First go at M33 battling the wind and cloud. Flats or no flats?

[RSM]

Finally, after two weeks of constant cloud, there were some clear skies over the South East of England at the end of last week. High winds were forecast but after two weeks of peering at the orange glow of the cloud, I was determined to get something out of the clear skies. After a session cut short to 5 mins on Thursday, I was more optimistic with the forecast on Friday. Nice clear afternoon, great sunset, and dark skies with no moon. Set everything up and took a quick look at M15 cluster before it dipped below the trees and then was going to switch to the Pleiades for some imaging. To the naked eye, they looked great, but for some reason I decided to see if I could find the Pinwheel Galaxy (M33) as the skies were good and dark, I'd seen some other great images of it on SGL and was up for the challenge. I checked my alignment on M31 which thankfully was pretty good so hit the button to swing around to M33. As my eyes were not adjusted to the dark, I focussed on a nearby star and then took a couple of 60 sec frames to see what I could see. I have to say my heart raced when I got the first frame of M33 on the camera screen. A distinct spiral of a galaxy pretty much dead centre frame. Yesssss! After a quick alignment to get the position I wanted, I started checking how long I could ramp up the exposure time. So far, my best attempts at alignment meant I could stretch to 45 seconds, but as I'd spent a bit of time aligning the reticle in my polar scope earlier in the week, I found I could get a 90 second exposure with no star trail.

Set the camera going to take my subs and feeling really good, popped inside for a celebratory cuppa. When I came back out about 15 min later, I found that most of my 90 second frames were blurred, smudged, ghost stars. Had no idea what was causing that until a strong gust of wind hit me in the back. Scope wobble I guess. To get something out of the session, I reduced the exposure time back to 45 seconds and started again only to then get an unforecasted band of cloud coming in! On and off over the next couple of hours, I managed to grab 27 half decent subs and when the cloud was thickest took my darks and bias (about 15 of each). Didn't bother with flats, had mixed results with them in the past and had some good results without.

Just processed what I managed to take in pixinsight this evening. Below is the result, which given the quality of the subs I took, I'm pretty pleased with.

Given I didn't use flats, does anyone have feedback on the difference taking flats and using them makes when shooting DSO? Maybe I'm not yet at the stage where the quality of my images needs the flats as I don't see a huge difference between having them or not, but am interested to hear more expert views than mine.

Thanks for reading. Can't wait for the next clear skies as I'm going to have a good go at M33 again.

Richard. 

[laudropb]

Nice capture given the conditions. Thanks for posting.

[Knighty2112]

Yeah, I could just make out M33 in my 15 x70 binos tonight when we quickly had a break in the clouds here too. Was going to get the scope out too, but the clouds rolled in before I could even get back in to get it. Was nice and clear and pretty dark too, especially as I am swamped by light pollution where I live.

Nice photo. Well done!

[Astrosurf]

Excellent job especially for such short exposures. I can understand your excitement when it appeared on the screen. I admire your perseverance!

Alexxx

[Starlight 1]

Was not a good as wind go but a clear sky it was, well done.

[cjdawson]

If you haven't altered the focus of your scope, there's nothing stopping you from taking flats and reprocessing to see if you can see the difference. I think the main thing is that you have to get the flats just right otherwise they can make things worse rather than better.

I understand that flats are supposed to compensate for dust motes and vignetting. However, I've only had one attempt with flats and frankly, I wasn't too impressed - maybe I was taking them wrong.

I suppose the real question here is are you happy with the result? If so, don't bother with the flats ;-)

[wimvb]

Congrats on a very nice image, in these cloudy times.

As for flats; I recently started taking flats to save a vignetted noisy image and I was very impressed with the quality improvement.

If you take flats and process them correctly (I use Pixinsight for calibration and stacking), you get a much flatter background.  I found that subsequent processing was much easier, even if getting rid of vignetting in PI is possible without flats.

For wide field images, I just point my camera at an evenly illuminated wall or door, keeping focus at infinity and the camera as Close to the wall/door as possible without creating shadows. It's quite easy to fire away some 40 frames to create a master flat. I don't care about colour cast in the flats as this will be taken care of in the DBE process in PI.

