Focusing - there must be a better way

[Tommohawk]

Hi all

I'm going to make the jump - I think - to mono for DSOs and planetary, but one thing bothers me. Actually lots of things bother me but one in particular.

Focusing - it takes me forever to get a focus I'm confident with, and if I have to spend 5-10 mins refocusing between filter changes I'm going to get some very odd Jupiter images I think. So how do folk do it?

I've tried turning down the gain, and turning up the exposure time (just for focusing)  - that definitely helps cos the image is less noisy, but still not great. I've tried the Firecapture focusing aid - not so sure that helps a lot.

Most of the best planetary images seem to be done with mono so its obviously possible. So whats the trick? Any advice really appreciated!

[LightBucket]

Focus on a bright star first, with a Bahitnov mask, then slew to planet....I find that much easier, but not if having to do it for every filter...

[tooth_dr]

I used to do it without a mask, but it took ages. Now I use the mask that @LightBucket suggests and it's a few seconds to get it spot on. Star gets crosshairs and you just line it up.

I made one from cornflake box and a scalpel, the other is cut out of steel.  Both work just fine! You can buy plastic ones relatively cheaply online.

 

All the best

Adam

[Tommohawk]

Yes I have a Bahhtinov and use that routinely for DSOs. I tried it for planetary but found that with v high mag it was difficult to slew to star and then back to target reliably. Not so bad with DSLR switching from full view to crop mode, but with small chip planetary cam it would be a bit of an issue I think.

Also when I did manage to relocate target planet successfully after Bahtinov on star, I wasnt sure that the focus seesd quite right. Clearly the distance to a star vs planet is different but not in focusing terms, though somehow it never seemed right. I think others have made this observation too.

Maybe the filters are completely parfocal?

 

[tooth_dr]

I use the mask on Jupiters moons when I'm imaging the planet, right or wrong but it worked ok for me, with a webcam and DMK and 5x Barlow. 

[LightBucket]

Focusing on a star and getting it correct should be fine, as for the difference in distances, maybe I am wrong, but that should not make a difference,  bearring in mind the distances we are talking about...

[Tommohawk]

Well one thing about dedicated cam with FC, as opposed to DSLR, is you  can rack up the gain and get a brighter image. I wondered if this might be helpful but havent tried yet - at least I cold slew to a closer star. But using a Bahtinov on Jupiters moons? I guess youd need to ensure the moon was good bit away from Jup otherwise the pattern would be a bit wierd?

[Tommohawk]

1 minute ago, LightBucket said:

Focusing on a star and getting it correct should be fine, as for the difference in distances, maybe I am wrong, but that should not make a difference,  bearring in mind the distances we are talking about...

Agreed - in theory!! But somehow I did get mixed results with this in the past. I'll try again next time there's clear sky. All these things to try and not enough hours in the day... err..  night.

[Tommohawk]

Leaving aside good focus its the refocus between filters which is the main issue cos of the time lag.

Do folk do this, or just try and ensure matched / parfocal filters and not bother with refocus?

[LightBucket]

I have a parfocal set of Baader filters, and used to focus for one and leave it for them all, but people say you still need to refocus, so can't really comment on that, as I have gone back to OSC for the time being for DSO imaging, as just not got the time or patience for Mono, and before Olly pipes in and says mono does not take any longer......when i'm attempting it, it takes a lot longer believe me....lol

so I think that is a discussion for the serious mono imagers and maybe some will be along to comment soon..

[tooth_dr]

15 minutes ago, Tommohawk said:

Well one thing about dedicated cam with FC, as opposed to DSLR, is you  can rack up the gain and get a brighter image. I wondered if this might be helpful but havent tried yet - at least I cold slew to a closer star. But using a Bahtinov on Jupiters moons? I guess youd need to ensure the moon was good bit away from Jup otherwise the pattern would be a bit wierd?

I'm not an expert just my own workflow. Not sure about a weird pattern, as every point of light creates its own crosshairs to focus on 

[Davey-T]

Generally I don't worry too much refocusing between RGB just make sure the L is focussed, this not written in stone for every target.

For narrow band I generally don't get enough clear skies to need to refocus as I just do one filter per night.

I use autofocus but some sort of motor focuser is a boon even if you control it manually, I used to use the FWHM in Maxim which also nails the focus used with a manual focuser.

In the end there's no substitute for spending more money for an easy  easier life.

Dave

PS: I don't think planetary and DSO imaging have much in common 

[Tommohawk]

12 minutes ago, LightBucket said:

I have a parfocal set of Baader filters, and used to focus for one and leave it for them all, but people say you still need to refocus, so can't really comment on that, as I have gone back to OSC for the time being for DSO imaging, as just not got the time or patience for Mono, and before Olly pipes in and says mono does not take any longer......when i'm attempting it, it takes a lot longer believe me....lol

so I think that is a discussion for the serious mono imagers and maybe some will be along to comment soon..

Olly's view on mono seems well known! He's probably thinking more about DSOs though, but even planetary specialists seem to favour mono. I know Kokatha man (Darryl?) like the ASI224MC, but he still prefers the 290mono I think.  

I've certainly been impressed with the general ease of use of the ASI290MC I've been playing with, and the interface with FC is so good I reckon going to mono should be a breeze (!) Its just the planetary focsusing thats bothering me.

