FMA180 with focus issue - Forums

Topic starter

Aug 28, 2023

Hi all,
I have some problems with focus with my FMA180. The focus is good for green and blue but red is complete out of focus. I have sent the FMA180 to my dealer and I got a new one for exchange. But the new one has the same issue and now the dealer says that this is normal with this scope because it is not a real color free APO.
Below you can see the result of a RGB-Image (with a OSC / QHY183C) where the channels are separated via APP.
Green - Blue - Red

Same happens with a dual narrowband filter like the l-Ultimate ... green/blue is sharp, red is totaly out of focus.

So my question is, is this really normal for this Lens ?

Helpful Engaging

Alan Tidey

Aug 28, 2023

I will follow this as I have the same scope. I use the ZWO ASI533MC camera with it so will check after splitting the channels. I've used it with and without the L-enhance filter.

AdamJ

Aug 28, 2023

So I use a mono camera and although I shoot LRGB I have not noticed any significant shift in focus position when moving between filters.

I have the original FMA180, is yours a pro? It looks like it's a long way out not just a little bit out.

Adam

Werner Stumpferl

Topic starter

Aug 28, 2023

So I use a mono camera and although I shoot LRGB I have not noticed any significant shift in focus position when moving between filters.

I have the original FMA180, is yours a pro? It looks like it's a long way out not just a little bit out.

Adam

No, it´s the "normal" FMA180. As far as I know, the difference has nothing to do with the lenses, only with focuser and a integrated rotator.
And yes, both lenses are far away from focus in red.

D. Jung

Aug 28, 2023 · Edited: Aug 28, 2023

I have used a borrowed fma180 and used it with the Antlia dual narrow band. I did notice a slight focus shift between red and blue, but not even close to the level you see. It was more in the level, yeah, a bit shifted, but no biggy.

https://www.astrobin.com/1datgq/
https://www.astrobin.com/mahksn/B/

Below is the red and blue channel from above image. You can see the focus offset, but not as bad as with yours. I recall that this was ultimately the reason why i decided against buying this lens. You can fix it easily with BlurXterminator, but the lens is a bit too expensive for this level of chromatic aberrations in my book.

Helpful

GregLatiak

Aug 28, 2023

I got my FMA180 last summer and have only had a couple of sessions with it using my QHY183C. I did not notice any problems with focus at either end of the spectra. My process for focusing any object is to use a focusing mask to get rough and finish using star image size. I do not have a power focuser on this scope. In my limited experience I would not consider this 'normal'. I am assuming you focus through the filter – which appears to pass a number of deep red emissions, so being out that much in this area is curious.

Helpful Concise

AdamJ

Aug 28, 2023

Werner Stumpferl:
So I use a mono camera and although I shoot LRGB I have not noticed any significant shift in focus position when moving between filters.

I have the original FMA180, is yours a pro? It looks like it's a long way out not just a little bit out.

Adam

No, it´s the "normal" FMA180. As far as I know, the difference has nothing to do with the lenses, only with focuser and a integrated rotator.
And yes, both lenses are far away from focus in red.

*So they are optically different from the diagrams. Here is a link to my friends image with a UV/IR cut filter.

https://www.astrobin.com/jt799b/

here is mine with SHO

https://www.astrobin.com/47y4ox/K/

Both sharp I am sure you will agree.

The issue comes when you stick a duel band on it and in that situation I know he has issues similar to yours.

I do think you are way way oversampling the scope with a 183mc pro, you got to remember that it's only for a 6um spot size and so the 294 is a better match.

@ChrisPeace may well be able to shed more light on this. The shot the springs to mind is a wide field of the Cresent nebula /SADR area.

Personally I always thought it was an autofocus weighting green issue. But not sure you are using one.

Adam

Werner Stumpferl

Topic starter

Aug 29, 2023

Hi Adam,

yes, I´m using autofocus and he is weighting green/blue and independent if over- or undersampled (and I´m undersampled not oversampled), the same occurs with a 294c, red is out of focus. So my question was, has anyone the same issue or do I have simply two bad FMA´s ?

CS
Werner

AdamJ

Aug 29, 2023 · Edited: Aug 29, 2023

Werner Stumpferl:
Hi Adam,

yes, I´m using autofocus and he is weighting green/blue and independent if over- or undersampled (and I´m undersampled not oversampled), the same occurs with a 294c, red is out of focus. So my question was, has anyone the same issue or do I have simply two bad FMA´s ?

CS
Werner

Ah yes the CCD suitability calculator, problem is it is wrong. It fails to consider two very important factors. 1) The Rayleigh / Daws Limits. 2) The actual resolving ability of the optics. In this case:

Daws = 2.9um Rayleigh = 3.45um and the Spot diagram for the scope is saying 6.8um. So the limiting factor in terms of resolving power is 6.8um.

You will never ever acheive 2.75 with the optic and so irrespective of anything CCD calculator is telling you in terms of sampling with its oversimplified model you are over sampled for the scope.

In my view its an expream effect and runs against my esperiance with the scope, which is that blue bloat happends not red bloat.

In my view its an interaction with the way the auto focuser is working and the optics and the bayer matrix that is resulting in a poorly optimied focus for a duel band filter.

In terms of, have i seen this before from the scope...yes I have but not on my own using a mono camera. @ChrisPeace would need to comment.

Adam