wookie1965 Posted September 25, 2018 Share Posted September 25, 2018 Had first light with the 5" Meade couple of nights ago and I am somewhat disappointed I know the glass is dirty which will be addressed at Galloway but now found more problems. Checked collimation and that although not spot on it is very close. The stars dont focus into peas like I am used to they are spiky whether it was the seeing I am not sure but this has happened twice now. In focus and out focus puts the star on the edge not in the middle I have attached two drawings to help explain. To the Frac enthusiasts any help appreciated. I have included a picture of the collimation. Link to comment Share on other sites More sharing options...
Peter Drew Posted September 25, 2018 Share Posted September 25, 2018 The images appear to show that the focuser is well enough aligned with the main tube but the out of focus star images, if I'm interpreting them correctly, suggests that the objective is well out of collimation. Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 The star test looks very wrong doesn't it ? The airy disk of the star should be central with diffraction rings around it, inside and outside of focus. Sometimes one side is clearer than the other. If the diffraction rings are not concentric then the collimation is out a bit. If I'm interpreting your diagram correctly, it looks as if you have a single large diffraction ring with the airy disk lodged on one or the other side of it (and a bit messy too). The laser is testing the alingment of the focuser optical axis, which looks more or less OK. A cheshire eyepiece is needed to check the tilt of the objective lens which could be miles off judging by the star test. If the collimation of the focuser is OK and the objective tilt looks OK then the next culprits are: - decentered elements (one or other of the lens elements has shifted so that it's optical axis is not the same as the other one - the lens spacing being off through a spacer being missing or in the wrong place - the lenses have been rotated so that they are not in the orientation they were when they were manufactured (there should be alignment marks on the lens element edges). Best thing to do next is to check the objective tilt with a cheshire I think. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 As I'm new to refractors I would not know what I am looking for using a Cheshire. The diagrams if I can explain better the scribble is the star all over the place the diffraction ring does not have the star in the middle its on the edge I will edit the pictures to show you. The blue arrow points to the star and the diffraction ring comes off something like a polar scope with polaris in the tiny circle. Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 My understanding was sort of how you have explained it. The diffraction pattern should look like this: I wonder if yours is showing coma due to sever miscollimation. Like this but a little worse ?: Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 I was going to say the stars have like a shuttle cock appearance does look like the second picture all be it only one ring. I am going to try take some pictures now with the Cheshire in I will post see if you can see anything. Thank you so much appreciate your help. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 When using the chesire to check objective lens tilt, you put the cheshire into the drawtube with no diagonal in place. Then shine a light onto the 90 degree window in the side of the cheshire, while doing so, view through the cheshire to examine the image of the 90 degree face that should be now relecting back off the rear of the objective. You leave the lens cap on for this so you get a dark background against which to see the relections of the cheshire 90 degree face. This post sometime back from me on this topic includes a link to a very useful piece by SGL member PhilJ which explains more about what to look for and how to adjust: Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 I did take the diagonal out apart from two pictures they were all taken with cap on, thanks John I will read this now see if I can get my head round it. Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 Try and ignore / look past the cross wires of the cheshire. They are not used when checking the tilt of the objective. Link to comment Share on other sites More sharing options...
jock1958 Posted September 25, 2018 Share Posted September 25, 2018 What my out of collimation Tal 100rs looked like through a Cheshire eyepiece, bluish / green double dough nuts. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 I have two separate donuts only 3 adjustment screws when I loosen one gets closer together but not on top and the two other screws don't seem to be having any effect. The screw I have loosened is nearly out feels loose. Mine is like Jocks except there is a couple of mm between the donuts. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 If the donuts are actually separate, the objective lens is tilted a lot and the scope is well out of collimation. It appears that your objective cell does not have the push-pull collimation screws that mine had and that PhilJ refers to. The screws you have pictured hold the objective cell in place on the tube I think. Some models have a 2-piece cell with the push pull screws in 3 places around the circumference to achieve this with precision. See the photo below - my Meade looked like the one on the left with the push pull collimation screws, yours looks like the one on the right. What you need to do is to tilt the objective in it's cell in relation to the tube. You might be able to achieve this by removing the objective in it's cell, shimming the tube top at a certain point, then re-attaching the cell. The shim should then tilt the cell and the objective slightly. It's going to be a trial and error thing I'm afraid. Link to comment Share on other sites More sharing options...
