Focus position versus temperature...and also side of pier?

[Tom Krajci]

See attached jpeg.

This is the analysis of focuser position/temperature from last night's log for the AAVSO Bright Star Monitor (BSM)...CGEM mount, 60mm Tak, MoonLite focuser.

Data was collected automatically using AcquireStar in FocusMax, with ACP Scheduler doing a focus run every hour for the entire night as it slewed everywhere...covering probably 150 targets..

I did not do a rigorous check to see what side of the meridian each focus run was performed...but the pattern seems to be that one side of the meridian gives a focus position different from the other side of the meridian...so based on that assumption is how I parsed the data set to east/west sides of the sky. (I know the first focus runs, at warmest temperature, were performed east of the meridian.)

...this may be an interesting trend that many GEM users discover about their rigs if they take enough data. But we can't answer that unless we get more data from more scopes and different types of mounts.

Anyway, I think I have a decent value for focus/temperature slope for this rig. (But does FocusMax allow me to enter a non-integer value into the temp/focus slope data entry box?)

Progress is made, but with some bumps along the way....

PS. Yet another reason to hate German mounts. I need to weld more forks. But yes, I also need to find and eliminate the flop/flexure/shift that the German mount exacerbates.

[Bill Mattil]

See attached jpeg.

This is the analysis of focuser position/temperature from last night's log for the AAVSO Bright Star Monitor (BSM)...CGEM mount, 60mm Tak, MoonLite focuser.

Data was collected automatically using AcquireStar in FocusMax, with ACP Scheduler doing a focus run every hour for the entire night as it slewed everywhere...covering probably 150 targets..

...this may be an interesting trend that many GEM users discover about their rigs if they take enough data. But we can't answer that unless we get more data from more scopes and different types of mounts.


PS. Yet another reason to hate German mounts. I need to weld more forks. But yes, I also need to find and eliminate the flop/flexure/shift that the German mount exacerbates.

Tom,

Personally I think you are tilting at windmills here. I can't for the life of me figure out how the "mount" has anything at all to do with the focus position. This is going to be a function of the optical characteristics of the instrument and that of the focuser. Until those are removed from the equation any suppositions made regarding the mounts contributions are putting the cart way before the horse.

I also think that any testing should be done at medium focal ratios and not at something less than f/5 or f/6. The focus tolerance then becomes less of a factor.


Regards

Bill

PS: You are completely entitled to your hatred of German mounts. But to state that a German Mount exacerbates Flexure in some way also seems disingenuous to me. What a German Mount does is to bring to light problems within the Optical System. All it does is *reveal* what is already there. Not the same thing. Furthermore German Mounts can be had at a given price/quality level and outperform Fork Mounts at that same level. Until someone decides to spend in excess of $10K - German Mounts present an attractive alternative.

In short I am firmly in the camp of those who believe that you need to fix the Optical System first.

[Tom Krajci]

I can't for the life of me figure out how the "mount" has anything at all to do with the focus position.

That's because you don't test anywhere near as much as I do. I've seen things you probably haven't. And what I've seen is not exotic or rare. I just pay attention to it.

Example: One C14 OTA here used to ride on a horrible German mount (Paramount). I'd focus on one side of the meridian...flip it...rats...focus doesn't hold...not even close.

That horrible German mount often will dip the OTA below the horizon during the nasty meridian flip.

That same OTA (unchanged, no tweaking) is now riding on a wonderful fork. OTA stays pointed skyward all the time. I focus on one side of the meridian, and after slewing to the other side of the meridian...focus holds.

An improvement, yes?

No more time-wasting meridian flips that point the OTA's nose in the dirt. Far less of a problem holding focus.

Sure, we'd all love to have a perfect system with zero shift/flop/flexure.

That's a childish myth. Stop believing in it. As you test more, and raise your performance goals and standards...yup, you're gonna find more problems that need fixing.

Don't worry, I fix the fast stuff fast...and keep a log of difficult items for next monsoon.

I also think that any testing should be done at medium focal ratios and not at something less than f/5 or f/6. The focus tolerance then becomes less of a factor.

What a silly notion. You test and evaluate the gear that you have...not some pie-in-the-sky wish list of gear that you don't have.

If you test your gear, you gain a greater understanding of its capabilities and limitations. Some of the stuff you can fix easy, some is harder to fix, and for some of it you develop work-arounds.

You like to call yourself data god (little g). Ok...bring on the data. We need more data from various rigs to better understand focus performance...versus filter bandpass, temperature, side-of-pier...etc....etc.

Don't show us that you know how to mash to go button on your rig. Show us that you are mastering your gear.

