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If anyone has the 8" RC from TS I would like to hear about your thoughts on focal length and backfocus on this particular version of the TS / GSO 8" RC. It's stated in the specs for the TS 8" RC, which I bought from TS a copuple of months ago, that the scope has a focal length of 1624 mm and 250 mm of backfocus. Doing af plate solve with my scope I got a focal length of only 1618 mm. Backfocus was arround 257 mm from the backplate. Comparing that to the specs made it clear that something was not right. After reading a bit on the internet I could see that there are different specs on this RC from different vendors of the same scope. Some state only 1600 mm. I then concluded that 1624 mm had to the right focal length. After all this is what TS write in their specs, right? I then adjusted the secondary almost 1 mm closer to the primary, which with a plate solve gave me a focal length of 1625 mm, which is as close as I think I gan get to the 1624 mm specs of the scope. But the strange thing is that I then have 271 mm of backfocus, which is 21 mm more than the specs state. What this makes me think is that the TS specs (FL: 1624 mm FL / BF: 250 mm) might not be right. Can anyone shed some light on this? Cheers Steen |
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Hi Steen, I am responding to your question, just to follow it. But, at the same time... is not it quite normal for an actual "platesolved" focal length to be slightly different from the official specs? Amateur equipment probably has some tolerances and the mesurements are not critically precise. Then, as you move the sensor back and forth to adjust the focus, you change focal lenght of the system is changing respectively. I think it is called the "focus breathe" in photography. I get the same with my Red Cat 51 (250 mm FL stated, 247 registered) and even my Takahashi Epsilon (528 instead of 530 mm). |
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David Nozadze: Hi David And thanks for your time. I know a lot of instruments are not sensitive to the exact spacing between mirrors/lenses as fex. plain Newtons and others where two mirrors are not required spaced very precisely to get optimum performance. Unfortunately it's the opposite with an RC in regard to spherical aberrations. But after reading "Star testing Astronomical Telescopes" by Harold Richard Suiter I have become aware of the importance of this particular problem. Also on the various forums I see others asking the same question. I have now asked TS to get their published specs verified with GSO, hope this brings new information. |
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David Nozadze: That is not exactly right, isn't it? "Focus breathe" occurs when the focal length changes with the position of the object being in focus and is a rather annoying effect essentially found in lens as they change inner lens position to adjust for different object distances. Telescopes, OTOH, are meant to focus at infinity and stay that way so they do not have this issue except when focus is achieved by varying the distance between objective elements, most notably mirrors in several Cassegrain designs (e.g. SCTs). As of the OP question I wouldn't change the position of the secondary if the scope was nullled with the a matched pair, which is the standard procedure for RCs. Because of this the specs are expected to change a fraction of % with respect to the nominal and the scope still being a diffraction limited one. Which is the only thing that matters. |
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andrea tasselli:David Nozadze: Hi Andrea Thanks for your very valuable input in regard to this topic. I have raised the question with TS and hopefully they can give some kind of answer to whether the scope is nulles with matching mirrors or if should just match a certain focal length. I just checked, the scope was delivered with a focal of exactly 1,615.92 mm according to a plate solve of one of the first shots. That's reasonable far from either 1600 and 1624 mm. |
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andrea tasselli:=14px Telescopes, OTOH, are meant to focus at infinity and stay that way so they do not have this issue except when focus is achieved by varying the distance between objective elements, most notably mirrors in several Cassegrain designs (e.g. SCTs) Hi Andrea, Thank you for the response. I think I absolutely understand what you mean. But, still, a vey slight focus cange does occur (at least in my) system, when I change a filter. Each wavelenght of light has slightly different focusing point. This applies even to my newt, as it uses a field flattener (so, technically, it's a catadiptric telescope). So, platesolve caluclates my effective FL, based on a [very low quality] filter and a particular sensor, which I happen to be using for that moment. And I am quite sure, those two are very different from what the manufacturer used/had in mind, when building the scope. P.S. I am not disagreeing with your answer. I simply try to learn more from more experienced people like you, by asking more [annoying and silly] quetsions. So, thank you for your time to read my comment  |
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Yeah like others have mentioned, the FL given by whatever program you are using might not be the most accurate thing since it calculates it from the image your camera takes. Also (and I am not sure about this one so feel free to correct me) if I had to guess, because the mirrors are mass produced there are tolerance errors for each set that is made. When GSO puts them through QC I imagine that they all have varying focal lengths to a degree because of that. Each one is going to be slightly different due to the uniqueness of the mirrors and their focal length needs to be set accordingly. |
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I can not find the post right now, but I read that telescopes are usually manufactured to a specific backfocus, in this case 250mm. Also, from my experience, 1600mm FL seems to be right. You can also try a Ronchi eyepiece and change mirror spacing to minimize spherical aberration. I am quite sure the platesolved FL should be exact enough for adjusting the scope. I know I am late, but I should mention that messing with the mirror spacing was probably not the best thing to do with the RC8. It will take a lot of time to get it back as well as it was from factory. I speak from experience. |