TBH some of this has to do with the fact that I'm at about F27, so pushing my luck a bit - the screen image is a bit iffy.

11 minutes ago, tooth_dr said:

I'm not an expert just my own workflow. Not sure about a weird pattern, as every point of light creates its own crosshairs to focus on 

Well - the simple thing is I need to try it! Your image posted above certainly shows a nice clear target on the EOS screen.

13 minutes ago, Davey-T said:

Generally I don't worry too much refocusing between RGB just make sure the L is focussed, this not written in stone for every target.

For narrow band I generally don't get enough clear skies to need to refocus as I just do one filter per night.

I use autofocus but some sort of motor focuser is a boon even if you control it manually, I used to use the FWHM in Maxim which also nails the focus used with a manual focuser.

In the end there's no substitute for spending more money for an easy  easier life.

Dave

Again for DSOs I'm not so fussed about refocusing if its necessary - there's time enough.

I tried a motor focuser on the SCT and TBH I didnt find it that helpful - I have a pretty steady hand.... on a good day!

What really does help is the autoalign in FC - it works beautifully.. keeps the image rock steady. So for me it's more about deciding when your at the right point rather than actually doing he adjustment.

 

[Davey-T]

14 minutes ago, Tommohawk said:

I tried a motor focuser on the SCT and TBH I didnt find it that helpful - I have a pretty steady hand.... on a good day!

What really does help is the autoalign in FC - it works beautifully.. keeps the image rock steady. So for me it's more about deciding when your at the right point rather than actually doing he adjustment.

 

Something that gives you a FWHM / HFD readout is invaluable much better than your eyeballs 

And a motor focuser can adjust at micron level, doubt if fingers can.

Dave

[newbie alert]

I have a mono camera and use baader filters..the red blue,green and luminance all have different point of focus..the green and blue aren't that far from one another but the red and luminance are ..are you using a crayford type focuser as you will need that fine focus adjustment..red blue and green all scatter and particularly the blue..as with Jupiter the blue is the important one...

I start with the red channel and focus on the limb or it's moons,I expand the image to get the best focus I can..touching the scope ,focuser and handset will add vibrations that will distort the image so let it settle down and make sure you have the best focus you can..with Jupiter and Saturn being so low it don't help with atmospheric turbulence.. .

I'm sure the way forward is a electric focuser ..im sure some have a counter so  say the red is in focus at say number  201 and green is 212,blue is 215.. they can just get on with imaging rather than doing it each filter change..theoretical figures..not actually based on what I do but that's how I understand it..

[ronin]

You need filters that are the same thickness and preferably the same glass type. The thickness is the main factor.

If I am right the change in focus is 1/3 of the glass thickness, so a 3mm thick glass substrate move the focal plane by 1mm, so having a mix of same 2mm and 3mm and maybe a 4mm filter in there somewhere really mucks it up. The refractive index of the glass also has an effect but as said it is more minor.

The other thing is to set it and force yourself to leave it alone (if the filters are the same thickness).

[Starflyer]

I use the Baader LRGB filters and none of them are exactly parfocal on either of my scopes, enough that I can see the stars are larger after a filter change.

I went the autofocus route and now have Lakeside focus on both scopes.  You can set up filter offsets in your capture programs, focus with a mask using the L filter and then when you change filters it applies the offset and you have perfect focus for that filter.

Not a cheap solution, but now I can forget about re-focussing and just let autofocus keep the setup in perfect focus all night.

[geoflewis]

On 21/05/2017 at 10:30, Tommohawk said:

if I have to spend 5-10 mins refocusing between filter changes I'm going to get some very odd Jupiter images I think. So how do folk do it?

For Jupiter, try focussing on one of its moons, but then there is no need to adjust focus the other filters. I tend to use R or IR filter to get best focus depending on seeing. Higher frame rates are possible with the R so that is my default for focusing, but sometimes the IR works better. B will almost always look a mess, so in my opinion don't try focusing with that filter. With patience and during good seeing it is possible to focus on the Jupiter's surface features. For Saturn I look for a sharp Cassini division. Best focus is constantly changing due to atmospherics when imaging the planets, so 'lucky imaging' using Autostakkert, or Registax will still grade for the best in focus images on each filter without the need to refocus. Good luck, Geof

[Tommohawk]

Thanks for the further comments.

I must say I can't see any difference in focus between the filters, and so for now I'm leaving it set the same for all. With regard to making the adjustments, I think my focusser is OK - the problem is more about deciding when its correct, rather than effecting the change.

TBH I think the biggest problem at the moment is that I'm way oversampled and the image just isn't clear enough to make the call. I'm using an ASI290 (2.9mic pixels) with skywatcher 200 and Powermate x5 (although its only 4.8 the way I have it set) so about F24 which is really too much. The plan is to upgrade to a different optical train at some point so this should improve.

I suppose my original question I was wondering if there was some more definite method for assessing image focus. Firecapture has a focus aid but TBH I don't find it works very well.

I used a Bahtinov for a while but got the impression that somehow it didnt quite get it right. Main issue is finding a star and then pointing back to target.

I haven't tried focussing on a moon - I like that idea. Hoping to have some sort of go at Saturn but getting a bit late now and with Saturn so low its a tough target. Looking forward to some lunar work when time allows and hopefully this should be a more obliging target. Thanks for all the comments!