I gave the processing of the one armed spiral galaxy NGC 4725 another try with the Astropixelprocessor, The smaller galaxy is NGC 4712. The result showed so much fine detail in the galaxy core that I decided to try a 3x drizzle on a crop of the image. As the last step of the processing I downscaled the result to its original pixelsize with PhotoZoom4. The final image radius is only 0.28 deg.......




Pentax K3ii, TS Photoline 130/910 ApoTriplet 40 x 300s @ ISO 800

When I compare the images that I shot with the motorfocus (Lacerta MFOC) with the ones before I see that they show significantly more fine detail now. Working exactly on the calibration curve of the MFOC with a lot of Bahtinov mask images I realized how much the focus shifts with falling temperature. And even though I had built up the scope (put away the shelter bags) outside long before the it was dark enough to image I noticed that the focus did not shift linear during the first half our or so of imaging. I concluded that there are still parts of the scope (e. g. glass) that need longer to acclimate and that its take the scope a lot of time to find a temperature equilibrium and then behave linear with the falling temperatures during the night.


In the end I could measure that the focus of my telescope (130/910 mm refractor) shifts 0.1 mm (110 microns) per degree Celsius with the temperature. This shift is enough to steal sharpness and details throughout the whole night if not readjusted frequently enough. So (if there is no autofocus) my advice to you guys is to test the behaviour of your scopes and determine a maximum rate of temperature change for the readjustment. Though it is a pity to loose imaging time I consider it better to loose some subs during the time for control and readjustment of the focus than to risk loss of details.

How do you guys handle this issue?

Cheers
Pete

Dang! Now I need a focuser?!?! ;-(

Good to know. My Stellarvue has a calibrated scale, but it's in millimeters, so I could get maybe 0.2mm precision at best. Still worth at least a try to develop a similar dataset for my scope too. Or maybe I'll just adopt yours and say "refocus after half an hour, and then every 0.5℃ thereafter".

I need a few of those and I need another MGEN II. I can't believe how easy it is to use now.

I've also found that temperature has a much larger effect on focus than I always thought. Some of my subs are awful.

As a response I've built myself an auto focuser, but it's not for the Pentax, as it works on HFR (half flux radius) as calculated by the computer. If the DSLR can't talk in real time to a computer then it's not going to work. I use it with an ASI1600MM-pro and Sequence Generator Pro software.

The focuser, by the way, is the Arduino ASCOM Focuser Pro2 DIY, (Google it) and was remarkably cheap. I'm hoping to get first light (if that's what you get with a focuser) with it tonight. I know it's a full moon, but it's clear here in Melbourne and all I'm after is a set of sharply focused stars.

Originally posted by gbeaton:

Dang!NowIneed a focuser?!?! ;-(

Just calm down.......that is not what I intended to say. In the past I fixed the focus once in the beginning of the session and then a long time later or even not at all anymore.I did not take into account how the temperature changed in the meantime and so focus became kind of a gambling game in longer exposure series. And what I did not take into account at all was the fact that at least my longer scope needed so much time to cool down in all its parts (somebody in the internet even talks of "internal seeing effects" before the equilibrium is reached). I here just wanted to plead for knowing or finding out the temperature characteristics of ones scope and develop a robust strategy to handle that issue. I think temp./focusshift behaviour is strongly dependent of the type and length of the system and the implemented materials. For example I remember that my trusted vendor said he would sell me no carbon tube because though it was of lower weight it would not react uniformly on temperature changes and twist/bend which leads to aberrations. Maybe in short focal legth scopes and within a reasonable temperature interval the effect is negligible but even knowing this for ones scope would be an advantage.

Knowing what I know now I would do the following in a system without auto(motor)focus like my TS 80/480 apo to get data and a first feeling for the characteristics:

- get me an exact thermometer
- in a night with expected significant temperature drop.....
- ....bring the scope out as early as possible but not as long as the sun is above the horizon to let it cool down
- paste a piece of tape onto the flat side of the microfocuser wheel
- wait until it got dark or at least 1/2 hour and slew the scope to a bright star that is above the horizon for most of the night. Let the scope track on it
- focus exactly with a Bahinov mask, mark 12 o'clock on the tape.
- take a reference image with and without a Bahtinov mask. Note temperature and imaging time.
- repeat in useful time steps with reference images w/ w/o mask until temperature has dropped significantly and defocusing is perceptible in the reference images. Do not refocus. Write down temp/time data
- leave the scope alone and come back in the coldest hour. Fine focus with the mask on the scope. Write down temperature, time and "hour value" of the mark on the tape
- transform the "hour value" of the coldest measurement into degrees of focuser rotation and focuser way (given the focusers gear reduction isknown)
- calculate the gradient of temperature-drop versus focus-shift
- decide which amount of defocusing is acceptable by examining the reference images ("naked" images and Bahtinov images)
- take 2/3 of the correspondend temperature difference relative to the starting temperature as a rule of thumb for correction intervals in the next imaging sessions.

Originally posted by VoiceOfReason:

Ineed a few of those and I need another MGEN II. I can't believe how easy it isto use now.

Nice to hear that the MGEN works as promised! If you think about acquiring an MFOC be aware that it does not work with all types of focusers.