Flats and the quest for clear images

[Datalord]

I have a problem with flats for my RASA. Clearly they make the difference between usable data and utter garbage, so I want to make great flats. With my WO this was the simplest thing, a piece of paper in front of the scope, a bright light and off you go. With the RASA, not so much.

I've tried the laptop screen with notepad, I bought an EL panel and I have used the sky. Each have their problems:

1. The laptop and EL share the problem that the camera needs to go down to 0.001s to not blow out the well, which has the fun effect of large horizontal bands across the image, which I presume is an effect of the refresh rate of the screen/panel lights. To remove the banding I have to go up to 0.01s, at which point everything is blown to white.

2. The sky has given me reasonable results, but unless it is absolutely clear or there are totally uniform clouds, the scope picks up tiny differences in light across the image. Those become visible in the stacked image, albeit minor. I also hate having to wait with taking the flats until morning or when the heavens give me an opportunity.

And suggestions to the predicament? I'm reluctant to build a box until I have a solution that will actually give me a reasonable result.

[vlaiv]

Maybe try piece of white cloth over scope when using flat box or sky to diffuse the light (in case of sky) or bring brightness to meaningful level with flat box so you can use shorter exposure?

[Tim]

Do you still have the EL panel? What about putting a piece of paper in front of it, or better a piece of semi opaque acrylic? Sign shops can be a good source of the acrylic.

Dimmable led panels are available too I believe.

Really you want to do your flats at the end of each session, things change all the time  especially dust motes :s

[wimvb]

I use a dimmable led panel (ikea floalt), with a few layers of white fabric to dim it further. This works for me (Sw 150pds and ASI 174MM).

[ollypenrice]

I'd also try a diffuser to slow the light panel exposures right down.

I've wondered, on the RASA and Hyperstar systems, whether there might be an issue with light panels if they were too close to the objective. Could they cause the camera to cast shadows? Just a thought. I don't know the answer.

Olly

[AKB]

22 minutes ago, ollypenrice said:

I've wondered, on the RASA and Hyperstar systems, whether there might be an issue with light panels if they were too close to the objective. Could they cause the camera to cast shadows?

I think I’m running into exactly that problem on a Hyperstar.  I first used sky flats, which worked well, but recently got a light panel and haven’t been able to get flats from that to work effectively... although, at first sight, they seem reasonable in themselves.  Must investigate further!

[Datalord]

interesting point. since I haven't got any usable flats yet, I can't say, but I have 7mm Perspex incoming today, so maybe I'll test it later.