4 replies to this topic

[bangzhengsun1997]

Hello everyone! Hope this is the correct thread to post this topic.

 

I've been thinking about a problem recently but couldn't find an exact answer. I've used several different telescopes (several refractors, a Newtonian, and an SCT, and some camera lenses for WF purposes). I always had to re-focus after ~20-30 minutes at the beginning of the session, and then the new focus could hold for much longer, sometimes over an hour (except for the Newtonian, which is more sensitive to focus change). Here, "hold" means that statistically, the PSF width of stars almost doesn't change, and it is not just visually okay.

 

I first thought that it was due to the temperature change when the night began. However, I recently did a session in which I was setting up the telescope (a 4-inch refractor) at 8 PM and waiting for clouds to go away until 1 AM - in this case, I suppose that this temperature shift wouldn't impact as much because the temperatures at ~1 AM should be relatively stable (it was indeed according to weather history). However, after the first focus, I still had to re-do it after just 20 minutes, and after that, I didn't touch focus for ~80 minutes, and it was still great. I kept thinking about what could be the reason. If we still explained the shift with the temperature shift in the first 20 minutes, then why didn't I have to re-focus in the next >1 hour?

 

Well, it might not be useful to crack the problem to the bottom because, nowadays, re-focusing is quite easy, but I'd like to hear everyone's thoughts. I really appreciate any help you can provide.

[archiebald]

Yes, it seems logical that the evening should start warmest, cool down fairly rapidly, then begin stabilizing sometime after midnight.

 

Not true according to the graphs I've seen since I added my Pegasus Power Box Advance with its environment sensor (at least at my location).  It logs and displays the temp / humidity / dewpoint graphically throughout the night and I have observed;

1. The drop-off is rarely consistent - often there are steps with sudden drops and plateaus in the graph.
2. It's not uncommon for the temperature to rise a few degrees at odd times as a warm front starts to come through (often with cloud following shortly or a few hours thereafter)

I also have a temp sensor on my focuser and although it doesn't graph the data, it does allow me to verify that the Pegasus data is reasonably accurate.

 

Plus I've even felt it myself sometimes where a slight change in wind direction can produce a several degree change in temperature, in my case that might be a warmer breeze coming up the valley from the river vs a shift to a cooler breeze coming down the nearby mountains, or vice-versa.  Temperature changes can be very localized too depending on the surrounding ground surface, not enough to show up in weather records data.

[bangzhengsun1997]

Yes, it seems logical that the evening should start warmest, cool down fairly rapidly, then begin stabilizing sometime after midnight.

 

Not true according to the graphs I've seen since I added my Pegasus Power Box Advance with its environment sensor (at least at my location).  It logs and displays the temp / humidity / dewpoint graphically throughout the night and I have observed;

1. The drop-off is rarely consistent - often there are steps with sudden drops and plateaus in the graph.
2. It's not uncommon for the temperature to rise a few degrees at odd times as a warm front starts to come through (often with cloud following shortly or a few hours thereafter)

I also have a temp sensor on my focuser and although it doesn't graph the data, it does allow me to verify that the Pegasus data is reasonably accurate.

 

Plus I've even felt it myself sometimes where a slight change in wind direction can produce a several degree change in temperature, in my case that might be a warmer breeze coming up the valley from the river vs a shift to a cooler breeze coming down the nearby mountains, or vice-versa.  Temperature changes can be very localized too depending on the surrounding ground surface, not enough to show up in weather records data.

Thanks for your reply! Yes temperature is still the only factor I can think of now. I'll I do have a temperature sensor but doesn't really graph the weather, maybe I should try and see if those are correlated in my case, too. 

[ngc2218]

Hello everyone! Hope this is the correct thread to post this topic.

 

I've been thinking about a problem recently but couldn't find an exact answer. I've used several different telescopes (several refractors, a Newtonian, and an SCT, and some camera lenses for WF purposes). I always had to re-focus after ~20-30 minutes at the beginning of the session, and then the new focus could hold for much longer, sometimes over an hour (except for the Newtonian, which is more sensitive to focus change). Here, "hold" means that statistically, the PSF width of stars almost doesn't change, and it is not just visually okay.

 

I first thought that it was due to the temperature change when the night began. However, I recently did a session in which I was setting up the telescope (a 4-inch refractor) at 8 PM and waiting for clouds to go away until 1 AM - in this case, I suppose that this temperature shift wouldn't impact as much because the temperatures at ~1 AM should be relatively stable (it was indeed according to weather history). However, after the first focus, I still had to re-do it after just 20 minutes, and after that, I didn't touch focus for ~80 minutes, and it was still great. I kept thinking about what could be the reason. If we still explained the shift with the temperature shift in the first 20 minutes, then why didn't I have to re-focus in the next >1 hour?

 

Well, it might not be useful to crack the problem to the bottom because, nowadays, re-focusing is quite easy, but I'd like to hear everyone's thoughts. I really appreciate any help you can provide.

The metal in the scope doesn't equilibrate with the ambient temperature but with the cold clear sky. My scopes tube temp is always lower then ambient. Was amazed when I Plotted the data... 

So even if the ambient temp is constant, the tube is not, so focus changes.

 

Anywho, here's my data: https://www.cloudyni...rmal-behaviour/

[bignerdguy]

Another possibility regarding temperature is the camera.  Since when you first set it up and leave everything sit the camera manages to cool to ambient temps but as you use it it heats up.  This can cause a slight change in focus and could cause you to have to refocus after 30 minutes.  That is about the length of time for the various parts of the telescope and camera setup needs to reach equilibrium with the outside air.  So if the camera was off until you started shooting then it could account for it since the camera will heat up once turned on and starting to take photos.  This is especially true for the DSLR.  I have one and i can tell you that it does need to warm up or cool down before you start to use it as the focus does change a bit during that time. Not sure about the others though.