LRGB Focusing / Imaging Order

[ribuck]

Hi All,

I've been curious for a while about what peoples views are on the focusing aspect of LRGB imaging using a camera with a temprature setpoint and assuming i have a motorized filter wheel.

Please focus comments on the above configuration of camera and motorized wheel.

Is it better to :

A) Select a single filter "L" for example, Focus then do a batch of subs and then repeat the process for other R,G,B filters

Focus once, then just do several Batches of LRGB

C) Focus at start, do a batch of LRGB, Then refoucs before starting the next LRGB batch and repeat.

D) Focus after every filter change (e.g L & Focus, R & Focus, G & Focus, B & Focus) and repeat.

E) Some other method ?

Thanks in advance,

Rich.

[narrowbandpaul]

i would focus.....shoot all the luminance data...

refocus through B, shoot the B data etc etc

grab the B when the object is at its highest that evening....blue light is more susceptible to seeing

though the effect isnt too great to be any more than a fleeting concern

but refocus after a filter change....

[brianb]

refocus after a filter change....

Depends on your filter set. Some e.g. Astronomik are parfocal.

[ribuck]

So assuming my filters are Parafocal, Maybe just refocus each LRGB run to account for atmospheric changes ?

[roundycat]

irrespective of whether your filters are parfocal or not the air temperature is not constant. The OTA will change in length to a degree and that will throw your focusing out. The longer the tube, the bigger the error. Also the critical focus zone depends on the f ratio. Set point cooling in the camera is irrelevant.

How well corrected is your scope? If it brings R and G to a similar focus but B is a mile away you need to re-focus. If you really are dealing with a true apo the problem is minimal. With my FSQ 106 I can focus in G and all the rest from Ha to B and L will all be in focus. However, when doing a three or four hour long imaging run it is, in my view, very risky to focus once and forget it. It takes just a few minutes to check focus and it has paid off for me many times.

Dennis

By the way, I tend towards 'D'.

[ribuck]

I have a 8" newtonian, so it's a long tube. I currently use MaximDL V5 as my imaging application and use FocusMax to do my focusing.

By the sounds of it, for automating the focus after each filter change I would need something like CcdCommander unless something can be doing in MaximDL v5.

Rich.

[brianb]

However, when doing a three or four hour long imaging run it is, in my view, very risky to focus once and forget it. It takes just a few minutes to check focus and it has paid off for me many times.

Agreed - but if trying to grab R G & B in a 2 or 3 minute window (because of rotation of the target) when imaging Jupiter, it helps a lot not to have to refocus, a parfocal filter set is pretty near essential in practise. With a SCT I usually find it's easiest to focus with the red filter in place (the seeing is steadier in longer wavelengths) so, when shooting planets, I focus in red then shoot R, G & B in that order without touching the focus again.

Over a period of more than 10 mins I'd want to check the focus because of possible temperature related changes, "creep" in the focusing mechanism etc.

[ribuck]

I think the way forward for me would be to focus using a single filter, do the RGB shots, then move to the L, refocus and shoot the luminance and repeat the process.

I dont think the temprature should vary enough in 15 mins to warrant more focusing.

[SteveP]

Ribuck

My preferred aproach is D for two reasons - it's a little quicker with no time lost in moving filters etc and secondly, I can keep track easier of what I've done if I image over more than 1 night - a growing issue with our recent weather. If however i'm taking loads of subs I might also have a quick look during a set of subs and tweak the focus if necessary. My scope is however f/5 and the critical focusing range is very small and does change over time

Steve

[GordonH]

Hi

Using parfocal filters is partly the best sollution. I now shoot all my images at f5, either with a BRC-250 or FSQ106 and I focus at the beginning of the evening using Maxim DL and I have never altered focus for the remainder of the night and for me the focusing has remained spot on, even on the long winter evenings where I will be focusing at 5pm and then finishing at 7am the following morning. This may be slightly unconventional compared to what other people do but I generally don't stick to conventions, eg I don't use flats or bias frames when calibrating

Best wishes

Gordon

[RobH]

Hello.

I use focusmax and robofocus on my TMB, and my WO ZS 80 is manually focussed. I will shortly be adding robofocus, or something similar to the WO also.

I focus through my luminance filter, and as I use parfocal Astronomik filters, as long as thay are screwed all the way into the filter wheel, I don't need to refocus for different filters.

If you do though, and you're using maxim with a motorised focuser, you can input a focus offset to deal with any difference in focus between filters.

Like Dennis says, I regularly check focus, as it can drift over the course of the night, certaily with the TMB, although the short WO scope stays pretty much the same unless there is a really big temperature change.

The way I shoot tends to depend on the weather.

If it looks dodgy, I will usually shoot the first 10 or so RGB subs one after the other, rather than in batches, as this then gives me enough RGB to make an image, evn if it's not a great one. If the weather is good, I'll set the filter wheel to shoot in batches, and will do red and blue first, as I can always synthesise the green channel if I need to.

As I usually use 2 scopes to image, while the RGB is being done, the other scope is continuously shooting Luminance, or Ha.

If I'm using one scope, I'll spend a whole night on luminance only, then the next night shooting RGB, and then come back for more luminance later.

Cheers

Rob