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Hello guys and greatings from Portugal to all of you. I have a technical question regarding fine tunning the backfocus to match the exact focal length of my optical systems. For example: I have a backfocus of 63.5mm measured by my caliper which is the recomendation by the manufacture for one of my scopes, but when I do the math using the Astrometry indexes as well the stacking in PixInsight, the focal length is 314mm. If I want to pull the focal length for the optical system which is 300mm, what I have to do? Put more spacing on the backfocus of the system or reduce the spacing? I'm not having problems in stars up to F/4, but like I said, I want to fine tunning all the telescopes. Regards and Clear Skies to all, Cesar |
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| Assuming you are talking about a refractor (as is), backfocus has nothing to do with focal length. |
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Hi Cesar, I'm not sure if I understand your question. I See you use the Vixen FL55ss, and the recommended backfocus for the reducer/flattener train is 63.5 mm, but only in that combination. A telescope doesn't have backfocus. Backfocus is only important for a reducer/flattener, you have to set it up exactly like that, to avoid getting image errors. And only after you set the distance between the reducer and the camera, you focus that fixed imaging train with the focuser. Now you measured your images and the calculated focal length was 314 instead of 300mm? But the backfocus is only important if you use the reducer, so how should the native focal length get affected by it, if you don't use it? I don't know if that clears things up, but whatever you meant, don't change the backfocus of the reducer. Johannes |
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andrea tasselli: And Johannes Möslein: Thank for your reply first of all. From what I'm understanding from both of you, you're saying that backfocus (I think this is the right term for the distance from the last rear element of a refractor for exemple, I'm counting with only a flatnner) does not change the focal length, well I have the living proof that it changes. I have the same optical setup with the same camera shooted with different targets and one gives me the exact calculation of 3013.99mm and the other gives me 302.21mm. The only change that this optical system had was the filter drawer, which when I did the change, using the a caliper I put in the 63.5mm. This means that it changed. But my problem is not this exact telescope (Yes its the FL55ss), it's a C8 + the x0.63 reducer/flatnner.. I'm having problems with coma on the edges resulted with the improper backfocus on the optical trem. Celestron stated that is 105mm, but it's not. I have the exact 105mm of backspacing to the sensor from the reducer/flatnner, and the stars in the corners are egg shaped. I'm not far from the perfect backspacing, but I need to know through the astronmetry files, which they are giving me 1299mm, if I need to put more spacing or remove in the optical trem or reduce it from the reducer/flatnner to achive the exact, or at least, the measure aprox. of 1280mm. My problem lies here. Regards to both of you  |
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Ok I see, thanks for clarifying. If the flattener/reducer is at the wrong distance, it changes the image, so a slight change in focal length can happen. Although it's so low, I doubt it would matter. But the backfocus has to be correct, no matter what. You can't change the backfocus just to correct some minor inaccuracies in focal length. And the measurement could be wrong, and is affected by other things too. Concerning your C8. The reducer has to be at 105mm. If you are having bad stars at correct backfocus, either your reducer is faulty, or, most likely, your telescope needs collimation. |
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Firstly, just to clarify, I explicitly stated "as is", so no intervening glass elements. Back-focus is traditionally the distance between the rear flange of an optical assembly and the focus point. So my assertion stands. Whether adding a reducer flattener would change the focal length with different flange to focus distances while possible is bad practice (it is what the photographers would call "focus breath"). But I expect downsides to that. The distance recommended by Celestron is for the optimum (and traditionally, at the nominal focal length) focus position measured from the back of the visual back flange. Any two-powered optical system, such as any cassegrain, will change the focal length with varying position of the back focus. Depending on the maker, focal reducer would/should carry a recommended positon for the visual back and a back-focal disnace (this time measured from the back of the focal reducer flange to the focal plane) so the question is: are you following those recommendations? And if you expect the same performance that you would have with an Edge HD you'll be sorely disappointed. |
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Johannes Möslein: I'm currently having 92mm of backspacing in the total imaging trem for the C8 + 12.5mm of the camera and ~0.5 for the 1.25" filter that goes inside. This suposedly gives the 105mm like you can check in the image.  Do you have any ideia which from the measure can be done? From the reducer/flatnner? It's from the point A or the point B? Regarding the colimation, it's spot on. I do every time I go to shoot using a brigh star as reference. The problem definitly is not from there.  Best regards, Cesar |
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andrea tasselli: *I put the question to Johannes above, but is also for you if you want to answer as well. That the manufacture stated that is 105mm the optimal backfocus.. What they don't say is where we measured from the reducer/flatnner. And that's what is causing me problems. Some website and cloudynight state that is 85mm counting from the rear glass element, others say it's from the shoulder of the reducer. Manufacture, in this case, celestron don't provide that info and my problem resides here. I'm not expecting an EdgeHD quality, but the results that I'm having regarding galaxies are pretty decent. I'm only have that problem of egged stars but in the extreme corners in a MFT sensor. Thank you for looking at this also. Cesar |
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| It's from the back flange on the scope, not from the 2" adapter. Assume 56mm from the rear element of the focal reducer to focus and see how it goes. Don't expect a lot as that focal reducer isn't suppose to flatten the field. |
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Should be A. Regarding the colimation, it's spot on. I do every time I go to shoot using a brigh star as reference. The problem definitly is not from there. Can you explain how you do this? I'm currently having 92mm of backspacing in the total imaging trem for the C8 + 12.5mm of the camera and ~0.5 for the 1.25" filter that goes inside. This suposedly gives the 105mm like you can check in the image. How do you connect the camera? Maybe there is some wrong calculation regarding backfocus. |
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I think the problem is solved, at least some of my issues (but the perfect backfocus adjustments regarding the distance from the focus point in the sensor remais), after I contacted the technical support from Celestron. From what they told me, the correct mesurament needs to be done from the back element of the real cell. In the photos above, they told me that is measured from the Point B, not the A. The optimal backfocus is 105mm and that focal reducer gives a 15mm circle. I'm using MFT sensors of about 23mm in diagonal, which is expected to have some coma at the edges. Like Andrea pointed at the beginning, this is not an EdgeHD, so the corrected circle is smaller. I connect the camera by screwing her in the T2 rings after the OAG with that optical system that I showed above. Well, thank you guys for all the support. Clear skies to everyone. Cesar |
