What is your typical FWHM?

[jimjam11]

https://www.cloudynights.com/topic/594258-whats-your-best-fwhm-imaging-challenge-optimize-your-resolution/

I read through this topic on CN and some of the results are amazing, but it motivated me to look at my image FWHM and see what they are like. 

Looking back through my data for the last few years and running it through the latest PI Subframe Selector:

2015 - 4.86"

2016 - 6.5"

2017 - 4.15" (Only 2 nights of data found)

2018 - 5.5" (Only 2 nights of data found)

2019 - 6"

The data I looked at ranges through a 500D, 100D, 80D and ASI1600MM Pro (2019 data), but always with a 150p with coma corrector @ 680mm. My best ever FWHM was in 2016 at an amazing 2.18". However, it is clear my average is 5+ based on the majority of the data.

1. Does this confirm my seeing is pretty horrible (Central Scotland) which means I am oversampling considerably (ASI1600 @ 680mm = 1.15" pp)?

2. It could be collimation but I check regularly and it doesn't seem to drift much.

3. Not sure if my mirror coating is breaking down which is leading to elevated FWHM:

This is not dust, dirt or anything easily removable; I have cleaned it a few times believing it was just dirt. The picture does make it look much worse though! My most recent (unfinished) image can be seen here and the stars look good, but the FWHM is allegedly 5.24" which seems really high!

https://www.astrobin.com/390632/?nc=user

4. Something wrong with the measurement in PI?

5. Poor tracking leading to inflated FWHM? This is definitely a possibility but my latest image had a guide RMS of 0.8" with both axes < 0.5". 

6. Poor focus, since getting the ASI1600 I have been autofocussing, my SGPro HFR seem pretty consistent and they dont inflate substantially when going from an 8s focus exposure to a 5min sub. I think this supports the idea that tracking isnt the issue (at least not in all cases).

7. Something else?

[bingevader]

Full width at half maximum.

Learn something new every day.

Sorry, I can't be of any help to you though!

[SamAndrew]

Only been imaging for a couple of months with my new setup but I typically see about 5" for 5 minute exposures. Last night imaging over the neighbours house I was seeing 6"+. I'll have to look back at the data to see my PBs

[Jkulin]

Looking at the example you posted, it would appear that either your guiding or Polar Alignment is off and the collimation.

You have oval stars and dual diffraction spikes.

I would address those issues first and see if your FWHM improves.

HTH

[SamAndrew]

Stars look fine to me in that image, if you go pixel peeping in the corners then it looks like the coma corrector isn't doing a perfect job, possibly spacing? but looks acceptable to me. Have you given your scope chance to stabilise thermally and do you check focus at regular intervals?

We'll wait and see what other values people share.

[jimjam11]

Eccentricity in the image is 0.438 according to Subframe selector so that seems pretty round to me? Agree that the field doesnt look perfectly flat, but it still feels like the best FWHM is too big (support images from FWHMEccentricity in pixels below)...

 

[jimjam11]

Similar plots for the best and worst 5min HA subs:

[jimjam11]

I revisted some of my better FWHM subs and noticed the Subframe Selector tool was identifying a lot of stars (3000+) vs 300-400 for my ASI1600 subs. By tweaking the Star Detection parameters (mainly the hot pixel filter) my stars detected reduced to 400ish and my FWHM rose to 4.02"!

I checked/tweaked my collimation, it was already ok. The weather has been poor but I managed a handful of 30s test exposures around M81 and my average FWHM was 4.3". I am going to try some tests with my ST80 and HA filter on the ASI1600 to see how my FWHM's compare. If the seeing is really this bad then I would be better with something like the 8" or 11" RASA...

I would still be interested to hear what others are getting in terms of their normal/best FWHM and what equipment they are getting it with.

[Lead_weight]

I get between 1.8-2.4 on my 102mm refractor. On my 6" RC I get between 4-6, though I feel like I've seen lower, I just don't always watch it that closely. A lot of it is seeing dependent. 

[SamAndrew]

Went back and had a look at my old data from my Quattro 10s newt. Seems my best nights hovered around 3.0" which is surprising considering how lax my focusing methods used to be. Also the thermal stability of that scope was immense! hardly any shift in focus over 2-3 hours. Typical values were 3-5" it seems, and that was from Berkshire.

[alexbb]

With a 150PDS, 0.9x SkyWatcher coma corrector and ASI1600 I got these days 2-2.6 px fwhm. Guiding through OAG went smooth at 0.4-0.5" rms, with occasional variations to 0.35" or 0.6" rms, probably depending on seeing. Subs were 300s long.

But my mirrors are new, the secondary even replaced with a larger one. Maybe your mirrors' coatings are too worn and light is spread in all directions. 

[jimjam11]

16 hours ago, alexbb said:

With a 150PDS, 0.9x SkyWatcher coma corrector and ASI1600 I got these days 2-2.6 px fwhm. Guiding through OAG went smooth at 0.4-0.5" rms, with occasional variations to 0.35" or 0.6" rms, probably depending on seeing. Subs were 300s long.

But my mirrors are new, the secondary even replaced with a larger one. Maybe your mirrors' coatings are too worn and light is spread in all directions. 

So your image scale is the same as mine (newer version of ota, same camera,  same cc) but your fwhm is approx 1-2px lower. Interesting.

[alexbb]

3 minutes ago, jimjam11 said:

So your image scale is the same as mine (newer version of ota, same camera,  same cc) but your fwhm is approx 1-2px lower. Interesting.

Translating from pixels into arcseconds, it means ~2.3"-3.0" at a 1.16 arcsec/pixel scale.

[jimjam11]

An update:

I am still not convinced the mirror isn't adding a small amount of blur but I havent had the weather to test this very well. However, I tweaked my collimation again and when manually focussed I was getting FWHM measurements as low as 2.4" (so 2.1px) which suggests the mirror is ok?

I got beginners luck with autofocus in SGPro and it worked from the first time. However, after tweaking my collimation it would never get back to good focus after it ran the routine. I eventually realised I had manually racked the focuser when recollimating and the skywatcher motor had slipped on the shaft introducing conserable play. This is now fixed so I need some decent sky to see if I can consistently get in the 2-3" FWHM range...

[jimjam11]

Another update:

I was desparate to buy a new OTA believing the mirror was the cause of my bloated FWHM measurements but I resisted and it would appear this was not the case.

A few nights ago during some very calm weather I averaged 3.16" over the course of two nights, with a good number of frames < 2.5" and 1 frame at 1.89".  Looking back at my original comments I think it was down to 2 things:

1 - Poor Seeing

As the subs were downloading I could see the seeing fluctuation and they would vary from my alltime low of 1.89" to 2.5" over the course of 20-30mins. The majority were coming in around 2.4" on one night and 2.8" on the other.

6 - Poor Focus

I cheaped out on autofocus for my 150p (using the Skywatcher DC Focus motor) and I think this was hurting my FWHM measurements. Autofocus works (I get nice V curves) but I dont believe it ends up in perfect focus very often (if at all). When I start imaging I therefore allow autofocus to run which gets me a set of HFR measurements (using SGPro) for the target/conditions and after it completes I manually tweak the focus until I beat/equal the best HFR during the autofocus routine. I use filter offsets and manually refocus if I see my HFR measurements drifting. This has significantly improved my FWHM measurements.

 

The CN thread has definitely helped my images; it has made me obsess over focus and really chase the best values I can obtain...