Flats and focus position

[Spaced Out]

Hi All

Just a quick one, I’m new to mono/filters so this may sound like a daft question !

If you have to make some tiny adjustments to focus for each filter, should you do your flats with those tiny adjustments made between each filter too ? I read that focus should be the same between filters and flats but is the point of focus absolutely critical for flats or could I get away with doing them all at the end of the night at one focus position ? Just trying to cut down on early morning faffing time, I know taking short cuts doesn't always work out tho !

Thanks

[tooth_dr]

Hi Gary. Focus isn’t that critical. I personally use luminance flats for all filters even though there is a little bit of difference between focus position. 

[Spaced Out]

Hi Adam, thanks for this. I'll just stick to a single focus position then it'll save time 👍.

I thought the point of flats was to remove imperfections (dust etc) on each filter so they need to be filter specific ?

PS - Loving your comet and noctilucent clouds there !

[endless-sky]

If you only have vignetting problems, you might get away with doing flats with only one filter (as stated above), or with just one focus position.

However, when trying to get rid of dust spots, if each filter has different spots, then you'll have to take flats for each filter. I also found that focusing was important (I haven't tried with a telescope, yet, but with my DSLR lenses, difference in focus between lights and flats meant that the correction spots of the flats would be either bigger or smaller than the ones in the lights, basically leaving a ring around where the spots used to be).

Shortcuts are fine, when you can take them. But when skipping 5 minutes might ruin 4-5 hours of imaging, I learnt the hard way that extra work always pays off at the end.

[Spaced Out]

2 minutes ago, endlessky said:

If you only have vignetting problems, you might get away with doing flats with only one filter (as stated above), or with just one focus position.

Ah, OK, I get it now...... Well my filters are very clean in an enclosed FW so maybe just the 1 lum flats for vignetting is all I need at the moment then ?

That sounds like a decent shortcut !

[endless-sky]

I don't have much experience with filters, because so far I only have one for light pollution (Optolong L-Pro). So I don't know if the amount of vignetting different filters produce is the same (and only depends on the rest of the imaging train) or if it is filter specific. I guess all I could think of is for you to try calibrating some lights you have already taken (so you don't potentially "ruin" your next imaging session by not taking flats that might be needed) with the flats of just one filter and see if the results are comparable to calibrating each light with the corresponding filter flat. If the results do not change by an appreciable amount, then you should be fine doing flats only with one filter. If that's the case, maybe do the flats with the filter that has a "middle" focus position between the filter that needs the "further in" focus position and the one that needs the "further out" focus position. This way you mediate the effects that a change in focusing might have in the resulting flat calibration.

[Xsubmariner]

All my filters were clean and clear of dust when I closed the filter wheel and yet all of them have collected some dust on the filters, some more than others. Note my kit includes Atik, ZWO and Starlight filterwheels, note none are immune.

It depends on what level of imaging you wish to achieve. I always create flats for each of the filters in the wheel that were used in the data collection. If I am too tired to create the flats at the end of the collection night, I generate them the next day before the system is used again and focus altered.

There are many threads on the forum that go into detail about calibration frames, it is worth reading a selection to understand better the role of calibration frames in image processing.

.

[Spaced Out]

34 minutes ago, Xsubmariner said:

 If I am too tired to create the flats at the end of the collection night, I generate them the next day before the system is used again and focus altered.
 

Thanks..... So you just use the last focus position you were using in that case, so using that same position for every filter ? 

[Xsubmariner]

Yes, I use the last focus position for all my filters. Yes it is a compromise to the perfect world, but one that is acceptable to the quality of image I am seeking. The problem is there have been so few ideal clear nights available these last couple of years and I am getting no younger.

[tooth_dr]

2 hours ago, Xsubmariner said:

It depends on what level of imaging you wish to achieve.

Hi Martin. I read that Olly off here uses one filter for a flat for all his filters (well most of the time) so if his standard is the bench mark then that is a reasonable level of imaging for me.

@Spaced Out Gary you will get differing views on this and some say one filter and some say all filters. I find dust appears to be consistent with all my filters so it must be on the camera or FF.   I have two scopes with 7 filters in each EFW.  Running off 14 sets of flats isn’t a 5 minute job with a camera that uses a mechanical shutter. With my light panel at one setting and using one piece of acrylic sheeting, my flats for lum could be 4-5 seconds. If I try then to take an Ha flat at the same configuration it can 30 or more seconds for a properly exposed flat. Then I need a corresponding dark flat.  For each filter i really need to run the flats wizard to get optimal exposure time.  For me it is much simpler and without negative consequences to have a single luminance flat to use for all filters.

Edited August 1, 2020 by tooth_dr

[Xsubmariner]

Hi Adam, absolutely agree that the choice of calibration frames collected is down to personal preference and what an individual wants from their images. 

 

[tooth_dr]

44 minutes ago, Xsubmariner said:

Hi Adam, absolutely agree that the choice of calibration frames collected is down to personal preference and what an individual wants from their images. 

 

Not that it matters since I’m still trying to get my dome slit pointing in the right place 🤪

[ollypenrice]

Here's my position for the record:

I do usually use a lum flat for everything and it nearly always works. On the rare occasions it doesn't I take flats per filter but that's very rare. This shows that dust bunnies are not generated by my filters but elsewhere - flattener or chip window. I can't speak for anyone else's setups but this seems to be the case for ours.

Also the lum layer will illuminate the rest of an LRGB image by definition, so a fully flattened lum layer will go a long way towards flattening the RGB anyway. This implies that narrowband imagers might want to do flats per filter because they're not necessarily going to use a luminance layer - though some do use Ha as luminance. Also NB data tends to take a harder stretch and have higher contrasts. For all that, I usually flatten my NB data with a lum flat but I don't use it to make NB images so much as NB-enhanced LRGB.

There are times when we cannot process all the data taken in one night here. A nice problem to have - but harmless shortcuts do have their place!

Olly

[tomato]

I’m fortunate to have a permanent set up now so tend to do flats the next day. I have looked at the flats from a set of LRGB filters and found the dust bunnies to be quite consistent across all of them, indicating as Olly says, that it is the dust on the sensor window that makes the biggest contribution. 
 

[Davey-T]

Just stripped down and cleaned the Atik314 and EFW2 on the SCT as the flats were starting to look like a tray of ring doughnuts now they're all similar except the red ones so looks like it's coming apart again.

Dave