Maksutov 127, asi224mc problems getting focus.

[LaurenceT]

In contrast to the precise advice from ZWO regarding the 55mm backfocus on their cooled cameras the information about the imaging train for the Mak and asi224 is sketchy and varied. I've seen a ZWO document that suggests simply putting the camera into the visual back is enough and also advice about leaving the diagonal in place and replacing the eyepiece with the camera. None of this seems to work for me and my lunar images are very poorly focused.

I came across this video:

https://youtu.be/DVB7R6xdxQY

and his imaging train involves using various Baader finetuning rings:

Baader Hyperion Finetuning Ring | First Light Optics

but he's very vague in describing exactly what his imaging train actually consists of although he does mention a barlow which I don't yet intend to use for lunar but will do so for planetary. As this setup seems to be a popular imaging solution I wonder if anyone can shed some light on this for me.

[Cosmic Geoff]

Getting an ASI224MC camera to focus with your scope is possible with your 127mm Mak, but is a skill that you will just have to master.

First of all, forget about 'back focus'.  It is largely irrelevant when imaging a planet or other object of small angular size, and is relevant for deep-space imaging with large-sensor cameras. 

You can use an diagonal or not - it's your choice.  Personally I find it convenient to use a diagonal and perform the necessary 'flip' in software.

You can use a Barlow if you want, but with a focal ratio of f12.5 or so you may find it does not improve results, and makes finding and tracking the target much more difficult.

The difference between eyepiece focus and camera focus will be enough to cause you some grief.  A trick you can try is to get focus with an eyepiece on some really bright object, then pull the eyepiece out by about 5mm and refocus, then center the target and substitute the camera.  The target should now be close enough in focus for you to see it, if you have the exposure time turned up enough. 

As for what to fine focus on, you can try to focus on planetary detail, or on one of Jupiter's moons (very handy) or on an alignment star.   I focus using the scope's standard focus knob.

You may also need an infrared-cut filter, and an ADC (q.v.).

 

Edited December 11, 2022 by Cosmic Geoff

[TheThing]

I had the same problem with my mak and ASI462MC.  I've 'fixed' it by using some of these parfocal rings: -

https://www.firstlightoptics.com/adapters/astro-essentials-parfocal-rings.html

Set up during the day (doesn't matter how far away the object is, as long as you can focus on it). Now I just focus with the ep with the parfocal ring on, swap for the camera and maybe a bit of fine focusing, and you're away!

[jacko61]

In addition to Geoff's comment ' First of all, forget about 'back focus'.  It is largely irrelevant when imaging a planet or other object of small angular size, and is relevant for deep-space imaging with large-sensor cameras. ',  It's worth understanding that Back focus is not a function of the camera and ZWO should be shot for their insistence on stating 55mm.  Back focus is a function of the optics which in many case 'can' be 55mm from the last optical part of the scope e.g. the threads on the visual back of an SCT but is often not 55mm.  The back focus of my C8 sct for example is 105mm from the visual back and is also 105mm from the threads of the 6.3 reducer if I fit that for wide field imaging.  Another example - the back focus for my Zenithstar 73 with a flattener is 66.4mm and with a reducer is 56.6mm.  In fact, of all my scope setups, the only one expecting 55mm is the altair 150edf fitted with a reducer (with a flattener it's 91.5mm).   

On the subject of focussing, Geoff's advice is spot on and I'll be using the 'pulled eyepiece' trick myself in the future. 

Graeme

[Owmuchonomy]

Good advice above.  Are you certain it's a focus problem with your images?  The pixel size of the 224 and your Mak's focal length are close to the sweet spot for the camera (f/15).  If you are using a barlow then potentially you will be over sampling a lot thereby degrading your images (among other possible reasons).  There is also little benefit in using an OSC for the Moon.  Much better results can be obtained using a mono cam. An IR pass filter combined with a good NIR camera such as the ASI290 will also combat to some extent the seeing issues in UK skies.

[AstroMuni]

23 hours ago, LaurenceT said:

In contrast to the precise advice from ZWO regarding the 55mm backfocus on their cooled cameras the information about the imaging train for the Mak and asi224 is sketchy and varied.

As others have said back focus is not relevant in your case. If you have attached a field flatner or Coma corrector then its relevant to help calculate the adjustment rings you have to put in between. Have a go at focusing during daytime and then you can tweak at night to get precise focus.

[PeterCPC]

Just to add that I find a flip mirror such a help when imaging planets.

[LaurenceT]

Thanks for the replies, I do now understand what back focus is and probably more importantly, what it's not.

I'll get those parfocal rings to see if they may help. I have managed to get focus during the day but it's not perfect which is the same as when capturing the lunar surface. I can focus enough to see craters and other features but the view is fuzzy and the results after processing are poor.

The view about mono cameras being better for lunar makes interesting reading, the only one I own is the asi120mm mini that is attached to my guide scope, I'm naturally reluctant to remove it as it's well focused but I suppose I could always take exact measurements with an electronic vernier calipers which could make reconnection easy without having to refocus. I don't think that I could sneak a new camera past the ever vigilant Domestic Supervisor....

Edited December 12, 2022 by LaurenceT

[Neil H]

Would a bahtinov mask and bahtinov grabber help , I was told that once eyepiece is at focus then put camera in and wind focuser in the camera will focus closer the scope , I have not tried my 224mc in my 180 MAK yet  but when it was in my 127/1200 refractor I had both extension tubes in to get camera to focus

https://www.bing.com/videos/search?q=bathinov+grabber&qpvt=bathinov+grabber&view=detail&mid=871F1C3677082B11BDF5871F1C3677082B11BDF5&&FORM=VRDGAR

Edited December 12, 2022 by Neil H

[Neil H]

https://astrophotography.app/APT_UnReg4_10.exe this is the download link for astrophotography tool and it has a bahtinov grabber in it 

[happy-kat]

If you have one you might try just using your 224 with an IR pass filter, a sort of mono experience. The atmosphere might just have been bobbing boiling around on the nights you've tried so far.

[AstroMuni]

17 hours ago, LaurenceT said:

I have managed to get focus during the day but it's not perfect which is the same as when capturing the lunar surface. I can focus enough to see craters and other features but the view is fuzzy and the results after processing are poor.

If you are not using a UV/IR cut filter then get one. This helps improve focus.

17 hours ago, LaurenceT said:

the only one I own is the asi120mm mini that is attached to my guide scope,

You could swap the ASI224 with the ASI120mm as they both have similar FOV and focus requirements and give it a try

Edited December 13, 2022 by AstroMuni