Help with flats frames on asi 1600mm pro + 200mm f2.8

Processing Calibration Narrowband H-alpha Deep Sky
17 replies•461 views
Ayrton Lopes avatar
Ayrton Lopes avatar
Hello everyone,I have several problems to be able to make the flats frames in the HA and SII filters im my setup (canon 200mm f2.8@f2.8, asi 1600mm-pro, filters optolong). I'm trying to use all possible types of ADU, but the image only gets worse.The method I'm trying to make the flats is to place the camera at 90 degrees, a white sheet of paper on top and a tablet with white light to illuminate.What intrigues me is that doing the same way, the flats in OIII worked perfectly ...Honestly I don't know what else to do, I've tried several different ways, all without success.Here is an example of an image applying the master flat in HA and another showing the purchase between the master flat of HA and OIII.
Follow the images in imgur.
compared to / without the master flat
master flat in HA/master flat in OIII

Thanks for helping!!
David Redwine avatar
David Redwine avatar
Flats should never be brighter at the edges.   I think you are seeing the filter cell itself in the HA flat.  Check to see if your HA filter is screwed in properly.
Well Written Insightful Respectful Concise
Ayrton Lopes avatar
Ayrton Lopes avatar
David Redwine:
Flats should never be brighter at the edges.   I think you are seeing the filter cell itself in the HA flat.  Check to see if your HA filter is screwed in properly.


Hi David, Thanks for the answer.
The filters are well fitted, I even opened the wheel to check but they are the same as the others ...
I think it may be a reflex, maybe something with a white sheet, I don't know. I'm going to test putting a white mesh in front of the sheet instead, because I honestly don't know what else to do.
I also think these white flats around are not right, but I don't know what to say, I accept ideas.
Morian avatar
Morian avatar
Hello Ayrton.
When making flats with ASI 1600 and telelense with SGP I use 12000 in ADU, some say between 12-18000 but 12000 works for me and I put a white t-shirt over the lens cap and use ipad as a light source, the app called BestNightLight and is free.That is it.
I usually make 50.
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Ayrton Lopes avatar
Ayrton Lopes avatar
but 12000 works for me and I put a white t-shirt over the lens cap and use ipad as a light source

Hi Morian!!
Im using a iPad too, but instead of wearing a white t-shirt I'm using sheets.
About ADU, in the other filters I use 15k and it worked perfectly, it's only in HA and SII that it didn't work.
I already saw that it may be a reflex, I will try with a white t-shirt, it will work ...
Morian avatar
Morian avatar
👍
Morian avatar
Morian avatar
Hey boddy
Im on vacation right now , but as soon as possibel I send a flatframe
Only one or one  of 50?
Morian avatar
Morian avatar
SORRY😄
Ayrton Lopes avatar
Ayrton Lopes avatar
Can you post a single Ha Flat?
YEs, follow in google drive: Single HA Flat - 15k ADU

im trying using a whit t-shirt but without success.. same results as using a white paper
Ayrton Lopes avatar
Ayrton Lopes avatar
In order to rule out the approach you are using (ipad on top of the optics), take a picture in Ha of the roof and then post it here.Let's see if it has brighter corners.

Do you want me to take a picture with or without the room light on? take a dark photo in h alpha it? as if it were a photo of the sky but without lighting, correct?
palaback avatar
palaback avatar
In my experience it is easier and more consistent to point the scope vertical and put white cloth over scope and a light source on top, iPad or in my case a cheap led tracing panel. and only soft  indirect light in room. Software like NINA or SGP can really simplify things. The flats wizard in NINA has made me much more consistent in making flats.  I aim for 50% in histogram.
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Tristan Campbell avatar
Tristan Campbell avatar
Did you ever solve this issue? I've been using 200 2.8 and sometimes get same doughnut like artefact from flats just like in your example. I suspect it's caused by the front stopdown rings - 72mm to 68mm. I'm going to try standard stopping down within the lens to see if that solves it.
Serge Caballero avatar
Serge Caballero avatar
I started using an ASI1600MM pro a couple of months ago and also had some issues getting good flats; part of that was years of using a CCD and switching to a CMOS. I've used an electroluminescent panel (USB powered,  adjustable light output, under $25 from Amazon; also called an art tracing pad) on top of the OTA mounted and pointed straight up. Like Morian, I use SGP flats wizard to calculate exposure for 13,000ADU +/- 200 ADU at Unity Gain.

Serge
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