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Hi, I recently had to disassemble my scope to re-glue the flocking material it came with. After removing the secondary and re-attaching it I was stumped. Coming from a f5 system the offset of an f4.5 seemed enough to test my patience. I can not get it collimated with even field illumination. I've tried googling for a guide but all the guides I find say to center the edges of the secondary to the focuser (like the image below), but this from my understanding isn't "fully offset" collimation and leads to illumination issues.  My questions are: 1. How do I go about finding the right offset using either a laser, cheshire or OCAL? 2. Is it even possible to get collimated and even field illumination on a "fast" system? 3. Ponder the picture above, should the edge of the secondary be moved inwards towards the tube like the image below?  4. I am at an absolute loss finding the right position for my secondary and don't know which tools I should use to get it collimated and evenly illuminated. 5. Using an OCAL, how should the image look (and circle line up) using software like Mire De Collimation? P.S Scope is a TS ONTC 8" f4.5 Please help me out! Much obliged, R8RO |
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| In an offset-collimated newt the appearance of the secondary as seen from the focus position via a sightube is of perfect concentricity between the edge of the sightube and the edge of the secondary mirror. It is best to remove the primary to avoid beeing fooled by reflections when doing this. Note that in the offset method the primary moves away from the focuser as well as toward the primary. Note also thatyou should concentrate JUST of the alignment of the edge of the secondary mirror to the edge of the sight tube and that the sight tube should have about the same "focal ratio" of the scope you're aligning (i.e., the ratio of length to diameter should be around 4.5). |
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| Note that I would not be too fussed about having the secondary simply centered onto the primary as the fall in illumination is rather marginal if the secondary is properly sized. |
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andrea tasselli: Thanks for the explanation Andrea, After achieving perfect symmetry with the secondary in the middle of the drawtube, my primary is very off center, moving it back to center, requires moving the secondary, therefore, eliminating the symmetry I just achieved. What should I aim for? Do I at this point switch to my laser collimator?   |
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This is as good as I can get it using the OCAL: After getting the above results using the OCAL my laser now says everything is off by quite a bit. What instrument should I trust?   |
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7.46
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First one question: Is the primary centered in the telescope tube? If it isn't or you don't know check because the assuption of an offet method is that the primary is centered in the tube. Furthermore, did you offset the secondary away from the focuser? Secondly, I don't know the first thing about this Ocal. I've only used sight tubes and used to those. Thirdly, laser should only used to center the secndary on the primary and nothing else. Proper collimation can be only achieved in auto-collimation. |
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andrea tasselli: The primary is centered as good as I can get it. I thought centering the edge of the secondary to the edge of the focuser was "offsetting" it according to your answer above. When you say away from the focuser what do you mean? |
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Dosn't really get dark here anymore but these are some test shots: After collimating with OCAL(sight tube) stars have tails and are very wonky 1s exposure:  After correcting with laser 1s exposure looking much better:  After correcting with laser 0.01s exposure (close-up on bright star, non stretched):  Skewed field illumination 15s (right side is way worse feels like I am outside of the imaging circle):  |
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The primary is centered as good as I can get it. I thought centering the edge of the secondary to the edge of the focuser was "offsetting" it according to your answer above. When you say away from the focuser what do you mean? It means the secondary needs to move away from the focuser, that is toward the main tube, usually by shortening one spider (or 2) and lengthening the other(s). There are websites that allows you to calculate the amount of offset both away from the focuser and toward the mirror. I might endeavour to write down the equations but maybe that way is faster. |
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1.51
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andrea tasselli:The primary is centered as good as I can get it. I thought centering the edge of the secondary to the edge of the focuser was "offsetting" it according to your answer above. When you say away from the focuser what do you mean? Now I am just getting more confused, so this: andrea tasselli: Isn't true? I believe my scope has a built in offset. The spider is in the same position as it were when it came from the factory. The edge of the secondary should be skewed "down" towards the primary? By how much, is there a visual I can use to measure this? Are there any resources that show what this offset looks like? Is this picture false then?  |
