Quick question about Focussing/FWHM

[Marky1973]

I have yet to be entirely happy with the focus on my pictures. When cropping them quite heavily (I use a DSLR, so smaller DSOs need a bit of cropping) the stars always feel a bit bloaty and look out of focus - much more than the image from the full frame. I have been using the focus aid in APT, checking out the FWHM tool. I don't know much about the science behind it, but my understanding is that the number needs to be as low as possible.

Last night I was focussing and tweaked it down to around 1 to 1.5 - obviously it was fluctuating with the atmosphere/interference, but I thought that it was a pretty good score. I locked the focus very carefully, making sure that I didn't change the figure - I usually tighten half-way, check the value and then re-do it (if needed) before locking it down completely.

I only twigged tonight that DSS shows a FWHM maximum on the light frames after it has checked them and, when I looked for the images I took last night, the FWHM figures were coming out at over 4.5. I am presuming that this is far from ideal focus, but is also a long way from what I thought it was.

Is there any relation between the FWHM measurements in APT and DSS? Is there a reason for the discrepancy or am I just going to need to be even more careful with focussing? Are there vital settings in APT that I need to set? Will check for more scope related settings next time I get out.

Might also need to get the Bhatinov mask out rather than relying on just APT Focus Aid.

Thanks, as ever, for any thoughts

[VikkiFord]

I use a Bahtinov mask and it was the best thing I could have done. I used it to focus on a bright star then locked the focus and slewed to Saturn, which was a little low in the sky (about 12 to 15 degrees). I recorded 1000 frames and shoved it through AutoStakkert and was amazed at my picture.

Taken using 200pds with x2 Barlow and ASI120-MC.

Vikki.

[Marky1973]

Hi Vikki, great photo! I have used a mask in the past and found it really useful, I have just been using the FWHM focus aid in APT because I am a little lazy and it is easier than getting the mask on and off the scope (which is really easy to be honest, I'm just impatient!) - might have to go back to using both to try and crack the Focus - there is a very small sweet spot with the focus, and it is vital to get it right, so I guess I need to spend more time getting the process right!

[Thalestris24]

 Hi Marky

I've found my focus has improved a lot since I installed electric focussers i.e. the SW autofocuser. It enables fine focus control and also doesn't require the tightening of the lock which invariably throws the focus out. Another advantage is being able to make adjustments remotely.

Louise

[Starlight 1]

You do have to set the FWHM in APT think it in the help files.

[Marky1973]

Thanks Louise - hadn't really thought about auto-focussers before, but will have a look as I just surprised myself with the relatively low price. No idea if there is one for the 70ED or is the SW one will work with it, but will do some research tomorrow! Thanks!

Starlight, I am definitely going to double check all the settings in APT and make sure there isn't something I have missed! Thanks.

[Starlight 1]

I did not know if he put this in the help files , but you can set a start and stop time for your run. on the camera  ,so you set every thing up and put a time for when you like it to start and end. only come by this one last week on testing Apt

[Thalestris24]

Thanks Louise - hadn't really thought about auto-focussers before, but will have a look as I just surprised myself with the relatively low price. No idea if there is one for the 70ED or is the SW one will work with it, but will do some research tomorrow! Thanks!

Starlight, I am definitely going to double check all the settings in APT and make sure there isn't something I have missed! Thanks.

Hi I'm not sure... I believe you can fit one to the Starwave 80?. I've fitted one to the 130pds and 150pds, both of which have 2" Crayford focusers. Also fitted one to the ST80 R+P. It's important to note that the SW is a simple geared DC motor, not a stepper motor. APT supports the more expensive true autofocus ascom stepper motor autofocusers + software, but not the SW directly. APT can be configured to broadcast fwhm and hfd values for custom non-ascom software to pick up. Alternatively you can simply monitor the fwhm/hfd values and adjust the SW motor manually in a similar manner to as you do now.

The SW Crayford focusers seem pants to use manually, at least with the locking screw - focussing has been a lot less frustrating since I installed the motors :} They hold focus very well too.