John Posted September 25, 2018 Share Posted September 25, 2018 The left hand image shows how the cheshire reflection donuts should look when the objective tilt is correct. Yours sounds as if it is even further out than the right hand image: Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 It is and I am not that confident to remove the cell even if I can. Its a pity I did not live closer to you may have been able to help me. Mine is like yours on the right how do I get the cell out and what can I use as a shim please. Link to comment Share on other sites More sharing options...
Peter Drew Posted September 25, 2018 Share Posted September 25, 2018 I'm nearer than John and would be pleased to sort it out for you if you can get over. ? Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 12 minutes ago, Peter Drew said: I'm nearer than John and would be pleased to sort it out for you if you can get over. ? Hi Peter, That would be very good of you your in Todmorden arnt you when could I nip up please. Would Thursday morning be OK I would need get back to St. Helens for 3.30pm do you think you could sort it in a couple of hours. Link to comment Share on other sites More sharing options...
jock1958 Posted September 25, 2018 Share Posted September 25, 2018 I can only echoe what John has said. I spent a great deal of time adjusting the objective cell on my Tal and managed to get it down to to a single doughnut, but to be honest the star test has never been completely satisfactory. I spoke to Steve Collingwood at SC Telescope and he was happy to bench test it for about £30 ish but as he’s on the other side of the country to me I gave up and settled for what I’ve got. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 25, 2018 Author Share Posted September 25, 2018 Kent is a bit far for me as well hoping Peter is free I go Galloway next week wanted this done before I go. Link to comment Share on other sites More sharing options...
jock1958 Posted September 26, 2018 Share Posted September 26, 2018 12 hours ago, wookie1965 said: Kent is a bit far for me as well hoping Peter is free I go Galloway next week wanted this done before I go. Good luck hope you get it sorted ? Link to comment Share on other sites More sharing options...
wookie1965 Posted September 26, 2018 Author Share Posted September 26, 2018 Well just had it apart the retaining ring was cross threaded and there were marks on the edge of the glass 120° away from each other. I have cleaned the glass then put it back with the marks lining up screwed the retaining ring on right this time the donuts are closer now touching but not overlapping, hoping Peter can sort it tomorrow I am not going to play with it anymore. Link to comment Share on other sites More sharing options...
John Posted September 26, 2018 Share Posted September 26, 2018 1 hour ago, wookie1965 said: Well just had it apart the retaining ring was cross threaded and there were marks on the edge of the glass 120° away from each other. I have cleaned the glass then put it back with the marks lining up screwed the retaining ring on right this time the donuts are closer now touching but not overlapping, hoping Peter can sort it tomorrow I am not going to play with it anymore. Thats shows that there is scope for improvement then. Sometimes loosening the objective retaining ring and re-settling the objective by pointing the scope upwards and "slapping" around the sides of the lens cell can improve things as well. The lens retaining ring does not need to be tight - just enough to stop the lens elements from rattling around. Over-tight retaining rings can cause lens pinching so you get a triangular star pattern. Sometimes people rotate one of the lens elements to try and reduce spherical aberration. Sometimes this works and sometimes it does not. You can improve SA and make CA a little worse !. A previous owner might have been experimenting with your scope, hence the lens alignment marks being 120 degrees apart. Link to comment Share on other sites More sharing options...
wookie1965 Posted September 26, 2018 Author Share Posted September 26, 2018 It certainly looks like that John but whoever did it was not careful there was dirt over both the inside pieces of glass and the ring was that tight I have had put a notch in it and tap it round. I am at a loss now what to do with it do I unloosen the ring and tap the side trying to spin one piece of glass (the front one) or wait and let Peter have a look, I was hoping take it Galloway next week so I am pushed for time and being a novice with refractor`s does not help. Link to comment Share on other sites More sharing options...
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