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How does this 80ED/DSLR Master Flat frame look?


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Hey fellow gazers

So i was testing out my equipment the other week, and was able to take some flats immediately after taking my lights, without moving anything. I took 60, and stacked them along with the lights and Bias frames in DSS as part of my test run on M31.

However, i noticed after stretching the Master Flat frame in PS, that there were what seemed to me to be quite a lot of blobs in the frame. Way more than i was expecting tbh. I've attached a picture below. 

So for all you DSLR guys out there, does this look normal to you? Or do i have a serious issue here with either the front lens of my SW 80ED, the 0.85 FF/FR, or the camera sensor itself?

post-27374-0-23703000-1445628770_thumb.j

Cheers!

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try a rocket blower first,works with my alpha, saves wiping. clear skys charl

Well, you're only wiping the dust shaker glass and between it and the sensor are usually at least two more glass layers before you even theoretically could touch the sensor surface. There is no risk i

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hi Xiga.

that are dust particles and 100% normal.

Thats exactly why we take flats for one part is to correct these imperfections.

Dont try to get rid of the dust, you will loose that fight :)

Mine look MUCH worse, and its pretty mich irrelevant to the final image if you take good flats.

Only if these would affect you while doing normal photography with your dslr you would have to clean.

Kind regards, Graem

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Ok, thanks guys, I feel a bit better about it now :-)

Ps- just realised I named the thread incorrectly. Should obviously say Flat, not Bias!

Fixed that for you :)

Yeah, that flat doesn't look too bad...

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When i started, i tried to clean all dust all the time. And soon realized it really does not matter (as i said, only it can affect you for normal daytime dslr if you are shooting high F's, then for example in the sky you would see the blobs)

Somebody told me this and it convinced me to let the dust be dust: If you have your DSLR attached to the scope, but one finger infront of it, and see if the image that you're taking has a huge difference. You'll see a very slightly dimmed picture (as you're catching less photons). But for a smallish sized scope, you're covering maybe 6% of the aperture. The dust - even if you have a lot - is maybe covering 0.1% of the aperture and will not kill you. Only if the dust is right on the sensor, then its annoying.

But looking at your flats, that dust is not on the sensor, then even at low f-values you would have stronger defined black dots in my opinion.

For me it looks like its on the Focal reducer (mine is FULL of dust, as it collects there when the scope is upright).

The reason for not cleaning your sensor if not necessary: Even if you're very careful, i'd not trust on that you're not damaging something.

Swabbing the sensitive parts that are collecting all those photons - i'd be very careful and only do it if absolutely necessary. For examle: I have never cleaned the front lense of my scope ever, neither have i of my DSLR. It's just not necessary yet (and its been running well now in this configuration for over a year).

Kind regards, Graem

Edited by graemlourens
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The reason for not cleaning your sensor if not necessary: Even if you're very careful, i'd not trust on that you're not damaging something.

Swabbing the sensitive parts that are collecting all those photons - i'd be very careful and only do it if absolutely necessary. For examle: I have never cleaned the front lense of my scope ever, neither have i of my DSLR. It's just not necessary yet (and its been running well now in this configuration for over a year).

Well, you're only wiping the dust shaker glass and between it and the sensor are usually at least two more glass layers before you even theoretically could touch the sensor surface. There is no risk in gentle wiping when clean sensor swabs are used. At some point you get enough dust on your lens or sensor that it's sensible thing to clean them. It may take years, but it will happen. I'd clean the topic starters sensor already :)

Edited by Herra Kuulapaa
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Another reason for not removing the dust is you will have to repeat the flats again as it changes the lens/flat  characteristics and when the dust again settles,  again and again etc, etc.

Derek

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If you don't like that flat you certainly wouldn't like mine!!

The main thing is that it's entirely credible, with even progression from dark corners to brighter middle.

It'll need debayering but presumably you have that covered in the software routine?

Olly

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I don’t know if you Olly meant me, but yes, I’m a clean person.

