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| @John Hayes Here is my focussed in view:  and here focussed out:  Here's the un-stretched in-focus view:  Any thoughts on these apart from the obvious focus tube thing? Lastly, here is a view of the stretched star:  It is very cloudy tonight, so maybe tomorrow would be better. Many thanks again. Andy |
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| The intra-focal was not taken at the same distance from focus of the extra-focal one which makes quantifying things somewhat more difficult. You have some SA and the primary is not too well collimated. I also suspect a small amount of TDE (but I wouldn't put my money on it, yet). You also seem to have a weird deformation of the mirror in two locations around 100deg apart, if I read the images correctly. |
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andrea tasselli: Andrea: thanks for that, but to be honest I didn't understand any of that. I am truly an amateur here and was just looking for some help.. I have no idea what SA or TDE is and you also say that the primary is not well collimated, in what way did you see that? Please help me rather than just throwing this stuff at me. I think you assume we know all these terms when most of us are just really starting on this journey. I know you are trying to be helpful, however many of us don't understand the acronyms. |
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As for the mis-alignment just look at the first image. It is obvious that the central shadow (the secondary) is offset with respect to the outer edge. It could also be quantified if the distances intra and extra focal are exactly known. SA stands for spherical aberration (how far your mirror is from a perfect paraboloidal shape, in simpler terms) and TDE stands for Turned Down Edge, which used to happen quite often in the past. It is because the stiffness of the mirror at the edge isn't quite the same as from the main body of the mirror and if pressure isn't reduced you end up removing more material than you should. The only thing that puzzles me is that the intra and extra focal images should be labelled the other way around. At any rate such questions should be really investigated without a CC in the midst as it complicates things. As for the deformed shape, just look at the in-focus image. There is no way that sort of un-symmetry is going to happen without something unsymmetrical going on, which seems to tie with the out of focus images. BTW, how many holding screws do you have in your focuser? Again, it would be better to test things without the CC and a different imager, maybe your ASI174MM. |
22.75
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I agree with Andrea but here's what jumps out at me right away: 1) Assuming that the secondary is well centered, the secondary does not appear to be aligned to make the axis of the defocused precisely coincident with a line drawn through the center of the focuser and the exact center of the primary. That's indicated by the fact that the shadow changes position as you focus. 2) Fixing that focuser sticking down into the input beam will clean up the diffraction pattern...a LOT. 3) You do have some spherical aberration but that isn't what is causing the stars to be out of round. From what I can see, I think that if you fix the focuser and work to get the alignment a bit closer, you should produce nice round stars. So, don't give up! Optical alignment requires a bit of patience but it gets easier with practice. Good luck!! John |
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11.01
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andrea tasselli: Thanks again Andrea. In answer to the question above: I have 2 holding screws with a brass compression ring. You were absolutely right about me mis-labelling the two images; sorry. The actual focusser positions were: 1500 for the focuser raked in 3836 for the in-focus star and 5500 for the focuser raked out Given my step size of 2.16 microns that means I focused inwards by 5.05mm and outwards by 3.59mm. In what way would having the focusser position the same in and out help quantify the issue? |
1.20
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Hi Andy I’ve struggled with trying to get perfect collimating myself, removing camera gear etc. to star test with an eyepiece is a non runner ( for me anyway ) Too much hassle. So I tried a defocused star and a bullseye overlay which I thought was good but there was still a lot of room for improvement. last night I remembered I had “Tri bahtinov mask “ for a 6”sct, so I gave it a go on my 130pds. ( balancing act + sticking tape ) A few small adjustments on the primary and the results were very pleasing. 😀. it made the whole process easier. ( Unfortunately, after a meridian flip things turned bad again, I’ll have look for looseness in image train. ) Tri bahtinov mask, is worth taking a look into. Padraig |
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Andy Wray:andrea tasselli: It allows me to simulate some (but not all) the aberration of your imaging system and the appearance of the extra/intra-focal images and hopefully from there you can quantify how much off you are from the optical axis. Now, on to the "squashed" appearance of the stars: it might be that your CC has very thin walls and if enough force is applied to the focuser's holding screws you might inadvertedly deform the lenses enough to cause the said appearance. Now, this is just a possibility but not too far off to the left field, if you see what I mean. A purely trefoil (caused by support-induced deformation) doesn't really show up in your images so it is something different from the classic 3-point pinching. Yet there is a lot of scattered light onto one side and TDE does not acccount for that since the scettering would be uniform. Again, my strong advice to you is to do a quick test without the CC but following the same guidelines from John. Make sure you have your star well centred (if you're using ASI software then set the crossline overlay on to centre the pattern). As I said before it would help you move the sensor to the intra and extra focal positions by the same amount. Makes easier to understand what's going on. |
