HFR is Off by a Lot

Not sure what to do here but since I switched to the ED 127 from the ED 102 my Auto Focus with my Moonlight has become untrustworthy.

When I run the Auto Focus I’m getting a perfect V shape but the HFR it settles on is way to high and I have to go in and manually add some steps in to get the HFR down to acceptable levels.

I know that all my info for the scope has been correctly placed in the appropriate boxes so I’m kinda lost.

I have it set for 9 data points and like I said it’s doing what it is supposed to do but just running the V too high.

If I could somehow tell the V to come down a bar or two it would be right on.

An example would be… Normally I get an HFR of between 72 to 80 but right now it’s giving me around 110 to 118

It’s probably something simple but I’m missing it. Any ideas?

Keep in mind that the HFR is an average of all the stars. How do they look visually?
Have a look at the solved image around at all the stars and see if they don’t have a few with smaller numbers and some with larger. 110 is not all that bad actually :slight_smile:

It might be due to abberations on the edges of the field and other such things. Do you use a field flattener?

I appreciate your answer but no it’s slightly out of focus. I’m not saying I get 70 to 80 every night. I’m aware that seeing and all kinds of things can effect that number. What I am saying is that when I had it on the ED 102 whatever number that auto focus kicked out was dead on for focus. No amount of tweaking was going to get you a better number.

Right now what ever number SGP kicks out as the best HFR on the ED 127 I have to go in and tweak it and always get it lower by at least 25 to 30 points.

I can physically see the stars tighten up. So something is off here. I just cannot put my finger on it. Any other ideas?

Have you tried putzing around with step size?

Yes and no. I was thinking that as well. Normally I use the method that they tell you to do in the book about defocusing it by 4 to 5 times and then applying the math to get your step size. I can definitely try that again. What’s getting me is if the step size was off then I wouldn’t think I would get a V but who knows you may be right.

The first thing it was doing was running a perfect right side of the V and then it would just give me stars straight out to the side on the left side as if the focusing tube was not moving at all. I talked with Ron at ML about it and he said it was probably the weight of the 3 inch flattener causing it to slip so he had me take an allen wrench and turn those 2 screws next to the focus lock bolt down just a tiny bit.

That solved that problem but not the problem with it being 30 or so steps high on the auto focus.

I’ll try the step size adjustment like you said and let you know how that goes.

Thanks for taking the time to talk with me.


PS. Just so you know what system I’m working with its a

Explore Scientific Carbon ED 127
Explore Scientific 3 Inch Field Flattener/Reducer
Moonlight 2.5 Inch Focuser
Starlight Express Filter Wheel
Astrodon Filters
Atik 460EX Mono Camera

Yea that was going to be my next guess, some kind of mechanical error in the focuser be it slip, slop, or backlash or maybe all three :wink:

If you have to tweak after your focus run, I would suggest that you pay attention to whether you always have to lift the focuser… If so, this is a hint that you still have some slipping or for some reason, the focuser can’t get the load back to the desired position after the focus run.

If that is the problem, you will have to figure out how to get that slipping out of your system if you want to successfully use the v-curves and focus offsets.

Good luck!

How do the HFR values vary throughout the image? When you tweak manually do you manage to improve all HFR values or just the center ones etc?

I’ve had problems with auto focus before which were down the spacing between the reducer and the camera. This makes it impossible to focus evenly across the field and will affect the overall HFR.

VI adjusted the step size and tightened up the screws that put tension on the focusing train and it helped.

I also determined the step size and it brought it into manageable numbers.

The HFR values are for the entire plane. I don’t zoom in on a star or a portion of the frame. The problem has been with the change of scopes. Up until then I run the same procedures every night like I have been doing for over a year.

You know how when you spend night after night out imaging the same way time after time? You notice it when something is off. Thats how its been these last few weeks. I pretty much have my system down to a science. It usually takes me 15 minutes to fire it up, get on target, focus, start guiding and imaging.

But lately I have had to stop what I’m doing and monkey with the auto focuser.

But things were much better last night. I gathered data on 3 targets last night and all the 20 minute exposures were sharp with round stars. Thanks for everybodies thoughts it was a big help.

I’ll be back out tonight and let y’all know how it goes.

What is the load on your moonlite focuser? I am also using moonlite focuser and load is ~8lbs, it slips when pointing to zenith, not big slip but noticeable.

If you run auto focus back to back, if there is a slip, then the V curve will shift to left. In my case, it shifts ~5 steps for each run, which is fine for my system. 127 is even slower so if your slippage is similar then it should be fine.

And you can also try to use slowest motor speed, it will help a hit.