Preparing for DSO subs

[tingting44]

i have not imaged a DSO for well over a year now and want to get back into it..... im ok with darks and bias, but when it comes to flats im not too sure, do i take them at the same exposure settings as the lights? also would my headlights on the car be ok as i dont have a laptop for out in the field?

this is how im thinking it will go, take 50 lights, 25 darks, 25 bias, 25 flats (with car head lights>?)

this was my best attempt last time, with only lights and darks with no guiding (1 min total of data)

Latest stack(4) attempt of M43-M43 (JPEG for WEBSITES) by tingting44, on Flickr

[frugal]

Flats should be taken with the same focal setup, but lower the exposure time to the appropriate amount for the light getting through. I just use a DSLR with camera lenses, rather than a telescope, but I take flats using the same F-stop, the same focal length with the same lens. All I change is to put the camera into AV mode to automatically chose the exposure time for that aperture. I must confess that I have only ever had luck taking flats against a clear twilight sky where the whole sky is a flat shade of blue. 

As I use the same lens a lot, I am slowly building up a library of flats as and when I can. As the flats only take 1/100 or so of a second I can take lots, so I tend to go for 100 flats to build a master flat from.

There is an interesting article on bias frames here: http://www.blackwaterskies.co.uk/2013/09/pixinsight-dslr-workflow-part-1-bias.html in which he shows how 200-300 bias frames builds up the true image of the signal noise. I had a go, as bias frames only take 1/8000s each it only took a couple of minutes to take 200.

[lensman57]

i have not imaged a DSO for well over a year now and want to get back into it..... im ok with darks and bias, but when it comes to flats im not too sure, do i take them at the same exposure settings as the lights? also would my headlights on the car be ok as i dont have a laptop for out in the field?

this is how im thinking it will go, take 50 lights, 25 darks, 25 bias, 25 flats (with car head lights>?)

this was my best attempt last time, with only lights and darks with no guiding (1 min total of data)

Latest stack(4) attempt of M43-M43 (JPEG for WEBSITES) by tingting44, on Flickr

They are called flats for a reason, a car head light is the last light source would give you even illumination. Use your laptop screen with a white back ground ( open Notepad and maximise it) and a piece of white cloth very taut and flat on top of the lens only a couple of centimeters away from the screen but do not allow it to touch the screen as the pressure causes moire patterns that you don't want. It is imperative that the focus is not altered and if you are using a camera lens use the same aperture as the lights. Set the camera to AV and let it take at least 50 flats. and good luck. I use an EL panel these days to do the job BTW as it is safer than handling a laptop .

Regards,

A.G

[PhotoGav]

If you have an ipad, there's a great free app called 'Color Softbox'. I've used that and it seems to work perfectly.

[ollypenrice]

Flats should be exposed to about a third of the way to saturation. Your camera will probably show you a histogram so you simply expose till the peak is a third of the way over to the right.

It is vital to calibrate flats with 'darks' or they will over correct. I put 'darks' in inverted commas because, happily, you can simply use a master bias as a dark for flats. It is insignificantly different from a dedicated dark at the exposure times involved.

It is also vital to remember what you are imaging in a flat. It is the light pattern of your system, so nothing in that optical system must change. No dust should be displaced, the camera and filters should not change in orientation, the focus should not be touched.

Have fun,

Olly

[tingting44]

Flats should be taken with the same focal setup, but lower the exposure time to the appropriate amount for the light getting through. I just use a DSLR with camera lenses, rather than a telescope, but I take flats using the same F-stop, the same focal length with the same lens. All I change is to put the camera into AV mode to automatically chose the exposure time for that aperture. I must confess that I have only ever had luck taking flats against a clear twilight sky where the whole sky is a flat shade of blue. 

As I use the same lens a lot, I am slowly building up a library of flats as and when I can. As the flats only take 1/100 or so of a second I can take lots, so I tend to go for 100 flats to build a master flat from.

There is an interesting article on bias frames here: http://www.blackwaterskies.co.uk/2013/09/pixinsight-dslr-workflow-part-1-bias.html in which he shows how 200-300 bias frames builds up the true image of the signal noise. I had a go, as bias frames only take 1/8000s each it only took a couple of minutes to take 200.

