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I recently brought out the OTA from my old Nexstar 8se and decided to put it to use this galaxy season. I have installed a new CGE Dovetail Bar, ZWO 533mc Pro and Starizona SCT Corrector. I shimmed my back focus to the 90.3mm as recommended. OTA collimation set by airy disk is very good and sensor tilt has been adjusted out, using my homebuilt laser collimator. I am guiding with a ZWO OAG-L and 174mm on a tuned SkyWatcher EQ6R-Pro with guiding averaging around 0.70-0.80 total at the 1450mm focal length. With all that said, I am getting tailed stars, BUT... only on one side of my image and only on the larger stars on that side! While not extremely pronounced, you can see in the tight crop below, they are enough to ruin a 300 second lights, 5 hour stacked image. Coming from imaging with Newtonian Astrographs and APO Triplets and not having this problem, this one has me absolutely puzzled. Thanks for any suggestions!  |
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Hi! This looks to me if you have somewhere on your image train a small scratch or something similar. I do have on the corrector a 15 mm small scratch, no idea when I got it, and I do have at bright stars some similar outcome. Not as much as you have, but otherwise almost same appearance. Just an idea! CS Rudi |
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Are you using any kind of dew-heater? i gebt similar artifacts when I run the dew-heater with too much power. |
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| Definitely check your dew heater. Those look like heat spikes. If you can, use a dew shield in addition to a heater. I had the Celestron dew heater ring which caused those spikes for my 8se. Went back to heater strip + shield, spikes gone, zero dew. I always use the shield and strip now. |
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Hi, I see you are using the Starizona SCT Corrector. I got similar results when I fitted it the wrong way round. I has threads at both ends so it's easily done. The correct way is to have the female thread towards the scope and the male towards the camera. Max |
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Thanks for the replies! I took a close look at the corrector plate, there are no problems and I checked the Starizona SCT corrector and it is in the proper direction. I have a dew shield installed, but did recently install a Celestron dew heater ring, as the humidity has been high with recent melting snow. With clear skies predicted tonight, I'll try imaging with it turned off to see if it eliminates them. Again, appreciate the replies everyone. Clear Skies! |
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When using the dew heater ring you have to use a controller (pwm dimmer, dew controller, whatever). Otherwise it wil heat with approx 25W and that is almost double the amount needed. I made the same misstake at the beginning. Also now using the 2x dew controller from celestron in auto mode lowest senstivity is still too much as it will (depending on the environment) unessesary heat with more than 15W. |
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| Right if you're going to use the Celestron Dew Heater Ring, make sure you use it with the Celestron Dew Controller. I tried lowering the power/heat level (with my Pegasus UPB) to the ring in conjunction with the shield but was getting dew on the corrector. Otherwise, dew shield+strap is the way to go. |
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One other thought. It could be a diffraction spike. For example, if your cable or some part of the dew heater is a little into the light path on one side, it would produce a spike. It might be only in the light path when you are at the edge of the field in one direction, so would only be seen on one side of your image. Or it could be your OAG mirror is too close to your FOV causing the spike. To troubleshoot the issue, put a bright star in the corner where the problem is and take an exposure that shows the problem. Then remove your dew shield or move your OAG mirror out, etc. until you find the problem. If you seen a dew heater wire sticking out, hold a pencil there in front of the corrector at that point and see if the spike gets worse (easier than removing the dew heater). Note to check if you are getting too much heat, unfocus a bright star a lot with a short exposure so you can see a big fat donut that is not overexposed. In the lit part of the donut you will be able to see any convection currents, if it is really bad it can look like this:  It likely would not be that bad, more often you will see lines moving through the donut--can be do to seeing, but convention currents tend to have a point of origin. If you see a point that is radiating, again put a pencil in front of the corrector to one side, you will see it as a shadow and that will tell you which side the heat is coming from. My practice is every night before starting imaging I check the donut to confirm collimation (usually stable with my SCT) and tube currents to make sure I am equilibrated. With an SCT I also go clockwise to the donut (mirror goes down), then counterclockwise to focus, mirror goes up. This also ensures the mirror is not getting tilted due to stickiness (as CCW pushes the mirror up, so you know it is in good contact with the focus ring, and spreads out the grease too, reducing stickiness. I have not personally noticed a change in collimation due to mirror flop after a meridian flip, but you might also want to check your donut after a meridian flip (I usually refocus then anyway). Also, if this is only one night that you saw this could even be wind, I have seen such tails when the wind blows the mount a bit, it may only be for a fraction of a second, not enough to show in guiding, but it can produce a tail. Wind blows in one direction, so these swings all tend to be in the same direction depending which way the telescope is pointed. Good luck Rick |
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Thanks again for the responses, they are much appreciated. I gathered a few hours of lights on NGC 3718 as a test last night, with only the dew shield and no heat. The dew heater ring was indeed the problem. Stars were good across the image with no tailing. Here's a quick process from last nights test. My thanks to everyone that took the time to answer. Clear Skies!  |
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| I have the same setup (except a different ZWO camera and am using the Celestron OAG instead of the ZWO OAG) - including the Starizona corrector, and having the same issue. I'm curious now that you have resolved the star artifacts, what you've done to address dew? |