Until someone decides to spend in excess of $10K - German Mounts present an attractive alternative.

Keep on repeating that until you believe it. (This is starting to sound like words from a religious German mount fanatic.)

I took a different, heretical path.

I learned how to (crudely) arc weld steel with a welder you can get for several hundred dollars.

That empowers. That opens doors.

I welded up and fabricated a fork this summer. It now carries the C14.

Bye-bye Paramount. Bye-bye silly meridian flip, wasted time, and exacerbated focus problems.

Life is lots better.

This first fork mount is not perfect. But you can be sure the second one will be better. ;-)

BTW, scope number six up here just reached operational capability. Each one of them has quirks and limitations...but I've spent far less on all them (mounts, OTA's, CCD's, software, piers, shelters) then the cost of just one Paramount. And scope's seven, eight, and nine are in the garage and basement - awaiting refurbishment.

Toodles!

[Bill Mattil]

Example: One C14 OTA here used to ride on a horrible German mount (Paramount). I'd focus on one side of the meridian...flip it...rats...focus doesn't hold...not even close.

Sheesh Tom you are blaming the mount for the OTA's failures. I was hoping for an honest discussion here.

That same OTA (unchanged, no tweaking) is now riding on a wonderful fork. OTA stays pointed skyward all the time. I focus on one side of the meridian, and after slewing to the other side of the meridian...focus holds.

If the optical tube were capable of holding the primary mirror in a fixed position that wouldn't happen. So based on this you agree that your dislike of Equatorial Mounts is because of your use of inadequate OTA's ?

An improvement, yes?

Not in my mind. I'm an Engineer. I'd fix the problem which is the OTA. Tell you what though. I'll take that crappy old Paramount off your hands and give you two Meade Fork Mounts in an even trade. Then it would be a win win for both of us.

No more time-wasting meridian flips that point the OTA's nose in the dirt. Far less of a problem holding focus.

Time wasting.... this *is* a valid point. I'll agree that a meridian flip takes longer. As for the focus problem that's already been addressed.

Sure, we'd all love to have a perfect system with zero shift/flop/flexure.

My system has No shift/flop. Period.

That's a childish myth. Stop believing in it. As you test more, and raise your performance goals and standards...yup, you're gonna find more problems that need fixing.

Don't worry, I fix the fast stuff fast...and keep a log of difficult items for next monsoon.



What a silly notion. You test and evaluate the gear that you have...not some pie-in-the-sky wish list of gear that you don't have.

If you test your gear, you gain a greater understanding of its capabilities and limitations. Some of the stuff you can fix easy, some is harder to fix, and for some of it you develop work-arounds.

You like to call yourself data god (little g). Ok...bring on the data. We need more data from various rigs to better understand focus performance...versus filter bandpass, temperature, side-of-pier...etc....etc.

Don't show us that you know how to mash to go button on your rig. Show us that you are mastering your gear.



Keep on repeating that until you believe it. (This is starting to sound like words from a religious German mount fanatic.)

I took a different, heretical path.

I learned how to (crudely) arc weld steel with a welder you can get for several hundred dollars.

That empowers. That opens doors.

I welded up and fabricated a fork this summer. It now carries the C14.

Bye-bye Paramount. Bye-bye silly meridian flip, wasted time, and exacerbated focus problems.

Life is lots better.

This first fork mount is not perfect. But you can be sure the second one will be better. ;-)

BTW, scope number six up here just reached operational capability. Each one of them has quirks and limitations...but I've spent far less on all them (mounts, OTA's, CCD's, software, piers, shelters) then the cost of just one Paramount. And scope's seven, eight, and nine are in the garage and basement - awaiting refurbishment.

Toodles!

Facts are facts. A mount is not responsible for the focus of a given OTA. That's the purpose of the OTA. Thinking otherwise is, well a complete waste of time. Now you think it's easier to replace perfectly good mounts instead of fixing OTA's that are poorly designed in order to meet some arbitrary price point then that's your choice.

But I'll spare others from having to wade through this. I'm done.


Bill

[Tom Krajci]

My system has No shift/flop. Period.

I don't care if you call yourself an engineer. I don't care what plaques hang on your office wall.

Engineers don't say that. They say something like "it's below what I've bothered to measure"

Before I bothered to measure this focus shift stuff...I was blissfully ignorant.

Now I'm not ignorant of focus shift...so I'm not blissful. :-(

But the rigs up here keep getting better. :-) ....uhhm, no thanks to you. :-/

[Bob Denny]

Hey guys, can we tone this one down?