Having a quick look through my raw, uncalibrated light frames, I have noticed a trend with elongated stars. I dither every 2 frames using NINA, and have noticed that each frame following a dither has elongated stars. Not disastrously elongated, but enough to want to throw out those frames. The following frame always contains pretty much perfectly round stars.
Could my camera be starting exposure too early after the dither while the OTA is still vibrating from the dither? Or worse, exposing during the dither? This is terribly frustrating as I have noticed this 2 nights in a row now and feel like throwing out 50% of my frames. I could just dither every, say, 5 frames instead, but I would rather fix the issue if I can.
Below on the left is a typical frame immediately following a dither. On the right, the following frame.
The right hand image stars are not perfectly round either. Take a batch of
subs without dithering, and if the stars are round, don't bother with dithering.
raymo
Can you lengthen the settle time? Are they always elongated in the same direction? What dither pattern are you using? Are they more elongated in longer subs, or the same?
Stephane,
A few reasons for eggy stars following a dither provided your stars are round without dithering
1/ Dither aggressiveness to high
2/ Dither recovery time too short for your mount
3/ Mount has tracking and guiding issues ( backlash issues etc... )
Does NINA has adjustable settings for item 1 and 2 above ?
Are you using PHD2 ?
Make sure your dither settings in PHD2 are “Random” and dither scale is 1.0 otherwise your dithers maybe to aggressive.
Most skywatcher HEQ5 ,EQ6 and EQ6-R mounts etc...require anywhere between 10 to 30 sec to recover from dithering before you take your next exposure
Both my EQ6-R mounts require 15 seconds to recover from a dither before the next exposure it taken. APT controls my dithering.I set my dither distance in APT to 1.0 which is the lowest and dither recovery time to 15 sec , has worked ok for years.
Also the PHD2 guiding graph visually gives you an idea when the mount has recovered enough to take the next exposure.
Generally a 3 to 6 pixel dither is sufficient to mitigate fixed pattern noise but the mount will let you know it’s limits , some mounts can handle a 10 pixel dither and recover in 15 sec others only 5 pixel , they’re all different.
Can you lengthen the settle time? Are they always elongated in the same direction? What dither pattern are you using? Are they more elongated in longer subs, or the same?
Markus
Agreed. It is likely that your not allowing enough settle time after a dither. And frankly I believe your dithering too often, increase the space.
Thanks for replies. Markus, they are all elongated in the same direction. I shouldn’t have said “perfectly round” on the right, but rather, “less elongated”. But all elongations are in the same direction and more pronounced after dither. I don’t think settle time after dither is possible but would love to be corrected. All subs are the same length, so I don’t yet know if exposure length makes a difference. As for dither pattern, I don’t know. I just click “dither”
Thanks so much everyone for your replies. I just realized I got a few in quick succession and missed them.
Martin, I am using PHD2 so I’ll have a look at the settle pattern. I don’t believe NINA has dither control. It does however connect to EQMOD & PHD2. Perhaps I can adjust the settings there.
Glen, I might also decrease the frequency with which I dither.
Thanks so much everyone for your replies. I just realized I got a few in quick succession and missed them.
Martin, I am using PHD2 so I’ll have a look at the settle pattern. I don’t believe NINA has dither control. It does however connect to EQMOD & PHD2. Perhaps I can adjust the settings there.
Glen, I might also decrease the frequency with which I dither.
When using EQMOD and PHD2 make your your guide pulse rate is set to x 0.5
Refer attached doc to check settings
Also your dithers are usually sent from your capture software which is the best way to initiate it , NINA does allow you to set dither commands
Thank you Martin & Elliot for your further comments.
Elliot, my settle time was set at 10 seconds! I think this might just be the problem. I have changed to to 15 seconds (based on Martin's advice) and lets see if that makes a difference.
Martin, I double checked my EQMOD settings, and they are all set to your recommended settings still, so no issues there. PHD2 dither settings are set to "random" and scale of 1.0. So I guess it must have been the settle time of 10 seconds which was just too short. I searched NINA and couldn't find any dither settings, but hopefully the settle time will fix my woes. Thanks again.
I'll also dither less often based on Glen's advice. Thanks to everyone.
