Is it dirt/dust or something else?

[RolandKol]

Hi guys,

Today is my happy day, as at last, I caught a few hours of the clear skies and

I properly tested my new 130PDS scope and mount. Previously I played around with my DOB and short exposures.

I was excited as I managed to get 2 and even 3 min exposures, but.... While stretching I saw some dark/black circles on the pic...

is it just a dirt/dust on the camera's filter or even sensor?

The form of the top one looks like a shadow of something... the rest two are dust (probably)

I use 130PDS, SkyWatcher Coma Corrector with CLP Filter on Canon 1300D... Quite disturbing....

Please advise.

P.S. I have stretched this picture to make these circles more visible

P.P.S just darks and bias, - I have not done flats
 

[Oddsocks]

Yep, 

Just dust donuts, calibration with flats will remove them.

Dust lying directly on top of the sensor usually just forms a solid black shadow, dust on a filter or the corrector casts a donut shadow, the bigger the donut the further the dust is from the sensor and generally the bigger the donut the fainter it becomes.

If you know the focal length of the telescope and the pixel size you can work out the distance of the dust from the sensor using the formula D=Pdf,  where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out which surface the dust is lying on.

HTH.

William.

[RolandKol]

Thanks a lot for a fast reply and advice!

as I was a bit nervous!

It will be the LP filter - 100% 

Coma Corrector is brand new, used it for the first time today

P.S.

Will need to start learning t-shirt technique  for flats

[Demonperformer]

Flats are the most fiddly part of the capture process, but probably provide the most benefit for the time spent. Well-worth getting into the habit of using them (or, more precisely, well-worth not getting into the habit of not using them, if you see what I mean).

Thanks for the formula, William, didn't know that.

[Juicy6]

5 hours ago, Oddsocks said:

If you know the focal length of the telescope and the pixel size you can work out the distance of the dust from the sensor using the formula D=Pdf,  where D= the distance from the sensor to the dust casting the shadow, P=The width of the dust donut in pixels, d=The width of a single pixel and f=The focal ratio of the telescope. If you use the pixel size in millimetres then the distance will be in millimetres too, if you use pixel size in microns the distance will be in microns so you just need to be able to convert accordingly. Once you have the distance calculated you can work out which surface the dust is lying on.

HTH.

William.

...or try online calculator: http://www.ccdware.com/resources/dust.cfm

Christer

[SteveA]

If you are taking flats remember that they need to be taken either prior to or after your imaging run...You need to acquire these with the imaging equipment (camera, filters etc) in exactly the position they were orientated when you took your lights...

Steve

[ollypenrice]

1 hour ago, SteveA said:

If you are taking flats remember that they need to be taken either prior to or after your imaging run...You need to acquire these with the imaging equipment (camera, filters etc) in exactly the position they were orientated when you took your lights...

Steve

This is absolutely correct but I find that, since I don't dismantle my kit between sessions, a flat may last for months.

They are a bit of a faff but make soooo much difference and the harder you stretch an image the more you need them.

Olly

[SteveA]

I found by far the easiest solution was using an EL panel...getting images of the sky  at twilight for me never happened.

If you are interested there is a short article by Steve Richards here...http://www.skyatnightmagazine.com/feature/how-guide/how-build-flats-panel-imaging

Steve

[RolandKol]

Thanks again, - I guess, I will start doing Flats at the start of the play... as I am sure, - I will be tooo lazy to do it at the end...

Furthermore, it looks like, my 130PDS focuser needs a surgery... 1.5cm off...

Just look at the top Dognut... and the brighter stars... Focuser intrusion is obvious  I thought I will avoid this if I will use all parts from Skywatcher (SW Coma Corrector, SW Ring/Adaptor for Canon)
 

Sad

[carastro]

The trouble is when you do the flats before the imaging session, you need to make sure you have focussed the camera.  Were you thinking of doing sky flats or getting an EL Panel.

When we are at camp, I normally do sky flats at the beginning of the 2nd night. 