I haven't done flats on images taken with my scope yet, but intend to do so as soon as those .... clouds move out of the way.

[RSM]

Thanks for the advice. I've taken them before by putting a white t shirt over the end of the scope and pointing a big white light torch at it, whilst setting the exposure time on the camera to get the histogram somewhere in the middle. I used the same ISO and focus as I took the lights.

[wxsatuser]

Given I didn't use flats, does anyone have feedback on the difference taking flats and using them makes when shooting DSO? Maybe I'm not yet at the stage where the quality of my images needs the flats as I don't see a huge difference between having them or not, but am interested to hear more expert views than mine.

Thanks for reading. Can't wait for the next clear skies as I'm going to have a good go at M33 again.

Richard. 

Don't need flats if you have PixInsight.

DBE and ABE can take care of vignetting, gradients and light pollution.

Gradients and light pollution need to be subtracted and vignetting needs to be divided.

[Pete Presland]

M33 is a tough target visually unless you have good skies, i have not seen it from my back garden. I also had a similar experience as yourself when i first imaged it. I knew i was in the correct place by the stars, but nothing better than seeing the first exposure to confirm it. 

For such short exposures that is an excellent image. What equipment were you using?

[Tim99]

Nice job Richard!  It is exciting when you see that image pop up on the screen (confirmation that you have done everything right,)  I am also just starting to work with Dark, Bias and Flat frames.  Anything we can do to subtract the noise will help our final image. 

Thanks for posting your image and story.  The wind can really cause problems, especially with bigger scopes.  The other night, I had a nice gentle breeze.  That was good.  It kept the skies clear - no dew that night.

Tim

[bendiddley]

nice image, i'd be pleased with that if i'd taken it.  i've only done m31 so far, I'd like to do m31 at some point. 

[RSM]

M33 is a tough target visually unless you have good skies, i have not seen it from my back garden. I also had a similar experience as yourself when i first imaged it. I knew i was in the correct place by the stars, but nothing better than seeing the first exposure to confirm it.

For such short exposures that is an excellent image. What equipment were you using?

Hi. I have a Skywatcher 200 PDS on an HEQ5 with synscan and the images taken on a standard Canon 500D. No additional lenses (barlows, etc used). All processing was done in PI.

[RSM]

Nice job Richard! It is exciting when you see that image pop up on the screen (confirmation that you have done everything right,) I am also just starting to work with Dark, Bias and Flat frames. Anything we can do to subtract the noise will help our final image.

Thanks for posting your image and story. The wind can really cause problems, especially with bigger scopes. The other night, I had a nice gentle breeze. That was good. It kept the skies clear - no dew that night.

Tim

Hi Tim, I find the DBE function in PI is great at getting rid of background.

[Tyson M]

Wonderful image! 

[Pete Presland]

Hi. I have a Skywatcher 200 PDS on an HEQ5 with synscan and the images taken on a standard Canon 500D. No additional lenses (barlows, etc used). All processing was done in PI.

Very nice set up, the same as myself. I am sure next time with less wind and longer subs you will be even more amazed at the detail showing.

[badger111]

Very nice

[Ouroboros]

Good image given the gusty weather. You should do really well with longer exposures on this target and a decent amount of imaging time.

Flats make a noticeable improvement on targets which fill the field of view of the image captured. Well worth doing and getting right IMO. But then I don't use Pixinsight.

[Knight of Clear Skies]

Nice image. I can't see any vignetting so I don't think it flats would improve it much.

I've started taking flats now by pointing my camera at a laptop screen but I'm having problems with them, more often than not I end up with a distorted or over-corrected image. I'm using Av mode on the camera but possibly the exposure time needs to be reduced.

[ollypenrice]

Well done on a good result.

Are flats imortant? Yes. Are they easy? No. They can be devils.

The harder you stretch your raw data the more imortant flats become, so if you are happy with a moderate stretch you may get away without them. If you are chasing faint data just above the level of the background sky you absolutely do need them and that's that, at least in my experience. In any event you should start to engage with them because, without them, you cannot move up towards difficult images.

Olly

[RSM]

Thanks Olly, that sounds like good advice. I'm reliant on DBE to do a good job of background removal, which it does well, but interesting to read your comments about how flats can help with more difficult images.