Why focusing during LRGB is important...

[Euan]

Unless you have an amazing super triplet scope

I tried my first LRGB creation, but it's a bit of a mess, mainly due to the fact that I focused in R then had fluffy stars in G & B

Here is an animation of the subs running through R-G-B, R being the brightest

Here is the result, not helped of course by getting blooming on the 383L+ at 3x3 binning

I may just have to learn the offsets and program it into Maxim for the filter changes

[shaunster]

I would also suggest its not helped by using 3xbinning

How would you program focus offsets as it changes each night?

[Euan]

I would also suggest its not helped by using 3xbinning ?

I should have put a disclaimer in above that I really don't have a scooby about LRGB and binning

I've gotten it slightly better by making an RGB imaging then applying the Luminance in PixInsight, but still have a fair amount of B and G bloating

How would you program focus offsets as it changes each night?

Would the offsets not be the same as long as you got an initial focus point on one colour?

[ncjunk]

Now...it may be argued that equal RGB unbinned is better than L unbinned and rgb binned combination.

I have recently been doing only rgb on the advice of Dennis and i think it is better than lrgb.

Light blue touch paper...stand well back...

[ncjunk]

I take it they are not parfocal filters? I had some cheap chinese filters and had to refocus after every filter change..

[Euan]

I take it they are not parfocal filters? I had some cheap chinese filters and had to refocus after every filter change..

It's a pretty new Baader (supposedly) par-focal set, although I did 20mins of Red, then 20mins of Green, then 20mins of Blue so it could even be temperature related I suppose

Now...it may be argued that equal RGB unbinned is better than L unbinned and rgb binned combination.

I have recently been doing only rgb on the advice of Dennis and i think it is better than lrgb.

Light blue touch paper...stand well back...

I was getting on fine with just RGB (see below), here was me thinking LRGB would be better, you know what they say... if it's not broken...

[TheThing]

I don't believe the cheap Baader filters are parfocal.

I received a set yesterday (thanks FLO) and from what I understand re-focusing is required.

TheThing.

Sent from my iPhone using Tapatalk

[JamesF]

Which ones do you mean by the cheap ones? The ones that come as a set for around £75?

James

[shaunster]

Which ones do you mean by the cheap ones? The ones that come as a set for around £75?

James

Yes.

[Euan]

I don't believe the cheap Baader filters are parfocal.

I received a set yesterday (thanks FLO) and from what I understand re-focusing is required.

It's the proper £200 imaging set I have, it only seemed to be the blue that was the problem with the 77ED, and the 101EDII supposedly has better colour correction

Next night out I'm going to run through them with the Bahtinov Mask and so what the difference is

[steppenwolf]

Parfocal filters are so dependant of the optics in use unfortunately so I always focus in between filter changes - it's not a bad thing to do anyway during a session as temperature changes as the night goes on can also affect focus so this makes you keep a check on what's going on.

[Uranium235]

This is exactly the thing I mentioned the other day. Unless I have some sort of super APO, then I always need to refocus when changing filter (especially when it comes to blue). The only way out of it would be to use a Newtonian or other design that brings all colours to focus at the same point.

The major downside to re-focusing is that you need to slew to a bright enough star, focus, then slew back... and as sure as eggs is eggs, you will need to re-frame all over again (easier said than done if working in NB!).... sigh.

I too recall roundycat mentioning something about just doing RGB (unbinned), then creating a false L channel by merging the RGB into a mono channel. Got to be worth a try

[steppenwolf]

The major downside to re-focusing is that you need to slew to a bright enough star, focus, then slew back... and as sure as eggs is eggs, you will need to re-frame all over again (easier said than done if working in NB!).... sigh.

After I have completed my first set of images with filter 1, I overlay a crosshair (Al's Reticle (sic)) on the brightest star in the image (doesn't matter where in the frame it is), slew to a bright star, change filter, focus, slew back and with the overlay crosshair still on top I adjust the position of the telescope until the crosshair is on top of the same star as before and I know I have exactly the same framing with a couple of pixels either side.

A search of the Internet for Al's Reticle will get you this great little program.

[ollypenrice]

I know UK imagers may not have my seeing but I refocus on any old star in the main image using Aik's Artemis Capture FWHM reading. Bin1, 3 or 4 second subs to average the seeing. Artemis also keeps the FWHM for the last focus star. Obviously the value is far higher than for 4 seconds once you are imaging in long subs but if you note the value you can see if it is moving.

The Baader interferometric LRGB set are highly parfocal at all FLs I've used, 328mm to 980mm, F3.9 to F7, doublets and triplets. But the sky and the scope change, as Steve said.

Don't use Bin3 except for finding and framing!!!

Bin 2 colour can be useful but not at short FL or if aiming to add NB at any stage. As we've said in earlier discussions it may well be best avoided. Dennis is well clued up on these matters and takes the time to measure and to share his findings.

Thanks for the Al's Reticle idea Steve. Looks good.

Olly

[Uranium235]

Ahhh... thanks! I still have the installer somewhere so I can stick it on the little acer Im currently using for guiding/capture. I havent used Al's Reticle it since I stopped using the DSLR, had totally forgotten about it until you just reminded me

[martin_h]

After I have completed my first set of images with filter 1, I overlay a crosshair (Al's Reticle (sic)) on the brightest star in the image (doesn't matter where in the frame it is), slew to a bright star, change filter, focus, slew back and with the overlay crosshair still on top I adjust the position of the telescope until the crosshair is on top of the same star as before and I know I have exactly the same framing with a couple of pixels either side.

A search of the Internet for Al's Reticle will get you this great little program.

Exactly the way I do it too, invaluable for my mosaic as well, find a star on one side of the frame put the reticule over it and stretch the reticule edge to edge then move the star to the other side of the frame....sorted. Handy for meridian flips too