Louise

[Atreta]

Hi Vikki, great photo! I have used a mask in the past and found it really useful, I have just been using the FWHM focus aid in APT because I am a little lazy and it is easier than getting the mask on and off the scope (which is really easy to be honest, I'm just impatient!) - might have to go back to using both to try and crack the Focus - there is a very small sweet spot with the focus, and it is vital to get it right, so I guess I need to spend more time getting the process right!

don't know if this is a bad idea, but you could use a mask, get it to focus perfectly and make a mark on the focusing tube with a marker pen, so it would be easier next time when you want to focus again.

i'´m planning to do this with mine, but i don't have a bahtinov mask yet.

Frank

[Starlight 1]

I have 4 little cards for each set up for focus with a line on each,

So top of card have  Sun +st120+ 1100d  and so on  . Then the auto -focus only have to made little moves .

[Marky1973]

Yeah, I've marked the guide scope for reference and have notes for the main scope... but obviously you do still need to make adjustments each time.

Seen a tutorial on YouTube showing how to fix the SW Auto Focuser to an ED80, so will have a look at the Crayford focuser on my 70ED and see if it is possible.

[VikkiFord]

When I got my first telescope it was a Sky Watcher Startravel 102 refractor. I had trouble focusing because the manual mount and tripod allowed the image to dart about like some demented bee high on honey. It was an absolute pain. I'd only just started then and I got a Bahtinov mask but didn't really understand it (probably done to demented bee syndrome) so I made my own electric focuser for it. Oh, at this time I only had my Canon EOS 1200D as my AP camera.

Initially I had the stepper driving the focus knob directly but even running it at 12V with the appropriate current limiting driver chip it couldn't turn the knob. I added a 4:1 gear system to it, as seen in the photo, and this worked very nicely. The controller I made (I'm an electronic R & D engineer so knocking up a quick micro-controller based do-dad is easy for me) allowed for fast movement at the touch of a button or single step movement with the flick of a switch. I'm a machinist too and have my own small lathe and mini-mill.

However, it wasn't without it's problems. My eyes are old and I couldn't really see if something was sharp or only just sharpish. The focuser was cheap and nasty and in single step mode the focuser would go past focus and waste 5 steps in the reverse direction because the focus tube moved laterally in the 'scope tube, yep, that also moved the observed up and down in the frame. Using the Bahtinov mask with it made life so much simpler as I didn't have to really on my idea of sharpness and any movement that was induced by the focuser was negligible compared to touching the focus knob.

This was at the start of this fascinating new hobby and now I have a 200pds with an HEQ5 pro mount. The 200pds focuser is low profile and has a 10:1 ration fine focus knob on it. The mask, the sturdiness of the mount and this soft touch focuser now make focusing a breeze.

I doubt I'll use my 102 much so I won't be updating the focuser controller. I was going to add limits to it so that if you passed good focus you could set a limit and then do the same for the opposite direction and keep lowering the limits closer and closer to the focus point until you were as good as in focus. That was before I learnt how to use the Bahtinov mask  properly. I may modify it to fit my 130p (it's a long story, I ought to go do a new member intro).

Upshot, electric focus is a big help but I won't be doing one for my 200pds because of the wonderful focuser it has on it.

Vikki.

[RikM]

I find I get the best focus using a Bahtinov mask and Bahtinov grabber software. I tried FWHM but found I kept chasing the lowest number and it took ages. With the grabber software and correct mask for your scope you can dial it in really quickly and know for sure when you are within the critical focus depth window. Even within that you can make very small adjustments looking at the focus offset in microns until you have it absolutely nailed.

[Thalestris24]

Hi

Just to mention that the SW autofocuser (and stepper autofocusers for that matter) are geared such that they can transfer a high torque to make very fine adjustments. Obviously they are designed to be able to operate the scope focus mechanism properly and easily. With the SW, as it comes, focus is done on a very basic controller and through a couple of push buttons for direction and a pot for speed. However, HitecAstro can supply a ascom compatible usb controller which allows controlling the SW from a PC/Laptop. I actually built my own, but presumably the HitecAstro one can be integrated with APT - though I'm not 100% sure on that.

Louise

[lensman57]

I have yet to be entirely happy with the focus on my pictures. When cropping them quite heavily (I use a DSLR, so smaller DSOs need a bit of cropping) the stars always feel a bit bloaty and look out of focus - much more than the image from the full frame. I have been using the focus aid in APT, checking out the FWHM tool. I don't know much about the science behind it, but my understanding is that the number needs to be as low as possible.

