Filter wheel inaccuracies

[DonPierce]

I am  looking into the ZWO 2" 5 position filter wheel. I have not had much luck finding reviews of this product for comparing to other filter wheels. My main concern is the following:

First i would image with multiple filters on the same night

Next, in the morning I need to generate flats for each filter.

I have read somewhere that some of the filter wheels do not position at the exact same location each time.

This would make generating flats frustrating as there would be alignment problems when you go to calibrate.

Anyone have any info on this situation and the product?

[Clarkey]

I have not used the 2" version but the 7/8 position wheel I have has been absolutely fine. I am not aware of any specific issues with the larger wheel but I am guessing most of the control is pretty the same. I do multi filter imaging and flats without a problem.

[jacko61]

Why not generate your flats after each filter run? 

Graeme

 

[Xilman]

12 hours ago, DonPierce said:

I am  looking into the ZWO 2" 5 position filter wheel. I have not had much luck finding reviews of this product for comparing to other filter wheels. My main concern is the following:

First i would image with multiple filters on the same night

Next, in the morning I need to generate flats for each filter.

I have read somewhere that some of the filter wheels do not position at the exact same location each time.

This would make generating flats frustrating as there would be alignment problems when you go to calibrate.

Anyone have any info on this situation and the product?

I can't speak for ZWO but the SX equivalent is very reproducible. A set of flats last me for weeks or months if I don't dismantle the optical train for whatever reason.

[malftobe]

Same answer as Clarkey above. I use the smaller wheel without issues - I am very new to filter wheels and dedicated cameras though.  
 

Just to add that in Asiair (and I assume in other software) you can select the filter wheel to only rotate one way. I read somewhere that this was best way to reproduce filter position . (I haven’t experimented with rotating both ways to see if it’s better - I just blindly follow others with my limited imaging time !!).  

Edited July 18, 2022 by malftobe
Spelling

[Richard_]

I imagine the repeatability aspect is due to potential back lash in the filter wheel mechanism. The ASCOM driver in some filter wheels allow you to change the behaviour of the rotation. You can choose between the filter wheel moving the shortest rotational distance to the next filter, or to only rotate the filterwheel in one direction (unidirectional). If you force the wheel to rotate in one direction only, you could eliminate any potential backlash in the mechanism and misalignment when shooting flats the next morning. 

Below is a screenshot from the EFW manual (v2.2) obtained from ZWO. Look at the lower right image where a check box says "unidirectional". If you check that box, the wheel will rotate in one direction. 

https://astronomy-imaging-camera.com/manuals-guides

[newbie alert]

It should have repeatable positioning... Loads of people use zwo fw,  so I'd assume that it's not the case for them all

You only need to redo flats if you move any component of the imaging train, ie take the camera off.. if you don't then no need to do them on every occasion ... 

[ONIKKINEN]

15 minutes ago, newbie alert said:

You only need to redo flats if you move any component of the imaging train, ie take the camera off.. if you don't then no need to do them on every occasion ... 

Have to disagree with this. Personal experience has shown me that with my newtonian the flats do have to be taken each time, and if OP is imaging with one (or an SCT) i would recommend to take the flats each time. Lower end refractors probably have some stability issues around the focuser too so with those the flats should also be taken that night.

Actually i would take flats each time anyway regardless of type of telescope, it takes 5 minutes at most and guarantees that the data is usable. No matching flats = No image.

[newbie alert]

1 minute ago, ONIKKINEN said:

Have to disagree with this. Personal experience has shown me that with my newtonian the flats do have to be taken each time, and if OP is imaging with one (or an SCT) i would recommend to take the flats each time. Lower end refractors probably have some stability issues around the focuser too so with those the flats should also be taken that night.

Actually i would take flats each time anyway regardless of type of telescope, it takes 5 minutes at most and guarantees that the data is usable. No matching flats = No image.

Interesting

What's your reasoning regarding this?

[ONIKKINEN]

3 minutes ago, newbie alert said:

Interesting

What's your reasoning regarding this?

Cant tell you an exact reason why this is the case, just the way i have noticed it with my flats.

But to guess a few: Temperature differences deforming the tube differently each night, focuser maybe in a slightly different position (also depending on the tube length - which is dependent on temperature), perhaps collimation was 1/10th of a turn of a knob different than last time. Many reasons with not-ideal scopes why flats suck. I have solved many issues, but not all of them so its a safe bet to just take the flats each night. With a modern camera it shouldn't take more than a few minutes anyway.

[Dan_Paris]

I had issues with the mini EFW (bad luminance calibration because the filters do not reposition exactly in the same position). My solution  is to shot the luminance flats at the end of the flat sequence, and then to start the image sequence with the luminance.  I read that enforcing unidirectional motion of the filter wheel would also solve the problem, but I did not try yet (in the fear of loosing a night of data).

[jacko61]

1 hour ago, ONIKKINEN said:

Cant tell you an exact reason why this is the case, just the way i have noticed it with my flats.

But to guess a few: Temperature differences deforming the tube differently each night, focuser maybe in a slightly different position (also depending on the tube length - which is dependent on temperature), perhaps collimation was 1/10th of a turn of a knob different than last time. Many reasons with not-ideal scopes why flats suck. I have solved many issues, but not all of them so its a safe bet to just take the flats each night. With a modern camera it shouldn't take more than a few minutes anyway.