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Torben van Hees: That is exactly right! The most important spec on any two mirror system is the back working distance (AKA "back focus distance" ). The EFL of any two mirror telescope is determined when the sensor is set precisely at the correct working distance. Small differences in the radii of the components that occur in manufacturing will change the EFL of the system by a small amount and it is highly unlikely that the actual EFL will ever be EXACTLY at the specified value. (This is just a difference between the design value and the "as manufactured" value.) Furthermore the uncertainty in plate solving is rarely much better than about 1%. The EFL value that OP measured, are correct to within 0.4% of the published spec and that's almost as good as it gets! Measuring anything to 1% is a bit challenging and it gets really hard to achieve 0.1% without getting really careful. Going beyond that precision requires a LOT of effort. It is very important to understand that plate solving for the EFL should never be used to set mirror spacing in an RC system. (I am completely agreeing with you here Torben van Hees.) The proper mirror spacing should always be set by adjusting the focal plane to be precisely at the specified back working distance. That's how the scopes are manufactured and tested in the first place. John |
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| Thank you, John, it was, of course, a post of yours that I was referring to. Also, I meant to imply that I am *not* sure about the platesolving. My RC8 came out at 1600mm FL. My 10“ CFF RC with GSO optics comes out at 2015mm. |
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| FWIW, my plain GSO RC8 came at 1614mm. |
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Sounds like your mirror spacing is a little off. I wouldn’t panic though. Take a look at this test. While there is only one correct distance you will note your back focal length can be off quite a bit before the spherical aberration impacts diffraction limited performance. Also note many high end RC focus by changing mirror spacing and/or some level of movement to accommodate equipment. http://interferometer-tests.blogspot.com/2013/06/2542000-10-ritchey-chretien-gso.html?m=1 specifically “Like all Cassegrain Systems, in the Ritchey-Chrétien ist the spherical aberration is dependent on the distance primary to secondary, there is only one optimal distance. At the same time the backfocus (distance focal plane to tube) also changes with that distance. If the secondary is brought nearer to the primary, for each 1mm the backfocus increases 10mm. Sometimes more backfocus is needed, so how changes the spherical aberration respectively? To get an idea about that I measured some primary-secondary distances, and just read the terms for spherical aberration alone to avoid influence from test impurities. Here are the resulting Strehl values, distances given are from secondary holder to spider (means I changed the secondary only, inverse to mirror distances): 6mm - 99% 8mm - 99% 9,5mm - 98% 12mm - 93% 14mm - 79% With decreasing distance secondary to primary, the undercorrection increases. Interesting, there ist no linear behaviour, first it goes slightly off, then rapid. If you ever need to adjust the distance, I advice to use a Ronchi grating, as zone errors can be deceiptive if you plan to use the star test.” |
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Hi, I just acquired a StellaLyra RC8 which I understand is a copy of the GSO design (as all of them?). Platesolving gives me a focal length of 1625mm and my backfocus is closer to 270mm than 250mm. Clear skies! |
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Hi, That's exactly how mine was adjusted when delivered by TS. The strange thing is that 270 mm BF is not what the specs say and the spec'ed 250mm don't match the 1626 mm. Therefore I have tried to increase the distance between mirrors to get a focal of 1600 mm which then gives exactly 250 mm of BF. I asked TS what to do and got the answer that specs never are the same on paper. They also wrote to check image Quality my self to see what focal length yields the best image quality (astigmatism/spherical abberation. As your scope has exactly the same factory adjustment as mine, it's hard to deny there shouldn't exist some reason for this. But 270 mm of backfocus is a lot and makes it a rather hard scope to balance with autofocuser, filter wheel and what have you. BTW what do you think of the focuser. I had to adjust it rather hard to make sure it does not slip. This causes a lot of resistance for the focuser though. |
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Before yesterdays shooting I adjusted the mirror distance 1.2 mm closer to each other and was very lucky to get exactly 270 mm of backfocus. Even with this backfocus, which is 20 mm more than the TS spec'ed BF, focal length does not quite reach the TS spec's of 1624 mm. I only got 1621 mm. There is obviously some thing going on and I don't know if it's inaccuracies in production at GSO or whatever. I found a very interesting artice on RC mirror design, testing, mirror spacing etc. Upon interferometer testing, the author of the article Tommy Nawratil suggests that spherical abberation effects of mirror spacing deviations of up to 10 mm are quite modest. As i wrote my scope came came with 1616 mm focal length which gives a BF of 250 mm. I think I'll stick to that since I am not able to do an interferometer test and since I am not able to see any difference in the image quality. After all we are talking about max 2 mm mirror distance adjustsments, which falls well within the margin of the 10 mm that are mentioned in the article. There is a link to the article here: http://interferometer-tests.blogspot.com/?m=1 |
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Steen Knarberg: Yeah that would be the article I posted a couple posts up with some discussion |
