22.04.2020 More net curtain twitching.

[Rusted]

60F and sunny but windy from the east again.
So I have draped two layers of shade netting across the lower slit for a bit of protection.

Better seeing, than yesterday, but was bothered by vignetting and PST etalon brightness variations.
So now I need to try and take some flats. 
My very first flat was a perfect negative of my solar image. Whoops! 

So now I have a hanky drying on the line for the creases to drop out.
I shall try that as a flat screen over the obj. I don't own a white t-shirt.
Not can I obtain a translucent, grocery bag while in self isolation.

A filament? Make of it what you will:

[Freddie]

For a flat, just defocus on an area of the surface with no features. No need to mess about with all that other stuff.

[Rusted]

6 minutes ago, Freddie said:

For a flat, just defocus on an area of the surface with no features. No need to mess about with all that other stuff.

Thanks, Freddie, but I really don't want to touch the telescope between exposures.
I ought to have made it motor focusing by now.  3m focal length is hard work!
I haven't worked out a way to support the drive motor with a helical focuser.
Which moves in an out at both ends. That means a very long, cantilevered motor support.

[Altocumulus]

28 minutes ago, Freddie said:

For a flat, just defocus on an area of the surface with no features. No need to mess about with all that other stuff.

A question on flats, Freddie, if I may.

I've recently started using cameras with a larger sensor, up from the 120MM.

I've allocated a size on the sensor.

So far I've been moving the ROI around as appropriate, on the sensor.

Am I right in thinking each new move requires a fresh flat? If so I'd guess, next, that it would be more efficient

to leave the ROI 'fixed' on the sensor, create a flat on that position - and then move the 'scope?

Do I have that thinking correct?

[Freddie]

You only need to do it once. So maybe get set up, set your gain and exposure to roughly what you will be using in your imaging session, defocus, take your flat and you are done.

[Altocumulus]

3 minutes ago, Freddie said:

You only need to do it once. So maybe get set up, set your gain and exposure to roughly what you will be using in your imaging session, defocus, take your flat and you are done.

Ok, thanks.....I think!

Does ROI not come into it at all?

[Freddie]

3 minutes ago, Altocumulus said:

A question on flats, Freddie, if I may.

I've recently started using cameras with a larger sensor, up from the 120MM.

I've allocated a size on the sensor.

So far I've been moving the ROI around as appropriate, on the sensor.

Am I right in thinking each new move requires a fresh flat? If so I'd guess, next, that it would be more efficient

to leave the ROI 'fixed' on the sensor, create a flat on that position - and then move the 'scope?

Do I have that thinking correct?

Yes, if the ROI moves to a different position on the sensor, that new position may have dust specks that the original position does not have. The flat is used to both even out illumination across the FOV as well as remove dust specks from the image.

I was aware that you can change the size of the ROI but I wasn’t aware that you could move a given ROI to a different position on the chip.

[Altocumulus]

3 minutes ago, Freddie said:

Yes, if the ROI moves to a different position on the sensor, that new position may have dust specks that the original position does not have. The flat is used to both even out illumination across the FOV as well as remove dust specks from the image.

I was aware that you can change the size of the ROI but I wasn’t aware that you could move a given ROI to a different position on the chip.

It was something I discovered yesterday morning, the ROI on the sensor is moveable. So that suggests it's best to move the 'scope and keep the same light path (as much as possible).

[Rusted]

I ruined a shot by trying to use a flat.

Here's the bare shot with lots of vignetting cropped out.

Some nice detail coming out. This is with the PST BF and WO 2x Barlow.

 

[Davey-T]

5 minutes ago, Rusted said:

I ruined a shot by trying to use a flat.

T shirt or hanky (creases don't matter) may be too opaque, I use cling film on my 6 inch refractor.

Dave

[Freddie]

Can you post the flat and the image where the flat was used and maybe we can see what went wrong.

[Rusted]

I ended up with a negative of my capture.

The next attempt was covered in bubbles like a bathroom window.

As Geoff has pointed out, my white hanky is too thick.

Edited April 22, 2020 by Rusted

[Freddie]

Are they the flats? I assume not as they shouldn’t look anything like that.

Go on, give defocusing a go. Surely it can’t be that difficult to refocus afterwards.

[Rusted]

42 minutes ago, Freddie said:

Are they the flats? I assume not as they shouldn’t look anything like that.

Go on, give defocusing a go. Surely it can’t be that difficult to refocus afterwards.

Thanks and yes it is!
The focuser is 4' from the fixed monitor on the pier!
I have to hold up a mirror just to see the screen!

I'm squeezing some nice, late afternoon detail out in ImPPG now, but the vignetting is killing me!

Edited April 22, 2020 by Rusted

[Freddie]

Ah ok, I see the problem now. That must cause real issues though as focusing must be a bit of a guessing game and hitting critical focus must be down to pure luck! It’s hard enough to focus with your head under a dark towel staring at a screen 30cm away especially as focus will keep changing as the scope heats up/cools etc. Have you thought about moving your monitor? 

I still think defocusing once during your image run will be worth it to get a decent flat. Just rotate the focuser one turn (check it is truly out of focus, rotate one more turn if not) and when done, rotate it back the same number of turns. It doesn’t matter how out of focus it is, as long as it is out of focus.

 

[Rusted]

If you'd like to nip round.. you can hold my 27" monitor while I focus.
Then, when I nods me head, boss.. hit it!

On the other hand, I could get myself a swivelling TV wall mount.
Though then it would mean a squeeze to get behind the screen even when it is flat on the pier.
My present wall mount is little more than 12mm deep and that took some finding.
And, I needed an extra dioptre of reading glasses even then! 👓

Or, at a pinch, I could start a "go fund you"  [a whip round] for a bigger home made dome.
I'm already flat against the northern wall and my wife has to come and rescue me sometimes.
By kicking my chair out from under me if I really do get stuck behind the focuser on my 2" Lacerta Herschel wedge.
A threat which usually keeps me from nodding off on my longer imaging sessions.
Which are often in excess of 21 hours, in mid summer, when it never gets dark oop here at 55° north.

At my age I should be seriously considering "meals on wheels."
But I'm terrified of getting the virus as they spoon feed me between my monthly baths. 👴🏻
It would have to be a plastic spoon because I'm not allowed sharp things.
You will tell me if I'm rambling again?