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After discussing in NebulaPhotos' discord I believe I've observed much more intense temperature sensitivity in focus with my SQA55 than when learning with my roki 135. The drop in the middle of the graph is the start of night 2. The first night I had my ASIAir in live mode so it did not automatically rerun autofocus. The 2nd night I used autorun mode and it appears to have autofocused after ~45 2min exposures. The ambient temperature dropped 6c over the course of that drift out of focus. What we believe therefore is needed with this scope to maximize its potential is to carefully acclimatize it before usage, such that the first focus run is at a stable temperature, and then aggressively rerun autofocus to 1c of change (I had my ASIAir set to 2 and it didn't pick up and rerun until at least 4c of change).  |
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I'm betting more for a shift of the objective in the tube this is what I've experienced from my side. huge shift in translation (this is why of course my last focuser movement is against gravity) medium shift in rotation too cheers |
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Thanks for sharing this! Will test this out once I get the AF installed.
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@jpsc01 can you detail what you mean and how you're mitigating it? I'm running autofocus once positioned towards the main target so I'm not sure how the objective would further settle against gravity. It should be clear enough for me to test the acclimatization tonight and/or Monday.
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That really seems like alot of shift. Have you ruled out the focuser slipping? Check your autofocus history throughout multiple nights, look for the focus point consistently drifting in one direction. Also, the Roki barely needs to autofocus more than once in a night. Even on nights with big temp swings, I've only ever refocused once using HFR as a trigger, the HFRs drifted about 0.01 over 6 hours. |
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@Brian Puhl thanks I'll check that too - what would I do for the focuser slipping, would it need to be tightened?
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Travis McGeehan: I'd just check for it first. You shouldn't be able to move the focuser by hand. Something this drastic would be obvious. You can also try moving the focuser by the opposite knob from the autofocuser. The autofocuser should stop you from going anywhere, aside from the usual backlash. Im not saying this is what it is, but I'd be looking towards mechanical issues. Seems like alot more than normal temperature drift. |
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I don't seem to be able to move the focuser by hand, but I did notice that the rotator is hard to keep clamped down and I do see more rotation over the subs than I'm used to from the roki, would the rotator slipping cause change in focus point?
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Brian Puhl: *Not if it is direct drive, only if you have reduction gears within the EAF. |
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Travis McGeehan: Rotation in your subs is more likely due to field rotation, poor polar alignment. If you can't move it by hand, I doubt it's your issue. |
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Ok, so here is all I've discovered with this APN objective. It's important to mention this is an APN objective as the focus consist to move the objective inside de tube here... -To reduce backlash you have to put the "fine focuser" ring in one of its course limit. Only one side is working to reduce backlash -if you have a medium to heavy camera (like me with the QHY600 + filter wheel), then the weight of the imaging train will make the objective to move in the tube which will make mandatory to guide with OAG instead of piggy back. -I've tried to build a lot of model with the 10micron 1000 and 2000 and all of them was catastrophic (30 to 60 arcs RMS), this was definitely showing lot of flexure in the tube which make guiding mandatory -you need to be sure the last movement of your focusing is against gravity when you are autofocusing -I've swapped the rotating ring by a simple ring CNC and it's a game changer (not for the flexure but more because I consider that the rotating ring is an anomaly for many reason)... -if you play with the extremity of the objective with your finger (when focused) you will see you can move the objective first up and next down and also rotate it.... Yes.. there is some play internally, you have to live with that (I'm suspecting some shitty plastic part not well adjusted inside the objective I'm talking about that on CN here https://www.cloudynights.com/topic/929708-askar-sqa55-55mm-f48-petzval-astrograph/?p=14030829 https://www.cloudynights.com/topic/929708-askar-sqa55-55mm-f48-petzval-astrograph/?p=13907345 Last but not least: sharpstar support is useless and pathetic. I've give up in the ticket I've raised with them.... cheers |
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Travis McGeehan: simple: with Nina autofocus, you can chose the direction of the last movement for backlash correction: take the one which is IN-focus (I mean pushing the tube against gravity) Also, I've set autofocus each 40 minutes max in my sequence and each 1°C diff cheers |
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I have the SQA55 as well with a ZWO EAF. I don't seem to be having the problem you are but I am running NINA with 3 focus triggers 1) HFR 10% 2) Temperature 3C and 3) Time 60mins. Below is part of the image history of my run last night spanning from around 7:30 pm to midnight and one of the focus runs:   |
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Ah ok thank you. I'm using an ASIAir plus to run everything, don't know if it has that option for the EAF.
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Travis McGeehan: Your backlash with a ZWO EAF on the A.A. is automatically zero. |
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https://www.astrobin.com/ghstby/ got 7 hours last night with the SQA55 acclimated to ambient temperature before imaging and the rotator more secured, and the FWHM stayed consistent at the bottom of the graph the whole session. The data is revealing a lot of background galaxies now! |
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Welcome to the world of longer refractors. I have noticed that our short FL rigs like Redcats and lenses seldom go out of focus and only set them to refocus every 2hrs. The Medium FL units (300-600) every 1-2hrs especially early in the evening when temp change is greater and our longer refractors (1000mm or more) every 30min for the first 3hrs and then once an hour. All aluminium tubes tube assy's are pretty similar, even Taks. There's a good reason many high end scopes utilise carbon fibre or some form of low expansion material. Another things is the better and faster the optics, the easier it is to notice the change in focus. Your SQA55 is both of these so refocus regularly (assuming other factors mentioned by others are all sorted) and enjoy the results. Your follow up image is a beauty btw Travis!
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I have not had an issue with losing focus of many degrees of temperature change with my sqa55. I sit along side my fried who uses an SCT and he is refocusing quite often. I set up focus and let my system run for hrs. I am paying attention and analyzing the images that come in but I do not seem to have an issue with focus on my scope. I have a zwo asi 533 mcpro and an asi air mini that I use and focus is done manually with a Bahtinov mask[b]. [/b]I will try and do some tests with my next clear skies to see if im just not paying close enough attention.
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