Most likely because the dust on my cameras sit on the sensor surface and I see the physical nature the of fibers etc very clearly J

But, I’m still a voter for a regular camera sensor cleaning. Reflector mirrors not so much though.

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I don’t know if you Olly meant me, but yes, I’m a clean person.

Most likely because the dust on my cameras sit on the sensor surface and I see the physical nature the of fibers etc very clearly J

But, I’m still a voter for a regular camera sensor cleaning. Reflector mirrors not so much though.

Not at all. I simply meant that my flats are far 'dirtier' and more vignetted than those of the OP. Here's my dirty washing;

O%20FLAT%20web-M.jpg

This calibrates out perfectly happily and should make the OP feel a bit better! CCDs, being sealed, don't generally suffer from contamination on the sensor though.

Olly

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Not at all. I simply meant that my flats are far 'dirtier' and more vignetted than those of the OP. Here's my dirty washing;

O%20FLAT%20web-M.jpg

This calibrates out perfectly happily and should make the OP feel a bit better! CCDs, being sealed, don't generally suffer from contamination on the sensor though.

Olly

Thanks Olly. I do feel much better now mate! :smiley:

I've since bought a Giotto Rocket Air Blower and have given the front lens & FF/FR a good seeing to :tongue: The FF/FR did seem to have some dust on it, but the front lens of the 80ED seems to have some specs of dust on the inside (really not sure how, i've only used it twice, and it lives in a soft foam padded case), plus, what looks like two fairly big dew spots. At least i think that's what they are, they look like giant finger prints (without the pattern obviously!) and they won't budge. Do i need to be concerned about these? I use a dew band so i'm surprised to see it there tbh.

As for the debayering, i just load all the raw .CR2 files into DSS. From memory, i think i use teh 'Bayer Drizzle' option, though i've no idea if that's even right!

Would love to see a DSS Guide for DSLR users if anybody has any good links?

Clear Skies.

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And switch off the buzzy sensor cleaning thingy :)

Dave

Hi mate, 

Why do you recommend turning this off? Is it not a good thing that the camera would do this at the beginning of each session? 

I can see that you wouldn't want it to keep doing it if you needed to power the camera off and on during a shoot, but what's the benefit of turning it off completely? 

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Just so. Do a manual cleaning routine once in a while but otherwise keep it turned off. Had at least one image ruined due to that mistake.

The spots on the OP's image are definitely on the glass somwhere just in front of the sensor, not on the reducer. You can calculate the distance using this:

http://www.wilmslowastro.com/software/formulae.htm#Dust

Just from experience I can tell you that they are the right size for the cover glass or similarly nearby. Dust on the reducer would cast a shadow so diffuse it would be invisible. Definitely don't worry about stuff on the mirrors or lenses. Not even a large central obstruction produces a visible shadow, never mind dust or dew marks.

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Just from experience I can tell you that they are the right size for the cover glass or similarly nearby. Dust on the reducer would cast a shadow so diffuse it would be invisible. Definitely don't worry about stuff on the mirrors or lenses. Not even a large central obstruction produces a visible shadow, never mind dust or dew marks.

My abortive first attempt with my astro-modded DSLR brought home that dust that is now about 1.5mm from the sensor is MUCH nastier than dust about 3.5mm from it :-(

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My abortive first attempt with my astro-modded DSLR brought home that dust that is now about 1.5mm from the sensor is MUCH nastier than dust about 3.5mm from it :-(

 Thats why I went full-specturm. My modding attempt left dust trapped beneath the filter glass so I ended up taking them all out and now I just clean the sensor directy (though not directly on the CMOS, but on its cover glass). I switch the camera on, hold it above my head (pointing down, so dust that is moved doesnt just land on another part of the sensor), flick it into live view - then use either the sensorclear pen (if its real mucky) or the rocket blower. Take a quick flat, if there is still some left I clean it again until satisfied.

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