EAA with Flat Frames

[PeterC65]

Last night I managed an EAA session in between the cloud banks. It remained clear as I let the scopes cool, then the first bank rolled in just as I was getting started. I had the Explorer 200 with x1.7 Barlow paired with the 72mm refractor, hoping to observe some faint galaxies and galaxy clusters.

Last time I used the Explorer 200 with the x1.7 Barlow I kept getting distracted by dust bunnies, so I’ve purchased an A4 light pad and this time I tried using flat frames. I’ve done EAA with dark frames before, with mixed success, but this time, with the flat frames those dust bunnies were gone, and I was seeing much more detail. The dark frames take a few minutes to capture but the flat frames are quick, and I captured them for all of the filters that I might be using.

As a test, I started with M43, just observing de Mairan’s Nebula with the Explorer, and it and M42 with the refractor. Using the flat frames I was able to see much more detail in the nebula. Quite an astonishing difference, so good that I captured flats for the refractor too. Not quite so much improvement with the lower magnification, but perhaps better colour rendition.

I then worked my way through a bunch of smaller galaxies and galaxy clusters.

All four of the Leo Quartet, NGC3185 / NGC3187 / NGC3189 / NGC3193, just fitted into the field of view of the Explorer 200 even with the Barlow.

That wasn’t possible with the Leo Triplet, so I concentrated on the Hamburger Galaxy, NGC3628, which is my favourite of the three.

The UFO Galaxy, NGC2683, was a new object for me and with the flat frames it showed me lots of detail with some nice dust lanes.

It looked like M81, another favourite of mine, would be a bit too big for the field of view with the Explorer 200 but it just squeezed in and was the best I’ve seen it, partly due to the flat frames and partly due to the higher magnification I think. The snapshot from last night looks even better with a bit of post processing (below).

M82 also looked good, if perhaps a little too purple for my liking. The flat frames seem to boost the blue channel for some reason. M51 also looked the best I’ve seen it.

Markarian's Chain is always a good place to go for small faint galaxies. I could see most of the Chain with the refractor and just about make out the strange shape of NGC4438 with the Explorer 200.

Last up was the Silver Streak Galaxy, NGC4216. This is a nice looking small galaxy that just happens to currently contain a supernova, SN2024gy. I’d observed it four weeks ago and wanted to see if the intensity of the supernova had reduced, which it had. This time with the flat frames I could also observe more detail, and with the refractor, its two companion galaxies, NGC4206 and NGC4222.

 

[Jonathan_Shields]

This is interesting. I have never used flats because frankly I can't be bothered and don't usually use darks as they seem to make very little difference with the ZWO585 as I think the amp glow is minimal on EAA length exposures (I am using a goto dob so I've never gone over 10s)

I think i should get off my backside and at least try flats as really it sounds a 5 minute task. I have a tablet i can just point at blankwhitescreen.com or similar. Do you think a white t-shirt is needed over the tablet or will that suffice?

[Xio1996]

Hi Peter,

Great images. It looks like you had a very enjoyable observing session.

Do you have any thoughts on why your flats improved the detail in your images? 

Have fun.

Pete

[PeterC65]

3 hours ago, Jonathan_Shields said:

This is interesting. I have never used flats because frankly I can't be bothered and don't usually use darks as they seem to make very little difference with the ZWO585 as I think the amp glow is minimal on EAA length exposures (I am using a goto dob so I've never gone over 10s)

I've used dark frames before and found they made little difference. Maybe slightly darker background, but they also seem to mess up the black offset (move the peak to the left, too close to zero). So mostly I've been using the SharpCap Hot Pixel Removal feature which seems to work well at removing hot pixels, which I do get with the IMX585 camera.

I've been using 15s exposures for EAA recently and that doesn't slow things down too much. For the same total exposure time I get noticeably more detail with 15s exposures than with the 4s exposures I was using previously. Taking 20 dark frames at 15s does take a while though. I had a spare slot in my EAA filter wheel which is now occupied by a dark filter and that makes life a bit easier.

While I do need to place the light pad over the objective when taking flat frames, the capture process is very quick, and they are actually much less trouble to take than dark frames. I might try just taking flat frames and not bothering with the more time consuming dark frames.

