ZWO 120mm for Guiding on ASIAir

[Horizon_Brave]

So I have the ZWO 120mm mini for guiding... and I have the lovely ASIair which I think has actually given me more problems than it's solved. Anyway I have an serious issue with guiding...

I can't manually choose a star to guide. When I enter "looping mode" and touch one of the stars, I do indeed see the green box and then a green circle around said star..but when I click to start calibrating, it basically says 'screw your choice' and it picks some other tiny faint star at random. I'm using this on iOS and Android and I get the same thing...

Calibration then fails after it picks it's crap choice of a star telling me 'it hasn't moved enough'

3. I don't know if I have a good focus... I have to attach the extra black piece to it, and it's hanging on by a thread literally out of the back of the scope that I have. Is this normal? This seems like a very very extreme positon as it barely has room to be tightened into the scope..Is this normal? This seems like a very very extreme position as it barely has room to be tightened into the scope..

I have this scope:

https://www.amazon.com/gp/product/B088D37YMZ/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1

4. The ZWO is 120mm, but it's attached to the above scope, which is 30mm.... In ASIAir's interface it asks for the guidecam's focal length...Is it the base 120mm? or is it changed by the 30mm scope I have it attached to? What would be the number that I put into the ASIAIR?

[iapa]

3. Let me look at more specs for that guidescope. To check focus, you can select  the guide camera as imaging camera and turn on real time view. Remember to switch 

4. Your guidescope focal length is 120mm.

Focal length (fl) = F-ratio x aperture.

 

 

Edited June 6, 2021 by iapa

[iapa]

1 hour ago, iapa said:

3. Let me look at more specs for that guidescope. To check focus, you can select  the guide camera as imaging camera and turn on real time view. Remember to switch 

4. Your guidescope focal length is 120mm.

Focal length (fl) = F-ratio x aperture.

 

 

 

 

Get a rough measure of 120mm from where the lens is, subtract 8.5mm, and you have a rough position for the front edge of the camera.

With the ‘scope being 112.2mm end to end, it does look like you will need that extension. Surprising that is necessary is it is supposed to support most common guide camera - most of which are the same shape and dimensions.

You can check focus in day time with a telegraph pole or some thing in the distance.

Something to do when there’s clouds, or in day time.

 

Edited June 6, 2021 by iapa

[iapa]

The guide scope has a focal length of 120mm. So, an object is in focus 120mm from the centre of the lens.

The distance from the front of the ASI120MM mini to the sensor is 8.5.mm.

To get the sensor at the focal point it needs to be 120mm-8.5mm from the centre of the lens to the front of the camera. 

So, if the front of the camera is around 111.5mm, the sensor will be at the focal point.

 

[Horizon_Brave]

Thanks for the response! So... my next question though is what do I enter into the guide software? It asks for the focal length of the guide camera....is it just 120mm? or some calculation of both the guide cam's 120mm and the guide scope's 120mm?

Edited June 6, 2021 by Horizon_Brave

[iapa]

 

12 minutes ago, Horizon_Brave said:

Thanks for the response! So... my next question though is what do I enter into the guide software? It asks for the focal length of the guide camera....is it just 120mm? or some calculation of both the guide cam's 120mm and the guide scope's 120mm?

120mm

[michael8554]

ZWO ASI120MM - 120 is ZWO's model number.

Cameras don't have a focal length, lenses and telescopes and guidescopes do.

 Cameras have a "Camera Flange Distance", the distance from the front mounting to the sensor,  8.5mm in this case as previously mentioned.

Your Guide Scope has a focal length, 120mm as previously mentioned.

As for ASIAir choosing it's own guide star, the brightest star is not necessarily the best choice for guiding.

The ASIAir choice will not be random, choosing a better star than your out-of-focus over-exposed choice.

Michael

[Davey-T]

Here's how mine is set up.

I use a parfocal ring so that the camera fits in the same position every time, then focus using the ring at the front

Dave

Parfocal ring

Camera inserted to ring

Focuser at front

 

[Mike Atkinson]

3 hours ago, Davey-T said:

I use a parfocal ring so that the camera fits in the same position every time, then focus using the ring at the front. 

Dave, just for my sanity, that parfocal ring is passive, yes? I use a small piece of electrical insulation tape to mark the focus position. I guess this achieves the same thing, albeit less pleasing on the eye? 

I do have trouble getting fine focus (using the front thread after loosening the locking ring). The single FWHM tool in Sharpcap works, but moving the focus invariably moves the FOV, moving the search box from around the chosen star. Guidescope is firmly clamped! However, I do find that for guiding precise focus isn't necessary. Indeed, I've seen recommendations to slightly deliberately defocus. 

Mike

[iapa]

I’ve got a one of those par-focal rings as well.

Passive, held in place with three grub screws around the edge which tighten after you have a decent focus.

[Davey-T]

As above just fixed with grub screws, quite cheap, pack of three £8.00.

I've never achieved perfect focus with the front focuser, it's easier to do it with the parfocal ring once the front gets it nearly right.

Dave

[SaberJeff]

Easy to loosen

[iapa]

1 minute ago, SaberJeff said:

Easy to loosen

That’s a bit obscure? 

[SaberJeff]

The guide camera is not firmly fixed.

[powerlord]

All same as dave here. They are a pain to focus.

As for OPs star selection question - don't pick brightest. You want a star that is medium brightness of ones you can see. It will then calibrate. If its not moving check your guiding is working - what mount is it ? Try changing to just north and get that working. Also bump up the aggressive on axis.

Lastly,  just try moving to a new bit of sky for a bit