And every time you take off the lens cover you're attracting new dust particles. As you say, 2 or 3 minutes to run a flats plan in APT so why wouldn't you take new ones every session?

Graeme

[Richard_]

Whilst a different brand of filter wheel, Rouzbeh talks about this with the QHYCFW3 filter wheel in the article below. Scroll down to the heading titled "Precision and Repeatability" which includes the discussion of setting the filter wheel to rotate in one direction only. 

https://astrogeartoday.com/qhycfw3-series-of-filter-wheels-reviewed/

I've also included videos where the author has rotated the filter wheel using both modes. You can see the misalignment caused by backlash when counter rotating. I wonder if the ZWO filter-wheels have the same difference? At least you can configure this in the ASCOM driver (the QHY requires a firmware update). 

Here's a video of his filter wheel running either way:

Here's a video of the filter wheel rotating in one direction:

 

[rnobleeddy]

Regardless of theoretical reasons not to, I haven't had any issues reusing flats with my QHY filter wheel. I always take new flats if the camera rotation changes, otherwise it seems to work fine between sessions.

 

I actually find flats quite time consuming, as I tend to do them indoors the day after. Practically, it'd be very hard for me to take a flat before every rotation of the filter wheel.

[Varavall]

2 hours ago, jacko61 said:

And every time you take off the lens cover you're attracting new dust particles. As you say, 2 or 3 minutes to run a flats plan in APT so why wouldn't you take new ones every session?

Because I like to sleep. NINA does all the work whilst I sleep, so taking flats that same night is not feasible.  However on the argument of temperature changes, my imaging could start at midnight at 30C dropping down to 20C over 4-5 hours, so taking flats before or after imaging, or indeed after each filter change, cannot avoid any effects temperature may have on the final image. Taking a flat after each sub may help, but that would be lunacy!

Edited July 18, 2022 by Varavall

[newbie alert]

5 hours ago, ONIKKINEN said:

Cant tell you an exact reason why this is the case, just the way i have noticed it with my flats.

But to guess a few: Temperature differences deforming the tube differently each night, focuser maybe in a slightly different position (also depending on the tube length - which is dependent on temperature), perhaps collimation was 1/10th of a turn of a knob different than last time. Many reasons with not-ideal scopes why flats suck. I have solved many issues, but not all of them so its a safe bet to just take the flats each night. With a modern camera it shouldn't take more than a few minutes anyway.

I'd question if flats are temp related? or a slight change in focuser position makes any real difference.. if you use the t-shirt or a luminance pad positioned on the end of the scope with the focus  is at infinity... If you take flats for the sun they're de focused and work very well indeed so why is it critical for deepsky.. majority of flat errors are because they're either over or under exposured ...can understand if using a newt that collimation might change if taking it on and off the mount.. if your thoughts are the same with a SCT then the collimation screws aren't tight enough... All well enough having those easy to adjust bobs knobs, but expect to collimate more often..

[ONIKKINEN]

3 hours ago, newbie alert said:

I'd question if flats are temp related? or a slight change in focuser position makes any real difference.. if you use the t-shirt or a luminance pad positioned on the end of the scope with the focus  is at infinity... If you take flats for the sun they're de focused and work very well indeed so why is it critical for deepsky.. majority of flat errors are because they're either over or under exposured ...can understand if using a newt that collimation might change if taking it on and off the mount.. if your thoughts are the same with a SCT then the collimation screws aren't tight enough... All well enough having those easy to adjust bobs knobs, but expect to collimate more often..

The temperature difference thing and its side effects are probably not a big deal, but its a problem i have control over - by taking flats each time, so no reason not to take that precaution. If for some reason someone doesn't want to take flats every time, then they have to figure out how to make flats work each time. I am guessing this is primarily a newtonian problem, but lower end scopes of any type probably suffer as well.

Taking my newtonian as an extreme example with flats taken indoors/in summer at +20c and the lights taken outside at -20c: 750mm primary to secondary distance changes by 0.66mm with a 40 degree temperature difference. 0.66mm is not nothing, its actually pretty large and the difference between a complete miss in focus and great focus. This means the focuser must be racked in by 0.66mm more to reach focus and if the focuser is not perfectly straight in its movement (i know mine is not) it will also move slightly sideways bringing the scope out of collimation (and causing flats/lights mismatch). This should be easy to test by taking flats right after lights, then racking the focuser in by 0.66mm and taking flats again. Calibrate lights with both sets separately and compare results. They should be identical if it doesn't matter at all, and im guessing they will not be identical.

Okay, well one could take flats in different temperatures and build a library. One could make sure their telescope is micrometer perfect in all mechanical aspects. Or one could take flats each time. Your choice, but i will be taking flats every single time i use my scope.

[jacko61]

17 hours ago, Varavall said:

Because I like to sleep. NINA does all the work whilst I sleep, so taking flats that same night is not feasible.  

I guess, because you're in Spain you don't have to worry too much about the weather suddenly changing through the night. I'm in Scotland so I don't have that luxury Regardless of what the BBC weather might say it's best to anticipate some rain through the night.  Anyway, I like to have a 'hands on' approach to my astronomy so I'll sometimes sit and watch the subs coming in and even if I've gone back into the house for a while I'll be back out to the observatory well before the end of a plan to check how they're looking. Means I can take my flats and dark flats straight after the session before I pull the roof back over my scope. 

Graeme