10 replies to this topic

[revans]

Do you have to shoot flats for each narrowband wavelength or is it OK just to do one luminance flat series and use it across the board?

 

I should know the answer, but I don't trust my memory and I'm doing some monochrome imaging tonight after a very long hiatus during which I only did OSC imaging.

 

Rick

[Enance42]

Flats for each filter. Each filter might have dust in different spots.

[revans]

Flats for each filter. Each filter might have dust in different spots.

It is a little tricky because I have a filter drawer setup and I have three drawers and four filters with the luminance. It is likely that putting them in and out is going to cause some degree of rotation and my dust motes might not line up well.  But it should be adequate if I post-process with care. The motes that show up best are probably the ones on the camera's protective window because it is closer to the sensor's.focus point.  Hopefully motes on the filters themselves won't be in focus and might dissipate.

 

Rick

[Oort Cloud]

Rick, if you're using a drawer, you should be taking flats after each filter change, not just for each filter.

If you want to reuse flats, the only way is to use a sealed, electronic filter wheel. Otherwise the dust motes can keep moving around every time you swap filters.

[revans]

Rick, if you're using a drawer, you should be taking flats after each filter change, not just for each filter.

If you want to reuse flats, the only way is to use a sealed, electronic filter wheel. Otherwise the dust motes can keep moving around every time you swap filters.

I see what you mean.  I was hoping that dust on filters would be far enough from the focus point not to be an issue and that practically speaking only motes and watermarks on the protective window would be real factors in blemishes showing up.  But I don't know that for certain and I tend to think your approach, being the more cautious, is more correct.  There goes my usual twilight/pillow case method of doing flats if I have to also do them during the imaging run because of the filter drawer setup.

 

Rick

[bobzeq25]

Flats do a lot more than dust. My approach is to shoot one filter per night, doing flats after each without touching the optical train.

[revans]

I suppose the most correct thing for me to do would be to take flats during the imaging run itself every time a filter is changed and before using it to image.  The problem there is to get the same illumination.  I'd have to use a white LED panel screen instead of twilight light as I usually do with OSC. 

 

Even though it may cause me some issues, I think that tonight I'm going to just do flats with each filter at twilight and call that good enough.  I think I could deal with any residual issues in post-processing.  It would really interrupt the work flow during the imaging run to stop and get flats with an LED panel each time I change a filter in the filter drawer.

 

It is a compromise but you have to start someplace and I'll see what happens. 

 

I'm glad I asked because my original inclination was just to do one flat using a luminance filter and use that across the board.  At least getting flats at twilight for each filter seems to be at least closer to moving in the right direction.

 

Rick

[archiebald]

Sorry for the question, I'm just curious why you can't add a filter wheel?

 

I keep looking at moving to a mono setup and the wheel itself is about the cheapest element considering the price of mono cameras and a good filter set.

 

Or is it a mechanical / space issue? Are you on a RASA / Hyperstar setup?

[Oort Cloud]

Sorry for the question, I'm just curious why you can't add a filter wheel?

I keep looking at moving to a mono setup and the wheel itself is about the cheapest element considering the price of mono cameras and a good filter set.

Or is it a mechanical / space issue? Are you on a RASA / Hyperstar setup?

I'll let Rick answer for himself, but I know he has a tendency to keep things simple. More electronics, more possible points of failure/things to troubleshoot. But the flip side is more complicated flats from introducing dust during filter changes. Personally, I love my EFWs. Right up there with autofocus.

[revans]

Sorry for the question, I'm just curious why you can't add a filter wheel?

 

I keep looking at moving to a mono setup and the wheel itself is about the cheapest element considering the price of mono cameras and a good filter set.

 

Or is it a mechanical / space issue? Are you on a RASA / Hyperstar setup?

There is no good reason for me not to add a filter wheel to the rig.  But I'll give you an example.  I added an autofocuser to one of my refractors to be more efficient.  It worked fine, but it took a long time for the device to find focus each time it made an adjustment.  I decided that I knew my temperature changes and my focuser well enough to do it quicker manually.  After a while, you get experience and know when to make a focus change as the temperature changes and you know how much to tweak the focus... and it just takes a second.  The issue was that I needed a numbers based system to know when I was at focus and I finally got that with the FWHM star averages for each exposure or in Live View in Astrophotography Tools. 

 

To me, the filter wheel is analogous.  It is easier for me to pop a filter in the filter drawer than to worry about another software glitch that could occur (although there are manual filter wheels and maybe I should go that route).  Ideally some people think you need the filter wheel because they believe it is best to run through the filters for each sub-exposure rather than to do them in batches.  Doing them in batches... if clouds come in... you might not get one of your filters into the mix. 

 

I think as you get older (like me) you distrust software and glitches and complexity and try to be as impervious to these potential misfortunes as possible.  Not that this is a good approach, but it fits in well with my character and approach to life   I won't win any awards, but I won't come in last either...

 

Rick

[Oort Cloud]

Rick, manual filter wheels are not sealed, so it won't change the fact that you'll need to redo flats regularly. Dust can easily enter through the gap that allows you to spin the wheel by hand.