3 hours ago, Jonathan_Shields said:

I think i should get off my backside and at least try flats as really it sounds a 5 minute task. I have a tablet i can just point at blankwhitescreen.com or similar. Do you think a white t-shirt is needed over the tablet or will that suffice?

As long as the light source does not have a gradient I think you should be fine. This is my second light pad as the first one did have an obvious gradient. I think the light source is so close to the scope that it will be way out of focus and that diffuses the light. I've read that some LED backlights use pulse width modulation for brightness control and that can be seen if flats are taken with short exposures, but I've not had that problem with my light pad which is operating at its minimum brightness setting.

 

[PeterC65]

1 hour ago, Xio1996 said:

Great images. It looks like you had a very enjoyable observing session.

Do you have any thoughts on why your flats improved the detail in your images? 

Well they certainly remove any optical pathway anomalies and that makes quite a difference in itself. The last time I was using high magnification (8" Newtonian with x1.7 Barlow) I was very distracted by the dust bunnies. In daylight the mirrors looked clean though, and I could only spot tiny dust specks with a torch. I think I will always use flats now when working at high magnification. Maybe not for lower magnification widefield observation.

As for the improved detail, it felt like the flats were stretching the available light across the display range. The histogram seemed to be showing that as the peak was much wider than usual and I didn't need to use the zoom check box to see it.

It may be that I was getting a kind of pre-stretch, or it could just be the removal of the optical anamolies.

 

[Xio1996]

1 hour ago, PeterC65 said:

Well they certainly remove any optical pathway anomalies and that makes quite a difference in itself. The last time I was using high magnification (8" Newtonian with x1.7 Barlow) I was very distracted by the dust bunnies. In daylight the mirrors looked clean though, and I could only spot tiny dust specks with a torch. I think I will always use flats now when working at high magnification. Maybe not for lower magnification widefield observation.

As for the improved detail, it felt like the flats were stretching the available light across the display range. The histogram seemed to be showing that as the peak was much wider than usual and I didn't need to use the zoom check box to see it.

It may be that I was getting a kind of pre-stretch, or it could just be the removal of the optical anamolies.

 

Thanks Peter, for the information. 

On my Altair Astro 294 colour camera I use a flat as the vignetting was too noticeable when I stretched the image, or as in your case, I had dust somewhere in the system. I do zoom the histogram in SharpCap for the colour camera but not for my mono 533 camera. It has a broader peak than the colour.

For the ZWO ASI 533mm sensor I don't use darks or flats. Like you, I find SharpCap's 'Hot and Cold Pixel' removal works very well. I also use 'Low Frequency Wave' background subtraction. Where the 533mm's field of view is much smaller than the 294 colour, I found that  I could get away without using a flat. The background subtraction method, to my eyes, improves on the 'flat-less' view in most conditions.  I will have to make a flat for the 533 and compare the view. I was so happy with the 533's image that I didn't bother to make a flat in the past. 

The only other thing I do, is dither every 4-5 frames using SharpCap's non-guiding dithering functionality. That really cleans up the image when combined with sigma-clipping. SharpCap is a wonderful tool 😃

I'm looking forward to seeing your next observing session.

Pete

[PeterC65]

7 minutes ago, Xio1996 said:

I also use 'Low Frequency Wave' background subtraction.

I've also used this background subtraction but I tend to use it only when I can see an obvious gradient, such as when the Moon is bright. I have some dew shields on order for the two Newtonian's which will hopefully reduce glare from the Moon and other stray light sources, and also reduce dew build up on the secondary which I think is a problem during longer sessions (I have dew heaters for the primary which solves the dew problem there).

One thing I have noticed is that if you leave the low frequency wave background subtraction turned on and then observe something bright like the Moon or Jupiter the background can turn a bright colour. This threw me for a while and I was thinking that there was something wrong with my setup.

It will be interesting to see if flats are useful without darks. The last EAA session I did, it took so long to setup, cool, and calibrate, that by the time I was ready to observe the clouds had rolled in. I need to be able to get started quicker.

