m51 from Bromley...

[JohnP]

OK - went out last night determined to sort out guiding... Made some progress so tried m51 again. Mn56 + atik 16ic & CLS filter. L=25X300secs + 4X300secs RGB. This is the first image I have done flats with & I  don't think I'll do an image without them again.... This is a lot harder to image from Bromley then Tuesnoad anyway here it is so comments welcome.

Cheers, John

[Mike]

Wow!! That's a fantastic image John.

[Daniel]

Wow! makes a mockery of the whirlpool shaped smudge I was quite chuffed with yesterday! Nice work man, got my dove tails for my guidescope today, so hopefully soon I can try for some exposures over 30 seconds

[MarkS]

That's an excellent image John.  I can't believe it was done with a CLS filter from Bromley!  The guiding can't be faulted, the image is very sharp overall (how was the seeing that night?) and you've got some nice red colour coming through there.

It would be truly awesome if you could do the same thing from a dark site without the need for the CLS ...

[Fay]

Gosh, John, thought you would be dissapointed withat one. It is really good!!!
How did you do your flats? Have you got a box?

[JohnP]

Thks everyone.

Mark - Yep I'm sure this would have been a really good image if taken from Tuesnoad - it's just unfortunate that 99% of the time I have to image from Bromley's crappy skies..

Daniel - I look forward to seeing your image - once you can do more than 30secs it'll really help..

Fay - I was disappointed initially - When I processed the L I had a great big fat dust donut right in the middle of the image. Fortunately, I noticed this on the night I imaged it so I intentionally left the camera & focusser etc. on the scope in the same place. Yesterday all I did was point the scope straight up early evening & put a white cotton T-shirt over the end of the tube. Then I selected exposures to fill 'Well depth' approx 50% - this worked out to be 0.07 secs as it was still quite bright. I took a series of 20 images & then just processed flat field in AIP4WIN. I couldn't believe the amount of dust that showed up etc. also vignetting which I wasn't aware of. When I processed the L again using the flat field it was unbelievable. The image had no sign of any dust donuts & was pretty much perfectly flat across whole image. I think I will make a point of using flats from now on regardless. I'll need to build a light box though... Below is a copy of the master flat I created - see how bad it is...!!

Hope that helps - Cheers,  John

[RobertM]

Hi John, Another lovely picture, well done.

It's amazing how the flat field evens out all the optical faults, I find it impossible to get a decent image without it.  I had thought of cleaning some of the filters and CCD window but my vignetting is just like yours (off centre too) which means I have to do it anyway.  It also looks like you managed to keep the guide star locked on the whole time which is no mean feat - what program/guider did you use ?

Cheers Robert

[JohnP]

Thks Robert - I use PHD & guide using my modified webcam & on this ocassion through my WO ZS66 refractor. Yep - I was doing 300sec subs & movement was minimal - it all worked for a change

Since doing flats on this image for the first time I will be using them from now on for all imaging. I will need to build myself a light box as it's tricky doing them the next day.. It was amazing how much easier it was to process  - apart from the obvious no dust donuts the whole field was a lot flatter with hardly any gradients etc..

Cheers,  John

[Fay]

How do you work out well depth?

[JohnP]

Fay - depends what software you use - But most of them tell you the ADU if you just run the mouse over the image. For 16 bit images max ADU count is 65,535 so you look for a count of approximately 30,000 when you put mouse over image. I use Artemis Capture for acquisition with the Atik 16ic & it just tells you value in margin at bottom. I'm sure Astroart is similar. I took my flats early evening & it was still quite bright - I only had to set exposure to approx .07 secs...

HTH,  John

[Tony G]

John,

As everyone has said so far, that image of M51 is stunning, then you started getting all techie with the flats and 'Well Depth' and all that stuff I don't know about yet, but if thats how you get images like that, I suppose I'll have to learn.
By the way, I still don't know for the life of me what Messier Object or Galaxy, that second image of yours is?????????????


Tony G

[JohnP]

Cheers Tony - You crack me up...

Sorry about techie terms.... As for 2nd image it's the infamous DDW aka 'dust donut widefield'... it's in an obscure part of the universe & is famous for having several black holes as evidenced by the dark rings - theory says that the extreme gravitational forces at the centre of the holes are drawing in interstella dust from the neighbourhood producing the characteristic donut shapes... all sounds like a load of *ollocks to me.... 

Anyways I'll try & keep it simple next time....

Cheers,  John

[MarkS]

Light box?  An LCD computer screen displaying a big white window works well ...

[Rocket Pooch]

Hi John,

Very nice, like the flat you can see the camera is not centred on the secondary hence the bright spot being off centre.

Great Image though the best from around here so far, and you can see the little galaxies.  I think there are 4 there, can you post a bmp of the fits file or e-mail me the fits so I can see how it compares to mine from last year, I think you have gone deeper.....

Well done....


Fay, the e-mail you send, all you need to do is expose the camera with a sock, tee shirt or something over the end so the readout on the bottom of the image is about 20,000 – 30,000, the camera and filters must be in the same position as when you images or they flat will not be subtracted from the image in the right place, there's no short cut to this process.

Chris

[Fay]

Robert, are you saying you can get flats by pointing scope at white computer screen?
Sounds more in my line of obtaining.

[Fay]

So, Chris, you put whatever white over the end, & the readout will be around 20-30 thousand. Do you then alter the exposure length to bring that number  down to what you want?

I am confused because I was running the mouse over my image but the number at the bottom changes all the time depending on if the mouse is on a white part or black.

[JohnP]

I am confused because I was running the mouse over my image but the number at the bottom changes all the time depending on if the mouse is on a white part or black.

Fay - yes of course it will the mouse is effectively measuring the intensity of light at the different positions on the image - if you put it on something black it is going to be a lot lower number then if you put it on something bright.... When you do the flats most of the image will be white just ensure that the value of the white is around 20-30,000 by adjusting the exposure length... It's really very easy to do...!!!!

John.

[Fay]

Ok, so put the mouse just on the white part of the image

[MarkS]

Fay,

Yes, I'm saying that pointing at a computer screen will work to create flats. 

You may need to do it for each filter you intend to use.  Is that right John?

And stop calling me Robert!!!!

Mark

[Fay]

Sorry Mark.............................they're coming to take me away ha ha ha ha to the funny farm!!!!!!!!!!!!!!

[JohnP]

Mark - Yes - You need to do for each filter as it's a different optical path... Also don't forget to take flat-darks as well....

Thks for the computer screen tip - I'll give it a go - only thing is to make sure screen is perpendicular to end of scope - do you literally put screen right up against opening of tube?

John

[MarkS]

Yes, bring the screen right up to the scope (and perpendicular!).

[Fay]

MARK,

I have just set up the scope indoors & had a practice at taking a flat fom the computer screen.

Not sure but it seems Astroart sorts out the exposure, could be wrong.

[Fay]

Will have to have another go getting image on screen

[Fay]

I am thinking I should not  have enabled FlatField Correction in AstroArt, am I right?

I have never used 2x2 binning, but the image was taken  in 2x2 as per AA.
When you have taken flat fields, do you have to process them  as you do a normal image?