Taking flats

[ChrisLX200]

In the expectation of receiving a new 12" scope (ODK12) I needed to do something about providing for a light source to create flats. I normally do this indoors on the bench, and with a variety of OTAs from 80mm to 127mm refractors, then 8", 10" and (now) 12" reflectors I needed somethng larger than the A4-sized EL panel I've been happily using over the past year or so. Large EL panels are expensive, and the emission colour of the one I have is decidedly Blue. That in itself is not too much of a problem but it did require callibrating the exposure times required to reach 40-50% on the histogram, with only very short exposures needed for blue and luminance filters, but very long ones for Ha and especially the SII narrow band filters. In fact for the blue end of the spectrum with the Moravian G4-16000 I needed to add 4 sheets of A4 80gsm cartridge paper to make it dim enough so the exposure length was long enough to avoid shutter artifacts (it needs to be at least a couple of seconds exposure).

So to cut to the chase, this time I went for an A3-sized LED light panel (purchased from Amazon, http://www.amazon.co.uk/gp/product/B00ISJG0C2?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00). The quoted size for the minimum dimension was 12.6" wide (and I confirmed that is correct by measuring it) - which is going to be a bit tight for a 12" aperture scope, but we'll see. The LED screen is a better colour, or at least it appears to be so - white. It is also dimmable, down to quite low levels of illumination so I hope extra sheets of paper won't be needed. The drawback though is that the dimming is achieved using a PWM power feed and not by varying the voltage, which means there will surely be a minimum exposure time required to avoid artifacts. However, by keeping exposures up around 3-5 seconds I'm not expecting problems.

One advantage of using SGPro for generating a flats library is that the whole process can be automated, the exposure times for each filter being put into a sequence and then simply run to collect the whole lot unattended. I normally use 50 frames for flats so that's a total of 350 exposures, with the EL panel some of these were up around 1minute (for SII) or 30second (for Ha) so it's nice to be able to walk away and just let it get on with it. The room this is carried out in can be blacked out so there's no problem doing the job during the day (or overnight of course) other than the camera needs to be cooled and maintained at operating temperature.

The photos below show my old A4 EL panel, and the new A3 LED panel at two brightness settings (about 30% and 70% brightness). At minimum brightness the LED panel shows only a glimmer of light and I can't think what other uses that might have other than being especially made for we astrophotographers who need to take flats! :-)  Usually of course, the remaining lights in the room are switched off - here they are on just to show the scope on the bench.

ChrisH

[libraryman]

Very nice, and a nice set up you have there, I knew about these panels I guess over a year ago when they were first spotted.

The price has risen somewhat now, wish I had bought one or two back then

Ray

[ChrisLX200]

Very nice, and a nice set up you have there, I knew about these panels I guess over a year ago when they were first spotted.

The price has risen somewhat now, wish I had bought one or two back then

Ray

Oh you can get cheaper panels than this one, less than half the price in fact, but beware that generally you get what you pay for and un-even lighting, dodgy cable connections, and some other issues will be waiting to bite you... ;-)

ChrisH

[davedownsouth]

Think I will get one of these panels as although the light box I have made is okay, it is a pain to position and i think the way you have got this set-up looks simple and very easy to do .  Order placed, should get it on Wednesday; got to love Amazon prime .

[ChrisLX200]

Hope it works for you Dave ;-)  This is the same item: http://www.amazon.co.uk/Huion-A3-Inches-Adjustable-Tracing/dp/B00DNPCZVG/ref=pd_cp_201_2?ie=UTF8&refRID=0FV8ANFGC4S45YJVQQG2with more reviews, a bit cheaper, but without some of the extras that you may or may not want...

ChrisH

[Astroscot2]

Chris, let us know how the flats work out for you. My Aurora panel has died on me and im concidering the Geoptic flat panel that can also be dimmed, its twice the cost you have paid so if this works well for you I may take the plunge and save some notes. I considered the Geoptic as it can be fastened to the front scope and I don't need to point the scope up and open the observatory roof !

[ChrisLX200]

Chris, let us know how the flats work out for you. My Aurora panel has died on me and im concidering the Geoptic flat panel that can also be dimmed, its twice the cost you have paid so if this works well for you I may take the plunge and save some notes. I considered the Geoptic as it can be fastened to the front scope and I don't need to point the scope up and open the observatory roof !

I've taken a full set of flats with it (350 images, 50 per filter) and it works great. Not much else to say about it really.

ChrisH

[davedownsouth]

My screen arrived last week and it is very nice, kids are desperate to get there hands on it for tracing artwork .  I have tried doing some trial flats with it and it looks like it will be very good.  Just need to get the scope properly focused before I do a real set of Flats...........

[Astroscot2]

Dave, which screen did you go for in the end ?

[davedownsouth]

Dave, which screen did you go for in the end ?

Sorry for not replying sooner, only just noticed your post. I got the one linked in the first post and although it is a very good screen, I have only managed to use it once because of weather . Might not be fair to blame this fact on the screen mind you

[ollypenrice]

Chris, let us know how the flats work out for you. My Aurora panel has died on me

So have mine. All three of them. Never again! The cables and connectors are pure tat. I would advise anyone and everyne to find an alternative.

I use a graphics panel now but don't find the dimmer reliable. It is inconsistent during a shoot. Instead I run it flat out and adjust the brightness with typing paper.

Olly

[swag72]

I don't use any panel at all - I park my scope facing my white obs wall and that's that

[tony210]

I have been trying plastic sheet panels over the end of the scope and the sky as a light source - seems to work well but I need a thick panel to get the right illumination-Tony

[gnomus]

I got hold of one of these light panels (the one mentioned by the original poster) the other day.  The problem that I am having is that it is too bright.  I have an Atik 383L which has a mechanical shutter, so exposures need to be of a certain length (I cannot see the shutter after a 3 second exposure, but I have been trying to aim for around 8-10 seconds).  The panel is hung on the observatory wall such that the objective lens of the scope is around 4 feet from the panel.  I can set a second park position with my Mesu so that I can have my normal park and 'Park 2' which I have carefully aligned on the centre of the light panel.  This would be a great set up for me, therefore, but .......   

With the straight panel on full the image is totally blown out at any exposure.  So I tried a few sheets of some A3 paper (actually some thick Hahnemuhle photo paper - it's all I had handy).  I needed to Blu Tak three sheets of Hahnemuhle to the panel before I could get a 3 second exposure at around 22000 ADU.  I also noticed that the light from the panel comes out the panel edges and illuminates the observatory somewhat.  I wondered if this defeated some of the light blockage of the paper - that is, was some of the light being reflected off the observatory walls back onto the paper and into the scope.

I can dim the panel, but it does not seem possible to reproduce the same setting from one session to the next (it is a hold-and-release-when-ready sort of thing).

So, I have a couple of questions:

1)  Should I put the paper on the panel or over the front of the scope?

2)  Is there something else I can try to get a longer exposure with the kit I have?

3)  If I can get this working is it worthwhile to use 'Flat Darks', or is this a waste of time?

Ta   

[PeterCPC]

I use a Tattoo panel and had the same issue in that it was too bright. I got a dark grey acrylic sheet and taped it to the panel to reduce the brightness. Seems to work ok.

Peter