Takahashi FSQ85ED dropping out of focus

[x6gas]

Hi all,

I've noticed that my Takahashi FSQ85 has been dropping out of focus during imaging sessions over the past few nights. Analyzing the subs with CCD Inspector shows the FWHM value doubling over the course of a few hours I've been imaging near zenith, so it might be that the focuser is slipping but I suspect that it is down to the thermal contraction of the aluminium tube as it cools.

Anyone else had this problem with the FSQ85?

I'm wondering if there is something to do with the high temperatures - ambient is changing from ~18-20 degrees C to ~10-12 degrees C.

Be interested to know the experiences of others.

Thanks, Ian

[Freddie]

Hi Ian, no experience with FSQ85 but my MN190 changes with temp.

[ollypenrice]

Hmmm. One of the big positives for me with the FSQ85 was its ability to hold focus. Not everyone I know finds the same, though, and my own scope, now Sara's, seems to shift a little for her when it didn't for me. Yves finds the same. His moves a little. Petzvals are certainly highly temp sensitive, though. EE Barnard found this in the 1920s! It may be environmental, then.

The focuser, assuming it's adjusted, will not slip. It's very good. I never used the lock screw. You can check it with a Vernier to see if it's slipping. I'd suggest using a dew heater even when there is no risk of dew so as to slow down the thermal effects. After that, well, you have to join the FSQ club ad check (or robotize) focus regularly. I'm now using an FSQ106N (older Fluorite) and finding that one or two refocus jobs per night is all I need but I do run the dewheater all night.

Olly

[x6gas]

Thanks for the reply, Olly.

Interesting that you never needed to use the lock screw. I "think" I need to use mine, but I left the focus in the same position and checked focus at the beginning of the session on two different nights, at the same ambient temperature (20 degrees) and it was bang in focus, but as the temperature dropped tonight I could see the focus dropping off. I must admit I'm surprised at the sensitivity... but it may just be that the scope has been very warm all day... we'll see.

Anyway, thanks again!

Ian

[ollypenrice]

I don't think it will be to do with daytime temp because the daytime temp here inside the observatories reaches 40+ in summer. Very odd.

Olly

[x6gas]

I don't think it will be to do with daytime temp because the daytime temp here inside the observatories reaches 40+ in summer. Very odd.

Olly

Ah well that's interesting to know; I have mine under a Telegizmo's 365 cover (no obsy) which I hope doesn't allow the temperature to get too high so, yeah, it is a bit odd. By the way, I do run a dew heater on about 20% power throughout the session and I have it on the tube right behind the dew shield. Not sure if that's the optimum position or if that might contribute in anyway.

Thanks again, Olly.

[ollypenrice]

If you're not short of electricity (ie not running from a battery) I'd try turning up the heat as the evening prgresses. I've also wondered about insulating my FSQ to slow down the temperature drift.

Olly

[swag72]

As Olly noted, I do get a little focus drift in the FSQ85, whereas he got none!! I'm using dewbands on full power from the moment I switch on and it has made a difference. Due to that I can only assume that it really is temperature related. I certainly notice a shift if I've been out in the evening and haven't let the scope cool down properly before starting imaging, so it gets a long cool down now!

[x6gas]

Thanks for the reply Sara. Interesting point about running the dew bands on full whack; power isn't an issue for me so I may try that and I think I'll take the scope off of the mount and store it somewhere cooler during the day.

This isn't an issue with my TS90 which had a carbon fibre tube. The light cone is also less steep so focus a bit more forgiving. I'm guessing this becomes a real issue with the reducer fitted to the Baby Q?

[swag72]

I suppose theoretically having the reducer on can make the issue more noticeable - I've not imaged without, so I'm just used to it now!

[ollypenrice]

Also worth remembering that Sara is imaging with smaller pixels than I was so her system's higher resolution is less tolerant.

I found the reducer perfectly straightforward to use, though.

Olly

[melsky]

Definately have to use the focus lock on my new FSQ85 and I've noticed considerable image movement when focusing at certain angles, is ther someway of tightening thing up?.

The focuser is very poor compared to the Crayford on my William optics FLT110.

Mel

[Brian Nordstrom]

My Sky90 started doing this a few weeks ago and it was a simple matter to take off the cover that covers the wheel adjusters on the focuser , tightening the 4 screws a tiny amount to stop the slipping .I removed the holder ( round about 15mm in diameter with 2 small holes in it ) with 2 pins and a screw driver , placed the 2 pins in the 2 holes on the round holder that holds the cover in place and used the small screw driver between them to unscrew the round holder , the 4 adjustement screws are under this cover just like any focuser is , only way better built . Mine took less than 1/8th a turn on each of the 4 screws , a tiny amount evenly on all 4 . Brian.

[ollypenrice]

Definately have to use the focus lock on my new FSQ85 and I've noticed considerable image movement when focusing at certain angles, is ther someway of tightening thing up?.

The focuser is very poor compared to the Crayford on my William optics FLT110.

Mel

Ouch, that's bad! Given the abominable nature of WO Crayfords (in my experience) I think your Tak focuser must need adjusting. They are normally second best to Feathertouch. I don't know how to adjust them but the focus lock does work. It produces a little shifting but if you run it tightish you can still move the drawtube. You're sure this is not thermal in origin? It really is drawtube movement?

Good post from Brian if it is slipping.

Olly

[x6gas]

Quick update from me: I took the scope off of the mount and stored it in the cool inside, slapped it on the mount last night and gave it ~an hour to cool down and the focus stayed bang on all night. This is quite a difference as on nights when the focus was moving off it kept getting worse all night long!

So a bit of an odd environmental effect going on for me, somehow... Anyway, autofocus solution purchase averted, at least for the time being.

Thanks for all the help and advice on this thread.

Ian