Thanks, yeah thats one thing i didnt do last time i dipped my toe in DSO AP was to make masters of darks, will start tho when i get the chance next

They are called flats for a reason, a car head light is the last light source would give you even illumination. Use your laptop screen with a white back ground ( open Notepad and maximise it) and a piece of white cloth very taut and flat on top of the lens only a couple of centimeters away from the screen but do not allow it to touch the screen as the pressure causes moire patterns that you don't want. It is imperative that the focus is not altered and if you are using a camera lens use the same aperture as the lights. Set the camera to AV and let it take at least 50 flats. and good luck. I use an EL panel these days to do the job BTW as it is safer than handling a laptop .

Regards,

A.G

Thanks A.G, a no,no for the car headlights then, i dont have a laptop, only a desktop so thats a no,no for a laptop in the field, would i be ok to take my flats when i get home again to use the PC desktop screen? the DSLR would have warmed up a bit tho from the lights and darks, or does it not matter about the temperature for the flats A.G ?

I will defo use your method i think tho, so ill open notepad and maximise then place a piece of white cloth over the lens (does the cloth have to be tight? or just kind of leave it flapping over the lens loosely? then set apertue to match the lights, set DSLR to AV and wham bam take my 50 flats thanks for the good luck, i think ill need it lol

If you have an ipad, there's a great free app called 'Color Softbox'. I've used that and it seems to work perfectly.

no ipad

Flats should be exposed to about a third of the way to saturation. Your camera will probably show you a histogram so you simply expose till the peak is a third of the way over to the right.

It is vital to calibrate flats with 'darks' or they will over correct. I put 'darks' in inverted commas because, happily, you can simply use a master bias as a dark for flats. It is insignificantly different from a dedicated dark at the exposure times involved.

It is also vital to remember what you are imaging in a flat. It is the light pattern of your system, so nothing in that optical system must change. No dust should be displaced, the camera and filters should not change in orientation, the focus should not be touched.

Have fun,

Olly

thanks Olly, all noted

[lensman57]

Hi

You could try and get the camera in without touching the lens barrell, that is what I used to do and found out that it did not always work.  focus would alter for some reason. It nay have had to do with scope being knocked and the focus point moved or even the warmer temp of the room altered the focus, citical focus is  temperature dependant BTW but I don't think that you have to worry about this at the moment. I even resorted to using the LCD TV set to one of the radio music channels with a white back ground, none of these were satisfactory for me. In the end I resortetd to using a small A5 El panel bought on Ebay and things have improved . If you do decide to do it inhouse  then make sure that you have no other source of illumination, such the celing light or LEDs from various power supplies lighting up the room for you. also others have given you excellent advice follow them and becomes an easy job. Also if you are using only one lens or scope with a single, LP filter for example, you may be able to ust use the flats froa previous session if you use the same aperture and iso as before and see how it fares.Hope all this works for you.

Regards,

A.G

[tingting44]

yeah i think that will be my 1st option, to try to get the DSLR back home without knocking the focus (about a half hour drive home, so hopefully it wont warm up too much), i could all ways take me daughters notepad out and try to do it in the field, its not like she will know as she will be in bed loool thanks for all the tips and advice, i cant wait to start trying to image more deep space! id be over the "moon" if i could capture a whirlpool galaxy

[lensman57]

yeah i think that will be my 1st option, to try to get the DSLR back home without knocking the focus (about a half hour drive home, so hopefully it wont warm up too much), i could all ways take me daughters notepad out and try to do it in the field, its not like she will know as she will be in bed loool thanks for all the tips and advice, i cant wait to start trying to image more deep space! id be over the "moon" if i could capture a whirlpool galaxy

On a couple of occassions I "borrowed" my wifes IPAD but the thought of holding £600.00 worth of her  belongings on top of the scope became scary and I put it back in quetly.

Regards,

A.G

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[tingting44]

life would not be worth living lool, good choice i might keep an eye out for a cheap, cheap notepad on the bay, anything will do really as long as i can have firecapture and notepad programs on there id be good to go