Last night I was focussing and tweaked it down to around 1 to 1.5 - obviously it was fluctuating with the atmosphere/interference, but I thought that it was a pretty good score. I locked the focus very carefully, making sure that I didn't change the figure - I usually tighten half-way, check the value and then re-do it (if needed) before locking it down completely.

I only twigged tonight that DSS shows a FWHM maximum on the light frames after it has checked them and, when I looked for the images I took last night, the FWHM figures were coming out at over 4.5. I am presuming that this is far from ideal focus, but is also a long way from what I thought it was.

Is there any relation between the FWHM measurements in APT and DSS? Is there a reason for the discrepancy or am I just going to need to be even more careful with focussing? Are there vital settings in APT that I need to set? Will check for more scope related settings next time I get out.

Might also need to get the Bhatinov mask out rather than relying on just APT Focus Aid.

Thanks, as ever, for any thoughts

The FWHM you get in a focus assist tool has little to do with the FWHM calculated by DSS or other stacking software . For a start APT uses the live view image to calculate the FWHM but here the exposure is very short indeed but your subs have exposures in minutes not millisecond. As the exposure is increased the FWHM value also increases as the light from the star is scattered more widely. I use an exposure loop of 3 seconds in Nebulosity for fine focus and this generally gives reliable results. The other problem as you mentioned is your local sky seeing and this is far from stable in most parts of the UK. There is a reason why they build the large telescopes on top the mountains as up there the atmospheric turbulence is less of a problem. I do use APT for focusing with my modded Canons and depending on the scope used I generally get values of around 2~3 for FWHM but I have increased the stack number to maximum.

A.G

[Thalestris24]

The FWHM you get in a focus assist tool has little to do with the FWHM calculated by DSS or other stacking software . For a start APT uses the live view image to calculate the FWHM but here the exposure is very short indeed but your subs have exposures in minutes not millisecond. As the exposure is increased the FWHM value also increases as the light from the star is scattered more widely. I use an exposure loop of 3 seconds in Nebulosity for fine focus and this generally gives reliable results. The other problem as you mentioned is your local sky seeing and this is far from stable in most parts of the UK. There is a reason why they build the large telescopes on top the mountains as up there the atmospheric turbulence is less of a problem. I do use APT for focusing with my modded Canons and depending on the scope used I generally get values of around 2~3 for FWHM but I have increased the stack number to maximum.

A.G

Hiya

I find it better to set up a focus plan in APT rather than using liveview. Perhaps it takes longer and you lose the instant feedback but I think it gives me a better result and I can take long enough focussing subs to even out the seeing

Louise

[Marky1973]

Thanks everyone for the comments and thoughts - plenty to be thinking about for the next venture outside - think I have adjusted for my cone error today as well, so hopefully there will be nothing to stop me now... well, apart from cloud, rain, atmosphere, light pollution and tripping over the tripod! 

The FWHM you get in a focus assist tool has little to do with the FWHM calculated by DSS or other stacking software . For a start APT uses the live view image to calculate the FWHM but here the exposure is very short indeed but your subs have exposures in minutes not millisecond. As the exposure is increased the FWHM value also increases as the light from the star is scattered more widely. I use an exposure loop of 3 seconds in Nebulosity for fine focus and this generally gives reliable results. The other problem as you mentioned is your local sky seeing and this is far from stable in most parts of the UK. There is a reason why they build the large telescopes on top the mountains as up there the atmospheric turbulence is less of a problem. I do use APT for focusing with my modded Canons and depending on the scope used I generally get values of around 2~3 for FWHM but I have increased the stack number to maximum.

A.G

A.G. - one very quick and stupid question - is that the LV Stack setting in the advanced APT settings? Seems to be on 20 by default, but should I be increasing that?

Hiya

I find it better to set up a focus plan in APT rather than using liveview. Perhaps it takes longer and you lose the instant feedback but I think it gives me a better result and I can take long enough focussing subs to even out the seeing

Louise

Hi Louise - do you mean simply taking  a series of images and adjusting the focus on the basis of the results? Probably would take me ages until I get more familiar with the equipment, but I can see how it would be a useful method. Can you also do me a small favour whenever you get the time, absolutely no rush! - can you measure the distance from the central mounting holes on the SW autofocuse baseplate to the elbow on the same bracket. The YouTube vid suggests I could use this on the Crayford I have, but want to check the hole distance as the holes on my focuser are different from those on the video from what I can make out - but no rush, I'm not going to be buying one today!   Thanks!

