FWHM value

[lensman57]

Hi,

I have a couple of bug standard budget 80 mm ED scopes, CCDCALC reports that with 0.8 reducer and my Atik 428ex I am imaging at 2.09 arcsec/pixel, what is the best practical value of FWHM that I should expect while imaging a 7th MAG star for 1s exposure loop please, I seem to be getting anything from 1.8 to 2.6, and over a 600s exposure this value goes up to about 4+. I know it is seeing dependant but how much tolerance should I allow?

Many thanks for your help.

Regards,

A.G

[ncjunk]

Your initial values of 1.8 to 2.6 seem about right. I get similar results some nights it won't go below 2.3 and other nights 1.something.

You'll also notice it shifting around a little bit as you try to focus, during the same night, so one time 1.8 and another 2.1.

Around 3am here seems to be when everything cools and reaches its most stable so it can be better around then.

Your final figure depends on tracking as well but i'm sorry to say i've never paid much attention to final values, i only check star shapes.

Sent from my GT-I9003 using Tapatalk 2

[lensman57]

Your initial values of 1.8 to 2.6 seem about right. I get similar results some nights it won't go below 2.3 and other nights 1.something.

You'll also notice it shifting around a little bit as you try to focus, during the same night, so one time 1.8 and another 2.1.

Around 3am here seems to be when everything cools and reaches its most stable so it can be better around then.

Your final figure depends on tracking as well but i'm sorry to say i've never paid much attention to final values, i only check star shapes.

Sent from my GT-I9003 using Tapatalk 2

Hi Neil,

Thank you for your reply, the only reason that I am concerned about the figure is to do with the final size of the stars as I think that my stars are usually bloated though I live in the vecinity of the airport and not far off the city center. You have set my mind at ease. Many thaks.

Regards,

A.G

[Martin-Devon]

I agree with Neil's comments above, FWMH values vary from night to night, narrowband filters usually give lower figures than LRGB as well. On a good night dark with no wind I can regularly get FWMH less than 1 with a H-alpha filter.

You do mention though that over a 600 second run, your FWMH value goes up to 4+ which is a little worrying. Over a period of several hours with a lot of temperature shift it's not unusual with some scopes for the FWMH to double or more over 3-4 hours, but to see this in 10 minutes suggests something other than temperature. Are you absolutely sure your focuser is not slipping, for instance have you locked down the focuser once you have a decent FWMH?

Martin

[swag72]

....while imaging a 7th MAG star for 1s exposure loop ....

Are you using 1s exposures in order to focus? You really should be using longer exposures as this will give you a more stable figure as it will combat the seeing a little. Perhaps by using short exposures you're never quite getting the focus bang on?

Try a 5s loop instead and see if that gives you a more stable figure, that will help a lot.

[Freddie]

The higher value you get on the 600sec exposure could be impacted by your guiding accuracy.

[lensman57]

The higher value you get on the 600sec exposure could be impacted by your guiding accuracy.

Hi Freddie,

I have an EQ5 PRO mount, the best I can do with it is to get the the PHD graph as flatt as possible and that is what I do, on most imaging sessions the GPUSB light only flashes green every 6 or 7 seconds at a time to make a correction, what I notice is that if I focus on a star near my target, it has a brightness value of roughly lets say 30000, once the long exposure sets in it goes up to about 60000+ and the FWHM value goes up accordingly. I have not been able to hit the focus dead on in my last 4 months or so of DSO imaging yet. I also believe that the seeing and the weather have a lot to do with this as I really do not have a peoper dark sky at the best of times, but in the last two evenings I attempted to image the western Veil Nebula, NGC 6960, the first night the clarity of the sky was amazing because the storms cleared all the mock away from the atmosphere, I only managed 5 subs of the target before the clouds moved in and out of the 5 subs only one survived, long story but not relevant here, the second night I managed 3 subs before it clouded over, I got much better result from stretching the single sub on first night than 3 subs on the second night and the difference is huge, the second night was what I call normal conditions for my location. I am at a loss as I can not improve on the FWHM figures without spending over half hour tweatking the focuser and then the end result is not what I'd expected.

Many Thanks and Regards to all,

A.G

Regards to all,

A.G

[Gina]

I use 5s for focussing until I'm about there then I increase it to 10s. I find longer exposures trend to give higher values but yours seems a bit excessive. Check your star shape with the long exposure. Round ones indicate the trouble is likely to be focus whilst oval ones are an indication of guiding errors.

[AndyUK]

Although not a silver bullet, do you have a Bahtinov mask? If so, you could use that with Bahtinov grabber.

Although I know many advocate FWHM focusing is best, I've never been able to get on with the seemingly ever-changing values on screen and Bahtinov grabber:

a) hasn't obviously let me down (yet!) and

b ) satisfies my focusing OCD as it not only displays concentric rings when it's wiithin what it believes is critical focus, but also displays how many microns it believes focus is out (+/-).

