ASIair Plus Focal Length Setting

[groberts]

I have been successfully using a William Optics GT81 (focal length 478mm) matched with a WO 0.80 F6-A Focal Reducer and ZWO ASI294mm pro camera and APT image capture for some time.  Of course, the addition of the Focal Reducer changes the focal length to 382mm, which is therefore the FL I use with the aforesaid image capture software and it all works very well.   

For convenience and greater flexibility, I recently purchased an AM5 mount and ASIair plus, which I am now just learning to use.  It’s been quite successful so far (blown away with the Meridian Flip and plate solve function!) but for some reason I am having a problem with the focal length used by ASIair plus.

At the start, when setting up ASIair plus I enter the aforesaid 382mm adjusted FL when setting-up the Main and Guide scope camera (same page that includes Latitude and Longitude etc) before proceeding to the main operating screen.  However, when I subsequently run an image schedule, for some unknown reason the focal length changes to 391mm – I have no idea where this number comes from or why it is self-populating the image settings.

Any advice / guidance would be appreciated on how to deal with this problem?           

Graham

[geeklee]

57 minutes ago, groberts said:

However, when I subsequently run an image schedule, for some unknown reason the focal length changes to 391mm – I have no idea where this number comes from or why it is self-populating the image settings.

Hi Graham - In theory, any plate solve action on the ASIAir will update the focal length it detects in the main options.  I've seen this myself to lesser degree, with a scope changing by a few mm when a plate solve is done.

You can plate solve the image yourself and get the focal length or feel free to drop a sub in here with your camera info and I'll plate solve it - see what the number is.

[groberts]

Thanks Lee, but don't uderstand why this is? 

As a matter of interest, although I've successfuly used the Meridian Flip function (brilliant!), which of course subsequently plate solves to frame the image again, but when I try to plate solve a previous sub to set up a new image run it fails.  Any ideas what could be the problem - I've scrupulously checked the focus which is good?

Graham 

 

 

Edited September 20 by groberts

[geeklee]

3 minutes ago, groberts said:

Thanks Lee, but don't uderstand why this is? 

If manually plate solving the image yourself agrees at 391mm, then that's your actual focal length.

If the FR was actually 0.818, you'd be at 391mm.  Marketing?

[Elp]

It's incredibly difficult to get the same exact specification every single time in manufacturing, you'd likely spend more time and money on verification and measurement rather than machining. So realistically achievable tolerances are made to work within. Also verification is likely done on random batch check rather than 100 percent every piece. If you want more accurate, better quality the customer usually has to pay for it (and they likely won't like the cost and associated lead times).

You will get optical variation as a result. The asiair calculates based on the camera sensor size and the FOV being imaged. If you're not sure on FL, you can input zero on the first screen, take a preview image in preview mode and plate solve, then it'll work it out automatically. You'll also get variation if anything has shifted slightly, one of my setups with minimal room for slack screw threads I know is 307mm pretty much all the time, the other night it plate solved to 306.

Regarding plate solving a prior image, you may need to press "sync to mount" or "sync to mount and goto" once the plate solve confirmation dialogue box comes up.

Edited September 20 by Elp

[Ags]

My FMA 135 is measured as being 138 mm focal length by my Air - I got 3 extra mm for free!

[groberts]

Thanks everyone, much appreciated - the AM5 + ASIair is a wicked combination, that I'm already really enjoying, as is my back!

One final question, if I may - I can't see Polaris from my location and used the All Sky Polar Alignment function, which is quite slick + I seemed to get reasonable aligment but, my stars are a little elongated.  Anything I could do to improve this - I should say that guiding is excellent, generally varying from about 0.40 to 0.75 RMS!  Just wondering if I've missed something? 

Graham  

 

 

[Budgie1]

Are the stars elongated in one direction or do they point towards the middle if you look at each corner?

If the latter then it could be the back spacing which needs adjustment. 

[groberts]

42 minutes ago, Budgie1 said:

Are the stars elongated in one direction or do they point towards the middle if you look at each corner?

If the latter then it could be the back spacing which needs adjustment. 

One direction.

[Elp]

Also every sub? What duration are the subs? What does your calibration graph look like (go into the guiding panel full screen, click on the top right icon, it'll show a calibration graph with a blue and red line).

[CraigT82]

2 hours ago, groberts said:

One direction.

If it’s all one direction and even across the frame then could be differential flexure in guide scope, or focuser sag something like that .

[groberts]

18 hours ago, Elp said:

Also every sub? What duration are the subs? What does your calibration graph look like (go into the guiding panel full screen, click on the top right icon, it'll show a calibration graph with a blue and red line).

On all 300 secs + calibration graph attached + distributed mainly top right down to bottom right.  Any thoughts?

I'm thinking the PA just needs to be a bit better, but it's tricky without seeing Polaris - though using drift align with a standard PHD2 on my laptop works well.

FYI I should also say that the WO GT81 rig is identical to what I usually use on my main EZ-EQ6 GT mount, except I've added another WO guidescope + ZWO120MM Mini camera so it works with ASIair plus (the other guidecope has a no-compatible Lodestar X2 but excellent camera).

Graham

  

 

[Elp]

Maybe the PA accuracy, if you also see similar on shorter subs could likely be the case. Are you also calibrating directly south around 30-40 degrees altitude (near the celestial equator?), I'd assume so having use of other equipment.

The PA routine has been simplified over the years, it now states good accuracy much earlier on within a few arc minutes, I don't think this is good enough, I always do within 50 arc seconds.

Also, HDs generally work better at 0.5s guide exposures but you need to balance it with the seeing youre having on the night.