TerryMcK Posted March 5, 2021 Share Posted March 5, 2021 (edited) I was out last night taking snaps of M45 and have just done some processing only to find what looks like pinched optics from the scope. The scope was out all day so had plenty of time to acclimatize. I have seen this intermittently before and now is the time to take action. Anybody know how to move the dew shield on a ZenithStar to gain access to the cell securing screws? Edited March 5, 2021 by TerryMcK Quote Link to comment Share on other sites More sharing options...
Carastro Posted March 5, 2021 Share Posted March 5, 2021 (edited) Doesn't look like pinched optics to me, what do your out of focussed stars look like? Examples of pinched optics: This is out of focus stars Edited March 5, 2021 by Carastro 1 Quote Link to comment Share on other sites More sharing options...
TerryMcK Posted March 5, 2021 Author Share Posted March 5, 2021 (edited) I'll have to wait until we get some more clear nights to find out Carole. Any idea what it could be? I thought it might have been the support screws just a little too tight hence the question. From memory I only appear to see it in the winter. Maybe it has creaking arthritic bones and it only likes warmer weather 🙂 Edited March 5, 2021 by TerryMcK Quote Link to comment Share on other sites More sharing options...
AstronomyUkraine Posted March 5, 2021 Share Posted March 5, 2021 10 hours ago, TerryMcK said: I was out last night taking snaps of M45 and have just done some processing only to find what looks like pinched optics from the scope. The scope was out all day so had plenty of time to acclimatize. I have seen this intermittently before and now is the time to take action. Anybody know how to move the dew shield on a ZenithStar to gain access to the cell securing screws? There are two possibilities for removing the dew shield. Either the dew shield unscrews from the ring, like my Esprit, or the focuser needs to be removed, and the dew shield sliding down over the focuser end of the scope. Hopefully it's the former. Quote Link to comment Share on other sites More sharing options...
TerryMcK Posted March 5, 2021 Author Share Posted March 5, 2021 I've been able to remove the dew shield. It was very easy in the end. Hold the aluminium ring where the clamping thumbscrew is and simply unscrew the shield and withdraw it off the main tube. Couldn't have been easier. Underneath the cell mounting is clearly evident and has a number of screws around the periphery arranged in two lines one per lens. I used a jewellers screwdriver to see if any were tight. Most of them were barely nipped up apart from one which was slightly tighter than the others so I have backed it off a hair. We are talking probably a 1/32 of a turn so nothing much. I have assembled it back together and will wait for a clearance in the cloud test. If it is something else then happy to listen. Here is a zoomed in view with PI STF - an hour at 60 second exposures. I have noticed this before and it seems to occur in the winter. It was about -2C last night but I had the dew heaters on around the cell area. If not the lens cell could it be at the other end where the field flattener is? Quote Link to comment Share on other sites More sharing options...
TerryMcK Posted March 10, 2021 Author Share Posted March 10, 2021 From what I have read so far some people believe it to be something called aperture vignetting - "where pixels away from the centre of the sensor are not illuminated by the full circular aperture of the scope or lens but some component of the optical train begins to obscure it" Although I have yet to find somebody who actually knows what it is and how to cure it. Somebody who has a guesthouse in France has the effect on two higher end Tak 106 refractors that he has. He calls it the lighthouse effect. I have bought a small artificial star torch to test my optics with the setup as is. That is the WO ZS73 fitted with the WO FLAT 73A 1:1 flattener. I'm going to try other scenarios too: WO ZS73 without any flattener - this will show if it is apparent with just the two lenses in place. WO ZS73 with my brand new WO FLAT 73R x0.8 flattener/reducer - saw it and couldn't resist it. Makes the scope a faster f/4.7with wider FOV and a focal length of just 344mm - needless to say it will be cloudy for a bit longer. Quote Link to comment Share on other sites More sharing options...
TerryMcK Posted March 15, 2021 Author Share Posted March 15, 2021 OK I have found out what the issue was with my William Optics scope - after quite a bit of testing with the artificial star and different scenarios. It was not pinched optics, nor aperture vignetting in fact it was nothing to do with the front elements at all. It appears that the field flattener had a slight burr that had been raised on the clamping ring probably in the factory. This burr was what could be seen on larger stars. I took a small file to it to remove the burr and all is ok again. No lighthouse effect at all. The burr can be seen on the tightening slot (slightly out of focus but try getting an iphone to focus this close 🙂 ) of the clamping ring. It could be felt quite prominently as a sharp raised point. The burr was the slight shark fin shaped lump in the corner of the slot in this close up shot. The miniature file smoothed it out, I blew the metal away carefully with a blower and then reassembled the flattener. Quote Link to comment Share on other sites More sharing options...
Carastro Posted March 15, 2021 Share Posted March 15, 2021 Great bit of detective work. Carole Quote Link to comment Share on other sites More sharing options...
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