Focus Problem or Optics?

[Caitnahs Swan]

First, not sure if this should be posted here or in the imaging area. I have the ES AR152 Achro and image with it. I am hoping to verify that my scope is okay.

I did a star test to see if the refractors collimation was okay. The star looks good. A couple times imaging with this scope, achieving focus was very difficult. I use a Bahtinov mask and other techniques. The images are not very clear. 

But I took an image of the moon by stacking five images and that came out perfectly focused.

My question is that since the Moon came out focused, am I safe to assume that the optics are OK?

[CraigT82]

I think your issue is do to the scope being an achromat, when using the camera the focus looks squiffy because the scope is not focussing the various wavelengths of light to the same spot. Visually it doesn't look too bad as your eyes aren't as sensitive to these various wavelengths as the camera is.

It looks OK on the moon as it's a less demanding target for an achro.  Getting pinpoint images of stars is very demanding on any optical element and, unfortunately, a big fast achro just isn't the best tool for that job

[ronin]

In a scope or more specifically a lens there is a depth of focus. This is a band where the focus is as good as it gets. It often desribed as where it is "in focus" but I think as good as it gets is better.

What happens is on an achro this depth is sort of longer, but that means the "good" is not as good as others will be. It is a bit of a trade off between finding the right place and having it pin sharp. If you get an ED or APO then this depth of focus is small, something too small, people talk of them "snapping" into focus. Hence 2 speed focusers, you need th fine one to simply allow you to stop at the right place. Equally people complain of never quite getting the focus right as they cannot stop at the right place, also any movement at the focuser end means it is out of focus again and back to square one.

I would guess this is the "problem". You will have a reasonable depth of focus but it will sort of never be pin sharp.

Also the AR 152 is likely to be too big a diameter, the edges start to have an impact and the edge will focus the same wavelength to a different location on the optical axis then the central portion does. Again blurring the image. Unfortunately not just different wavelengths will focus at different positions, so will the same wavelength. The faster the acromat is the worse this effect will be also.

[Caitnahs Swan]

6 hours ago, ronin said:

 

What happens is on an achro this depth is sort of longer, but that means the "good" is not as good as others will be. It is a bit of a trade off between finding the right place and having it pin sharp. If you get an ED or APO then this depth of focus is small, something too small, people talk of them "snapping" into focus. Hence 2 speed focusers, you need th fine one to simply allow you to stop at the right place. Equally people complain of never quite getting the focus right as they cannot stop at the right place, also any movement at the focuser end means it is out of focus again and back to square one.

 

The movement you are referring to is probably adding to my problem. I image from inside and use Team Viewer with my iPad, remotely from my inside laptop. The rig is on a 2nd story deck and while stepping near the scope you can see the change. When I was using my 80mm ED scope I would remotely focus and once achieved, I could lock the focuser down.

With the AR152, I don't lock it down since, I go back inside to check focus with SGP. Failing to lock it down may be part of the problem as I step away from the scope.

Am considering electronic focuser...soon.