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The offset method relies on mechanical settings to achieve its purpose, which is to have even illumination and a centered system, with the primary coaxial to the tube. It is to be stressed that setting the right offset precedes the collimation part. You need to make the mechanical adjustments so that the secondary is in its right position before actually collimating the telescope. The picture you posted above this message is what a centered but not offset system would look like. It would still deliver even field illumination but at the loss the coaxiality between the telescope tube and optical axis of the same. Frankly, the offset is too much faff to bother about and I'd go stright to this method instead. |
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Dosn't really get dark here anymore but these are some test shots: vignette and lack of third clip present in the star indicates collimation shifted left. Use defocused star and a circle reticle. You'll have her lined up pretty quickly. |
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Hello! However, if the telescope only needs to be reassembled, it is safe to assume that the offset will already be set correctly when the secondary mirror is reinstalled, even if it was set beforehand, right? Best regards Markus |
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Yes, this scope comes with a built-in offset. I guess you are using the original secondary? I also have the 8" f/4.5 ONTC and my secondary has a small ring which when centered indicates the correct secondary position including offset. Look at page 2 in these notes: https://www.teleskop-express.de/shop/Bilder/shop/ts-ontc-unc-newton/unc_ontc_zusammenbau_kolli_en_v20150427.pdf |
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| Note also that an economy laser collimator + single clamp receptable is not the most precise arrangement, which you can see for yourself by rotating the laser or releasing the clamp tension. Unless you pinch the primary you will also have slight changes when moving the scope from the horizontal orientation to a realistic working position. I only use such an arrangement for approximate alignment when the scope has been disassembled. Then it is practical as one can see the laser dot move towards the intended position when turning the primary adjustment knobs. |
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What focuser are you by the way using? Mine came with this unit, which I changed to a Steeltrack. https://www.teleskop-express.de/shop/product_info.php/info/p974_TS-Optics-2--Low-Profile-Rack-And-Pinion-Newtonian-Focuser---8-kg-Payload.html The drawtube extension mechanism has an additional clamping arrangement which can also tilt your laser beam I guess. |
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| Trust the OCAL, I had nothing but trouble getting perfect collimation until I got the OCAL. I have a couple of lasers, even the Holtech can't really be trusted. The only source of truth is visually confirming everything is centered. It's normal for your laser to show you're off center, it's why lasers aren't the best tools. They too must be perfectly centered and collimated. |
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Also, with the OCAL, you don't have to do anything special for the offset due to the focal ratio. Just line everything up like normal, you'll see there is an offset once you're done, but it will fall into place once it's centered. Now for the OCAL itself, make sure to enter your serial number to get the offset for your idividual camera. They have an excel spreadsheet with all that info posted. Without that, you'll be pretty far off, it wasn't a small change when I entered their offset based on the serial. After that, this thing is worth its weight in gold. It's the only solution that actually works so far. |
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This is as good as I can get it using the OCAL: you wouldn‘t believe how mich of the exact same problems I am having after a disassembly - so very curious for answers inthis thread what I have learne so far - centering secondary in focus tube is essential - primary collimation can be done with laser, but only here you can laser collimate - my laser collimator was badly uncollimated, ie rolling it lead to a rotating laser dot on the wall where I struggle at the moment that I cannot get the primary fairly centered in the secondary - leading to asymmetrical star images when checking out of focus help and advice greatly appreciated! |
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Arny:This is as good as I can get it using the OCAL: If you're using compression fittings like the majority of us, you may notice that your field shifts depending on how you tighten the fitting. This is something I don't believe there is any way to avoid. Those same things happen with the lasers when they're inserted. You will need a collimated laser regardless, so if you can't get it fixed, you'll need something else. I have a farpoint. The trick I use is I keep the laser loose in the compression fitting. I then 'roll it around' in the fitting to draw a circle with the laser on the mirror for aligning my secondary to the primary. Now of course we don't want a circle, but you can use your imagination to figure out where the center of that circle is. It's a simple as that. You can use this trick as well for aligning your primary to the focuser, but it gets a little more hectic, since the distance traveled has now doubled and the circle is much bigger. I have disassembled my scope twice now and used this trick and have been dead on both times, so I know it works. The same things must be considered when you put your imaging train back in as well. Do not overtighten the compression fitting. I know we all want to because our expensive equipment is hanging from it, but it actually hurts us in the end. Hope it helps. |
