Advertisement / Annons:
15: Temperature and focus
All optics are affected by temperature changes. The focus position changes. When setting up the telescope/lens for a night's photographing, it must have time to adapt to the outdoor temperature. It takes different time depending on the construction. Some telescopes have fans to speed this up. A lens is more enclosed and may take a little longer to temperature stabilize, I usually expect about an hour. From an indoor temperature of +20 degrees Celsius to an outdoor temperature of around 0 degrees Celsius, a delta T of 20 degrees Celsius. Now I will analyze this more carefully what time is needed for the focus position to became stable.
Pentax 645 300 mm f/4 ED:
This is how my lens Pentax 645 300 mm f/4 ED behaves, a massive lens of glass and metal. To do this analyze I use the astro edit software Siril. It has built in tools to analyze the quality of the stars and it's free to download.
I took two series of 60 images, in total 120, with 60 seconds exposure each. They are taken from the balcony and the seeing isn't very good. I normally don't take a serie much longer than one hour. Between the series it do an auto focus, plate solve and calibrate the auto guider, the last extra calibration can be skipped when photographing the same object.
The first photo is taken 40 minutes after I took the equipment outdoors. Photo number 60 is taken 2 hours after setup. Photo number 61 is taken after a new auto focus, plate solving and auto guider calibration, this time it took 13 minutes to do. The last photo is taken 3.5 hours after setup. Even at sequence two it can be seen that the focus isn't stable the first ten minutes.
From this I can see that the focus isn't stable until after 2.2 hours, that's a long time to wait and loss of exposure time. I can also see that the focus is better after the second auto focus compare when I started the photo session. I can shorten the series and do auto focus more often but then I loose a lot of exposure time. When I drive to a dark place the temperature in the trunk isn't 20 degrees, much lower and the temperature stabilization will go faster. It take me normally at least 45 minutes to setup the equipment, first after that I can start astrophotographing. Now I see that's too early.
It's not only the temperature that changes, the objects height over horizon changes too. In this case the altitude of the object is 44 degrees in the beginning, in middle 48 degrees and at the end 54 degrees above the horizon. That small change in altitude shouldn't change the focus much. In this graph I have plotted the roundness of the stars, higher value is better. The roundness is more related to out of focus, some astigmatic problem maybe.
The last graph is the wFWHM, Siril doesn't say much about this more than it's an improved variant of the normal FWHM.
From all this graphs I now know more precise how I shall handle temperature changes. For a delta temperature of 20 degrees C I have to wait at least two hours before I start taking photos, or doing more auto focus after 30 and 60 minutes. If the car's trunk hold +10 degrees Celsius and outdoor temperature of 0 degrees C maybe I only have to wait one hour. During the night with small temperature changes, less than one degree C per hour it's enough to do an auto focus each 1.5 hours. My focuser has a built in temperature sensor which can move the focus point, but when i try to use that it's always fail because it's complicated to get a working calibration. Metal and glass react very different to temperature changes. So I prefer to do auto focus.
If I could speed up the auto focus process I could do it more often. As it is now the DSLR camera slow it down a lot, each image need 15 seconds to transfer and normally an auto focus need 20 exposures. Read more here: Canon slow transfer.
Next time I do a two hours exposure and at a dark place with longer sub exposures I test this setup :
With that I will use the time much better and get more exposure. I normally do a dither every 3rd photo. The KStars/Ekos that control my equipment is very advanced, I can setup it to overview the FWHM and do an auto focus when it's necessary. I need to practice this first so it don't start to do auto focus too often and what happens when a cloud pass by ?
An example of a normal photo taken from a dark place, M81. Something I can improve in the future.
Auto focus during a session:
A clear night and I can test how to setup the equipment to do an auto focusing during a session. It wasn't easy to find instructions how to do this. I found that it isn't easy because there are so much that can disturb the HFR reading and then cause a forever focusing loop. Stellarmate has a page with instructions how to setup the focus limits: Stellarmate.
You find this setup at the CCD tab and limits.
First I start an auto focus procedure to find where the seeing is this night, it's from the balcony and not very good. Find the best focus to be HFR = 1.2.
Enable the Focus limit and set the limit to 1.5, well above the HFR 1.2 I got.
Starting a photo session of 120x60 seconds exposures. To my surprise I found that it over write my setup. Tested several times. It can be because of the bad seeing I have at the balcony.
I change the focus control to do an auto focus every 40 minutes, much safer. Maybe I can change this to 90 minutes after two hours when the lens have find its equilibrium. My idea that only do an auto focus when it's necessary failed here.
The Analyze tab has a lot of information to tell. I have enabled the HFR, it's the blue line, but no graph there. The guiding error is very big this night.
At the Setup tab there is a profile of the quality of the focus.
Strange, no trace of an auto focus, it should have been around image 30, 60 and 90 with 40 minutes apart. What I can see is that even after 150 minutes the lens isn't in equilibrium. The outdoor temperature goes from -6 degrees Celsius in the beginning to -8 degrees Celsius at the end. Next time I take the car for a nights astrophotographing I will leave the lens on the balcony many hours before I go to let it cold down. The object I photographing is the global cluster M92, I will upload it later.
One more try the night after, still no trigger for auto focus, it's set to do an auto focus every ten minutes. This time I at least got it correct focused at the start of the photo session. Half the FWHM diameter.