16 minutes ago, Xio1996 said:

The only other thing I do, is dither every 4-5 frames using SharpCap's non-guiding dithering functionality. That really cleans up the image when combined with sigma-clipping. SharpCap is a wonderful tool 😃

That's interesting that you use dithering. A couple of months back, I put a lot of effort in to being able to control the mount in such a way that I could use the SharpCap non-guiding dithering function, but when I tried it, it didn't make much difference. How do you use it? I often have two scopes and cameras on the same mount and I think that probably makes dithering impossible, but I could use it on just one scope.

 

[Xio1996]

Hi Peter,

That's interesting, I find dithering gives a smoother, less noisy background.

Usually, I dither every 4 frames with a movement of 15px. Though, I think the 294 used 30px.

I would be interested what you think when using it with one scope.

Pete

[PeterC65]

1 hour ago, Xio1996 said:

That's interesting, I find dithering gives a smoother, less noisy background.

Usually, I dither every 4 frames with a movement of 15px. Though, I think the 294 used 30px.

I would be interested what you think when using it with one scope.

I'll give it another try with the settings you suggest. Is it more useful for widefield / narrowfield / both?

 

[Xio1996]

Hi Peter,

I'm far too lazy to unscrew the f6.3 focal reducer from my scope!

A CPC-800 at f6.3 is always the ideal solution for all observing sessions 😄

Have fun dithering.

Pete

[Jonathan_Shields]

On 17/02/2024 at 17:49, PeterC65 said:

It will be interesting to see if flats are useful without darks. The last EAA session I did, it took so long to setup, cool, and calibrate, that by the time I was ready to observe the clouds had rolled in. I need to be able to get started quicker

I tried taking flats just using a tablet with a white screen and following the sharpcap instructions. I adjusted the exposure so the histogram peak was at about 50%.

I then had an EAA session looking at some of the Leo galaxies.

I noticed some "reverse vignetting" where the edges of the image were brighter. However given what i am interested in is in the middle usually it wasnt so bad. 

I didn't use darks just hot and cold pixel removal as usual.

The results were certainly not worse.

I will persevere maybe trying a white t shirt or some sheets of paper over the tablet next time.

Edited February 21 by Jonathan_Shields

[Martin Meredith]

I believe the reverse-vignetting (overcorrection) will disappear if you use darks and flats together.

[PeterC65]

I read recently that you should always use darks when using flats, so I should probably make the effort to capture both each time I do EAA.

The issue for me is that the setup process for EAA is getting longer and longer. I have to setup the kit from scratch each time, then I usually let the scopes cool before focusing so that I just need to focus once, then there is the capturing of darks and flats. My last session was visual and the setup for that was much faster, no cables to deal with and I could use the scopes right away as I was sat next to them to focus. OK, I didn't see as much but I'm thinking that visual is my grab and go.

[Xio1996]

39 minutes ago, PeterC65 said:

I read recently that you should always use darks when using flats, so I should probably make the effort to capture both each time I do EAA.

I agree with Peter and Martin. Flats and Darks go together, or much weirdness will happen in SharpCap. I believe there is a post in the forum on the subject by Robin.

Talking of Robin 😀 Here is an interesting quote, about people such as myself, that use background subtraction as a flat replacement.

Quote

I must admit that I didn't realise people were trying to use background subtraction for vignetting correction/as a flat substitute. Yuck! It wasn't ever designed to do that and because it's purely subtractive then it won't do a great job at all. Still, I suppose it might be better than nothing for some situations.

Robin, then goes on to discuss synthetic flats New Feature : Background Subtraction - Page 6 - SharpCap Forums I hope he implements the solution. Then I won't feel so yucky using background subtraction 😁

If I ever get a clear sky without it blowing a gale I want to try out a dark/flat on the 533mm. However, I would much prefer to use Hot and Cold pixel removal and a synthetic flat.

Have fun.

Pete

[Jonathan_Shields]

Thanks all.  Ok maybe next time i will try with darks as well.  If i just stick to one exposure time its not so bad. 

I agree with Peter though too much preparation time kind of sucks the life out of it but its worth experimenting to see what kind of difference these things make.