Thanks again everyone!

[Thalestris24]

Thanks everyone for the comments and thoughts - plenty to be thinking about for the next venture outside - think I have adjusted for my cone error today as well, so hopefully there will be nothing to stop me now... well, apart from cloud, rain, atmosphere, light pollution and tripping over the tripod! 

A.G. - one very quick and stupid question - is that the LV Stack setting in the advanced APT settings? Seems to be on 20 by default, but should I be increasing that?

Hi Louise - do you mean simply taking  a series of images and adjusting the focus on the basis of the results? Probably would take me ages until I get more familiar with the equipment, but I can see how it would be a useful method. Can you also do me a small favour whenever you get the time, absolutely no rush! - can you measure the distance from the central mounting holes on the SW autofocuse baseplate to the elbow on the same bracket. The YouTube vid suggests I could use this on the Crayford I have, but want to check the hole distance as the holes on my focuser are different from those on the video from what I can make out - but no rush, I'm not going to be buying one today!   Thanks!

Thanks again everyone!

Hi

The widest distance between screws is about 32mm and the narrowest 22mm, to nearest mm (As you can see from the vid, there are 4 screws, one at each corner of the mounting plate). I suppose one could always drill extra holes if needed and if you have the equipment to do it precisely. I'm not sure about the availability of any alternate mounting plates for the SW. Other brand autofocusers may have more choice.

Yeah, see 'focus plan' in APT. Of course you can do a combo method - get close with liveview and fine adjust with focus plan. There is a bit of a knack to it compared to simply twiddling the focus knobs. I'm fortunate in that I don't have to refocus very often - a quick check with a new session plus maybe a tiny tweak seems sufficient but I have a semi-permanent setup - in my living room! I did have the idea of trying to do a true autofocus in software but the atmosphere here precludes that I just have to get the best I can.

Louise

Edit: Oops! think I misread... It's about 44-45mm from the holes nearest the L of the bracket to the outside of that bracket - is that what you wanted?

[Marky1973]

Thanks Louise - I probably didn't explain it very well. The video shows a bracket with for corner holes and four in the middle - like this

My focusser doesn't have the four corner screws that were used in the video to remount the focuser and the auto-focuser plate. However, it does have four that match up to the two larger and two smaller centre four (if that makes sense - I'm not describing this well!) - I was thinking I would have to fix the plate to the focuser via those holes - so was wanting to check the distance from the centre of the centre holes to the bend in the AF plate, to see it it would fit - as per the arrow!

Sorry, I didn't really explain it very well, but really appreciate you measuring for me!

Thank goodness I'm not trying to explain drift alignment to anyone!

[Thalestris24]

Hi

I'm afraid I think you need the four corner holes and the kit includes a set of 4 longer screws to attach the bracket through the corners. Under each corner on the original SW focuser there is a small o-ring and these need to be in place. So I can't say if it would be possible to mount the SW autofocuser on your scope. Maybe someone else here has managed it or found an alternative? It's unfortunate that the forum is quiet now with many apparently having packed up until August.

Louise

[Marky1973]

Ah, thanks Louise... looking at it again, I guess the centre holes are for access only, so it may not be a goer. Unless I can use heavy duty Velcro!

[Thalestris24]

Hiya

There's a short vid on fixing one to an Equinox 66 - maybe similar? Just a bit of drilling...

Louise

[Marky1973]

Drilling!

[lensman57]

" one very quick and stupid question - is that the LV Stack setting in the advanced APT settings? Seems to be on 20 by default, but should I be increasing that?"

I can't remember the number for the love of god but I have set it as high as it gets.

Regards,

A.G

[Marky1973]

" one very quick and stupid question - is that the LV Stack setting in the advanced APT settings? Seems to be on 20 by default, but should I be increasing that?"

I can't remember the number for the love of god but I have set it as high as it gets.

Regards,

A.G

Okay, sorry - I just thought there might be a "best-fit" setting that most people use - I'll have a fiddle next time I am out and about.

Cheers!

Mark