Those values still fluctuate but on most nights I can get down to +/- 10 microns or so at the start of session... However, as noted, as the temp drops, then the focus does get worse - When it drifts about 20%, (estimated by FWHM values) then I'll refocus...

[ncjunk]

I use 2.5 seconds and focus max with a stepper motor so when it stops theres no movement in the focuser.

2.5 second is fine for determining focus. The fwhm values look like a "V" if you plot them on a graph against position but the point of the v is not sharp and theres no exact focus point. It looks more like a "u" so theres an area where focus changes make little difference (depends on your setup), its a small area.

Focus max calculated where the point of the v would be based on the slopes of the v and where they would intercept if they carried on to a point.

Are you manually focusing?

If so are you locking down the focuser after you've focused?

I found, pre-stepper motor, that when i manually focused it didnt always stay put. Then i noticed a locking screw but that shifted the focus when i locked it.

I finally realised i needed the locking screw set almost tight enough to stop movement (movement a bit stiff but ok) then it didn't shift when i tightened it off.

I would guess there may be a bit of slack in the focuser?

Hope you get it sorted. I should be trying to focus my 428 on an ed80 for the first time tonight as well.

Sent from my GT-I9003 using Tapatalk 2

[lensman57]

I use 5s for focussing until I'm about there then I increase it to 10s. I find longer exposures trend to give higher values but yours seems a bit excessive. Check your star shape with the long exposure. Round ones indicate the trouble is likely to be focus whilst oval ones are an indication of guiding errors.

Hi Gina,

Thanks for your advice, I attach a quick stretch of an image of the 6960 from last night, I only managed to do 3 subs last night as the clouds moved in but it demonstrates the point, the bright star is the one that I am talking about. The subs were 600s each.

[lensman57]

Hi Gina,

Thanks for your advice, I attach a quick stretch of an image of the 6960 from last night, I only managed to do 3 subs last night as the clouds moved in but it demonstrates the point, the bright star is the one that I am talking about. The subs were 600s each.

Very sorry I pressed the button before it loaded up, here is the sub.

Regards,

A.G

[ollypenrice]

Put your focus star at one of the four intersecions of the 1/3 lines of your chip (the four lines parallel with the edges of the chip and 1/3 of the way to the opposite side). When you have done this once you will never ever focus on a central star again. Read this post twice and then thank Peter (Psychobilly) who began the campaign.

Olly

[lensman57]

Put your focus star at one of the four intersecions of the 1/3 lines of your chip (the four lines parallel with the edges of the chip and 1/3 of the way to the opposite side). When you have done this once you will never ever focus on a central star again. Read this post twice and then thank Peter (Psychobilly) who began the campaign.

Olly

Thanks Olly, the law of 4/3 s, I need to remind myself at times that I used to do fine art photography in my younger days, a better focuser wouldn't go amiss either.

Regards,

A.G

[ncjunk]

Put your focus star at one of the four intersecions of the 1/3 lines of your chip (the four lines parallel with the edges of the chip and 1/3 of the way to the opposite side). When you have done this once you will never ever focus on a central star again. Read this post twice and then thank Peter (Psychobilly) who began the campaign.

Olly

What is this 1/3rd witch craft of which you speak? Whats the theory and why would i never focus on the central star again? Does it bewitch me and cotrol my mind?

Seriously though, whats the idea behind it? I assume to do with variation within the optics and trying to pick a zone in the middle of the min and max variations?

Sent from my GT-I9003 using Tapatalk 2

[Gina]

Your focus plane is rarely truly flat even with a flattener. One third in will give the best compromise between centre stars and edge stars.

[lensman57]

What is this 1/3rd witch craft of which you speak? Whats the theory and why would i never focus on the central star again? Does it bewitch me and cotrol my mind?

Seriously though, whats the idea behind it? I assume to do with variation within the optics and trying to pick a zone in the middle of the min and max variations?

Sent from my GT-I9003 using Tapatalk 2

As Gina posted it is to do with field curavture, I wonder if this is relevant with RC type scopes.

Reagrds,

A.G

[ollypenrice]

What is this 1/3rd witch craft of which you speak? Whats the theory and why would i never focus on the central star again? Does it bewitch me and cotrol my mind?

Seriously though, whats the idea behind it? I assume to do with variation within the optics and trying to pick a zone in the middle of the min and max variations?

Sent from my GT-I9003 using Tapatalk 2

Yes, that's it. Give it a whizz. The big bonus for me is that focus lasts longer during cool down. (Maybe!) Of course, I prefer to measure my chip sides in Rods and Perches since they are so much more accurate than these new fangled millimetres and you only need a stick of imperial hornbeam to do it... Let me know your focal depth in fathoms and I'll work it out for you!

Olly

[Daddystu]

I've found a good way of focusing in APT. I switch on liveview on a sufficiently bright star then switch to negative mode, zoom in and use the focus aid tool. I try and get as high a number as possible on the tool and that seems to do the trick.