My 130PDS leaks light so I need to do my flats at dusk, then if I don't time it right I run out of daylight.  I really need to get a replacement EL panel for the 130PDS, sky flats work fine in the day time for my refractors as no light leak.  I have tried masking the rear of the Newt to block light but never seems to be successful.

Roland, if you are bringing an EL panel to astro camp, please make sure you have a cover to block out the light from the other campers.

I constructed this with my previous EL panel (which died on me after a while), it shields stray light.

Carole 

[RolandKol]

yep... forgot about the focus...
It seems, no other option, but only after the "play" or in between them...  I will try a white T-Shirt today to see what it shows.

later maybe this http://tinyurl.com/yb8pxnc8 

P.S.

And I hate to think of a Jigsaw and the "circumcision" of the focuser.

I will try to collimate the scope by tightening up the bolts to the limit at first.

 

[celestron8g8]

I've always taken flats differently I was taught you take 5-7 images then take 1-2 flats and then images and then flats . Reason being flats work better taken as your camera heats up and ambient temp changes . Those changes effect your images as well as the flats . I've tried the ways of taking all my images and when finished then taking my flats but sometimes that ambient temp changes quite a bit from beginning to end and where I live can fluctuate a lot throughout the night . But maybe in your location it's best take flats after your finished imaging for that particular image . 

[Demonperformer]

That is a new one on me. Are you using a cooled camera? 

[ollypenrice]

27 minutes ago, celestron8g8 said:

I've always taken flats differently I was taught you take 5-7 images then take 1-2 flats and then images and then flats . Reason being flats work better taken as your camera heats up and ambient temp changes . Those changes effect your images as well as the flats . I've tried the ways of taking all my images and when finished then taking my flats but sometimes that ambient temp changes quite a bit from beginning to end and where I live can fluctuate a lot throughout the night . But maybe in your location it's best take flats after your finished imaging for that particular image . 

I'm not convinced by this and have never heard it suggested before. The temperature affects the noise in the flats but the way to deal with that is to take plenty of them and to calibrate them by subtracting a master bias from them at the stacking stage. I wouldn't spend time that could be spent taking the data on any kind of calibration files.

Olly

[feilimb]

16 hours ago, SteveA said:

If you are taking flats remember that they need to be taken either prior to or after your imaging run...You need to acquire these with the imaging equipment (camera, filters etc) in exactly the position they were orientated when you took your lights...

Steve

Can I just ask on this point - does the focus point need to remain exactly the same also, or if you accidentally bump the focuser to a slightly different position, and take flats the following day would that be OK / not matter about change in focus? 

[Demonperformer]

Yes, the focus should remain the same, else the image of (say) a dust bunny will alter in size and the result will produce a wrong correction (and actually make the photo worse!).

[SteveA]

Its a pain I know...especially when you are tired and want to hit the sack,but this is essentially why the process of taking flats just needs to  become a routine task ...However on the bright side they don't actually take that long to do, especially when you have worked out the correct exposure length, which is normally quite short. Of course if you use a mono camera and use filters you need to take flats for each filter.....the more filters you have shot through..the more flats you need.

Steve

[alacant]

16 hours ago, celestron8g8 said:

take 5-7 images then take 1-2 flats

? I think that that would make more sense for dark frames which are more temperature sensitive. Do you really set up a light source in between light frames?

[Demonperformer]

32 minutes ago, SteveA said:

Of course if you use a mono camera and use filters you need to take flats for each filter.....the more filters you have shot through..the more flats you need.

Leaving aside @ollypenrice's comments (always dangerous) about only using the L filter for flats (I haven't done this, but will definitely try that at some point), each set of flats should be taken with each set of lights, not all at the end of the evening (another good reason for an independant light source). Even filters that are nominally parfocal can require a slight adjustment to focussing.

[SteveA]

1 hour ago, Demonperformer said:

Leaving aside @ollypenrice's comments (always dangerous) about only using the L filter for flats (I haven't done this, but will definitely try that at some point), each set of flats should be taken with each set of lights, not all at the end of the evening (another good reason for an independant light source). Even filters that are nominally parfocal can require a slight adjustment to focussing.