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What focuser are you by the way using? Mine came with this unit, which I changed to a Steeltrack. I use a TS RAP2DEL, did some more collimation based on the defocused star method, didn't have more time to test though and been clouded out since. Looks like tomorrow is my next opportunity. Will report back. Arny: At this point I almost feel like getting the secondary right using my OCAL is the best way, laser is a bit hit or miss since it may not be collimated itself. Will do some more tests when I have the chance. |
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What focuser are you by the way using? Mine came with this unit, which I changed to a Steeltrack. You nailed it! The secondary is impossible, in my opinion to eyeball. All the cutesy collimation caps and lasers just don't get us over the finish line. Once you can trust the secondary is centered, just aim the laser at the center of the primary, from there you'll be golden. I've done star tests after following this method. |
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One of the more significant points of confusion for me was in doing a defocused star test and expecting the shadow of the secondary mirror to be concentrically centered in the pattern when it should be offset to one side, reflecting the offset of the secondary mirror. I don't know why it took me so long to realize that. Also, if you don't have an OCAL, you can accomplish the same using a mini guide camera with a CS lens pointed down the focuser along with the new reticule utility in ShapCap. There's a good writeup on the process of collimating a Newtonian using the new reticule system: https://forums.sharpcap.co.uk/viewtopic.php?t=5610 |
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I managed to solve the issue by going on "gut feeling" over literature. The secondary should *NOT* be centered in the focuser as my images from the OCAL show above. I went out and moved the secondary *OUT* of the tube towards the aperture by tightening the middle bolt by about 0.3-0.5cm. This solved my illumination issue and gave me perfect collimation after hitting it with the laser again. @Arny might be helpful for you too. |
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There are three items I use for collimating my 10" F4 Newt with great success. A concenter eyepiece (normally only required when installing the secondary)(must easier to use than a sight tube), catseye cheshire (for collimating the primary) and catseye autocollimator (for collimating the secondary). Starting with the concenter eyepiece. This alone can achieves a "rough" collimation by centering the focuser with the secondary and the secondary to the primary. It's an iterative process of firstly adjusting the up-down then rotational position of the secondary until centered with the focuser (As seen through the concenter). At this point, it's normal for the primary to be non-centered with the secondary as seen through the concenter or a cheshire. From here, I'll then iteratively cycle between collimating the primary (using the catseye cheshire) and secondary (using the autocolimator) until honed in (can also be done with the concenter but I find it easier using the cheshire and autocollimator). At this point, insert the concenter and access how centered the primary is to the secondary. For me, it's usually off center in the left-right direction. To fix this, it's a matter of slightly rotating the secondary (requires loosening the collimation screws)(placing a metal washer between the central stub and the collimation screws helps a lot here), then re-collimating the secondary (concenter first if way off then autocollimator to fine tune). If the primary is better centered then you know you rotated the secondary in the right direction. Once the secondary is properly positions, collimation becomes quick and trivial with the cheshire and autocollimator. As for secondary offset, this is something that must be mechanically fixed in place before installing the secondary. For my 10" F4, when I bought a new secondary (larger one than the undersized stock secondary), I placed the central mirror support stub 7mm off-center along the major axis of the secondary (about what's needed for 90mm secondary at F4) before installing into the OTA. Secondary offset is not something that needs to be done super accurately as it's not required to achieve collimation and has no optical consequences. However, as I've discovered from experience, not using an offset means that, when collimated, the optical axis is not co-linear with the mechanical axis of the tube. One consequence of this is that that the OTA may obstruct the light path but, more importantly to me, you'll experience appreciable cone error, which causes a field rotation to occur after a meridian flip that results in star diffraction spikes not lining up when registering images so you end up with X-shaped diffraction spikes in the final image. Hope this helps! If you lived in Perth, Western Australia, I'd say bring your scope over to my place and I'll get you sorted. Done that for a number of astro friends who were struggling with setting up their newts. |