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Stupid question time? I'm assuming that if I'm properly collimated the centres of the two circles below should align? |
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| Indeed they would. |
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11.01
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John Hayes: Thank you ... you do inspire me to keep at it. In my haste to get the diffraction patterns last night between clouds I forgot that I'd been working hard on the OTA during the day trying to work out how to extract the stuck primary and was probably a bit rough with it. I didn't check the collimation when I put it back on the rig before taking the out-of-focus shots. I checked this afternoon and my laser was about 15mm away from the primary centre spot and Bob's Knobs were a bit loose. So, I have now re-collimated and will (clouds-permitting) re-do the star test tonight. |
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11.01
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Thank you all for your help with this (in particular John and Andrea). Below is just a quick 10 sub-frame image put through abberation inspector. The stars are now looking nearly round (certainly not as triangular as before). I still need to get better at collimation and need to get my RA guiding under control, however I'm not going to worry about pinched optics for the time being. Also nice to use star tests at last. I know the stars are still a little bit ugly compared to the best images on this site, however it is one step at a time. Also, this was just 10 sub-frames shot and integrated immediately: no sub-frame selection or sub-frame checking. As always, if you can see something that needs further improving based on the below, please let me know.  I know the stars are ugly, but they are sort of OK at a reasonable scale:  |
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Andy, That's better but I have to reiterate that if you get the system properly aligned and you fix the focuser sticking into the input ray bundle, you should be able to produce very round stars--much better than what I see in your image. Good luck with it! John |
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11.01
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John Hayes: I'll take that as a 5 out of 10, could do better ;) I do need to replace the focusser, but that is a budget thing for me right now so will have to wait a while. A decent focusser would cost more than my OTA originally cost, so then I question if a new OTA would be a better investment. I'm going to focus on my collimation/alignment skills for the time being. Between you and Andrea you have helped me a lot in that regard. |
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Andy, Just to ask again, but is there a reasonable way you can shorten the tube without destroying the focuser? Point added: it's nice to see that you are getting there! But more importantly, learning how your optics work and gaining the skills to fix issues. |
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11.01
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Alan Brunelle: I could just take a hacksaw to it to be honest I guess ... if I wanted to sell it on, that may be a problem as it would knacker it for visual use. I am tempted to buy a new hacksaw ;) |
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To be honest, you would be doing the buyer a favor. A saw, preferably set up on a mitre, to make the cut square, followed by a light touch with fine sandpaper and a dab of black paint and no one would know. Also, it sounds like this OTA is not an expensive unit, so not sure how much you would expect to get for it. When the time comes to part with it, maybe a donation to a local school and take a tax deduction. |
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Andy if your using the skywatcher cc which I believe you are Astrobloke on YOUTUBE had a bit of the cc turned down so he could push further into the drawtube which then made the drawtube push further out to focus on his 130pds also you need an aperture mask as your getting reflections from primary mirror edge I bought mine for my 200pds from pavel at Deepskydad it fits a treat and designed very well to stop refl ections
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Not perfect, I know, but this is what my stars look like now. A little bit squished and a little bit fuzzy, but actually better than I have ever managed before  :  Fiddling with my scope and captured the Wizard |
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First, I should say that this looks great. Nothing is ever perfect, but one consideration is do you want perfect, or are you wanting to achieve that which is necessary for viewing a great image at its intended size/magnification/etc? Perfect can be very costly, time wise and morale wise. Having said that, I am seeing color in your stars very slightly. Red at 2 o'clock and less obvious, blue at 7 o'clock. If I am correct, you are doing LRGB and not OSC, so maybe something in your recombination. Edit: just noticed you are doing SHO. Not knowing that process, not sure how relevant my comment is here, but I am see in a color gradient. End Edit. But a reflective optic should not generate refractive color. So this points again to your cc because of the lense elements. Since it shows on stars in the center of the field, it does not support an issue with incomplete correction, say at the edges or even cc to sensor distance. My guess is some slight tilt in the cc as it sits in the focuser, but if you have the cc sitting flush, then tilt in the focuser on the optical tube. Sometime this sort of color, if left uncorrected is more noticeable when viewing astrophotos at intended scale than star shape. Especially since it often shows up strongly on mid to bright stars. Sometimes slightly mishapened stars improve if you practice star reduction to varying degrees. Alan |
