using the focuser at all - no lasers, no Cheshire eyepiece. Just a piece of card with a hole in it. This thread: tells the story of how this method unfolded and why. The end result was a successful technique which I've written up here: https://www.dropbox.com/s/avpu2vn6s3ynsz5/Collimating GSO Ritchey Chretien with a plastic disc V2.pdf?dl=0 Alternately, I see that you are a couple of hours away from Cambridge and an option would be to contact Es and take your scope to him for collimation and tuning. This would have the additional benefit of getting Es to do a Ronchi test to check focal length and a knife edge test for the quality of the mirror. Es has recently subjected my new RC10 to such tests. Well worth it. You could PM me to discuss more. David