Although I guess I can see the logic behind that, I've never done it that way ie. interrupt an imaging session to take flats. With my setup I orientate the scope in the vertical direction and lay the EL panel across the open end of the OTA and so I would have to relocate the target and frame it between lights if I were to do that. So I tend to always take flats at the end of the session. Mind you I dare say its not beyond me to find a way of taking the flats without having to do this. Having said that, its been a while since I managed to grab more than one set of lights on any particular night, so its somewhat academic really....thanks to CLOUDS..

Steve

[ollypenrice]

1 hour ago, Demonperformer said:

Leaving aside @ollypenrice's comments (always dangerous) about only using the L filter for flats (I haven't done this, but will definitely try that at some point), each set of flats should be taken with each set of lights, not all at the end of the evening (another good reason for an independant light source). Even filters that are nominally parfocal can require a slight adjustment to focussing.

In my systems the focus shift between filters is far too small to affect the flats in any perceptible way. When we do shoot flats 'per filter' here we just leave the focus where it was at the end of the session and run through the filters. I can't recall any of our regular visiting imagers, many of whom are very expert, doing their flats at the specific focus of the filter in their own systems. The theory is correct - there will be a small difference in the size of dust bunnies and vignetting - but it will never, in my opinion, be enough to justify wasting the chance to add a few subs on the target. And it would go down pretty badly at star parties! (And at my place. )

[Demonperformer]

I think all this goes to show just how much of a hassle flats can be.

With my small frac, I have attached a cardboard ring around the (circular) panel and it can therefore be sat on the end of the OTA in position - gravity holds it there.

I suppose I am becoming aware of this as I am moving into a situation where I will be using filters from different sources, so "parfocal" is likely to be a long way from reality and so it would be more important. Or maybe it's just my OCD playing-up. However, if I start doing L-only  flats ...

Thanks.

[ollypenrice]

1 hour ago, alacant said:

? I think that that would make more sense for dark frames which are more temperature sensitive. Do you really set up a light source in between light frames?

Regarding the darks, I disagree. It would be far more effective to get more subs. And if you only take a couple of darks at 'temperature x' they will add more noise than they remove. Alternatively, if you stacked all your variously-shot darks together I doubt that the resulting master's noise profile would be significantly more appropriate to individual subs than a master made at the end. Even with set point cooled CCDs I'm not a fan of darks anyway and, with a DSLR, I would follow Tony Hallas and use a 12-ish pixel dither and a master bias as dark.

What is the one thing all the best images have in common? Lots of exposure. Doing calibration files in dark sky time would be crazy.

Olly

Edit: regarding the use of only the L flat, it's a compromise. I don't advocate it as anything other than a way of saving time. Our record here, for data collection in a single night, is precisely 24 hours. Getting through the reduction of that lot does not take five minutes!!! 

[carastro]

Totally agree with Olly, I never takes flats during an imaging session, I always take them either at the end with a light box or the following day as sky flats.  I only ever heard one person say that flats were temperature dependent and I have never cooled my flats.  

Yes flats are a pain but they make so much difference to the end result and ability to process.  Also agree with Olly if you have to tweak the focus ever so slightly between filters, it is not enough to make any difference, so long as it's only slight.  

Quote

http://tinyurl.com/yb8pxnc8 

That was an interesting find Roland, I have shot some questions off to the Chinese retailer as that is a good price.  I do think the cost of some EL panels are grossly expensive, and I would have bought the same again as I bought last time except I think the electrical "joint" to the panel is somewhat susceptible to getting damaged which is what I think happened to my last one.  

Carole 

[spillage]

All I can add is that I have ditched darks for master bias and only bother using my L flats, I only have the baader filters so not sure how parfocal they are. I just point my scope skyward and use elastic bands to hold the lcd panel in place for taking the flats.

I you have to strip down your kit do you need to take new flats every time. I would have thought that as your focus is going to be the same you could get away with using the old ones. I guess you have the chance of moving bunnies or new ones being added to the imaging train though.