Backfocus
When I first heard about the focusing plugin Hocus Focus (HF) I was unsure if it was something I wanted. NINA's built-in autofocus seemed to work perfectly. Then I watched one of the Patriot Astronomy videos that demonstrated what else HF can do and I realized I not only wanted it, but I needed it!
HF (created by George Hilios) features what he calls Aberration Inspector. Much of what it does is beyond my ability to intepret meaningfully, much less act on. But it has one key ability: to measure error in backfocus. Having proper backfocus is a problem when using focal reducers.
If the backfocus isn't right, you won't be in focus across the field; if you focus on stars at the field's center, stars in the corners will be out of focus. This is why an old fix for this problem is to focus on stars some distance out from the center. Like most compromises it's hardly a perfect solution: stars in the center and corners will be slightly out of focus.
The better remedy is to get the backfocus as close to correct as possible. Usually this means taking images, examining them closely, guessing the distance of the correction to make, making the correction, then shooting more images, etc. It's time consuming and inexact. Fortunately there's a better way--use Hocus Focus.
HF can quickly estimate the magnitude and direction of your backfocus error during a special autofocus session. Then you can make the correction if you have the proper spacers on hand. And you're done.
I have a Takahashi CR 0.73X reducer That I want to use with my FSQ-106. The CR wants a backfocus of 72.2mm. My camera and filter wheel add to 32.5mm, so I need my adapters to provide 39.7mm.
For my first run of the Aberration Inspector I had this:
- M56 to M48 adapter, 12.1mm
- M48 to M42 adapter, 16.5mm
- M42 spacer ring, 10mm
These add to 38.6mm. Close, but 1.1mm too small. The Inspector told me I needed to add 3 focuser steps to the backfocus, which is the right sense of change, but the magnitude seems off.
One of my focuser's steps is about 0.004mm (30mm/8000steps), so 3 steps is a mere 0.012mm. My assumption is that somewhere in how I set up NINA or HF a factor of 100 error sneaked in. If that's correct, then the correction it's suggesting is to add 1.2mm.
So I added two thin spacers totaling 1.2mm and ran the inspector again. Here is the result
As you can see (if you click the image to enlarge it), the Inspector now says the error is zero steps and the difference in star quality between center and corners is almost imperceptible. I think it's safe to say that I'm now within 0.1mm of having correct backfocus!
This means the next clear night I'm going for a larger target, maybe the Elephant's Trunk, M31, or the entire Veil!
Some incidentals for those of you who like miscellaneous information...
- All of the goodness of fit (R squared) values were 1.00
- This was performed without polar alignment or guiding, the exposure time was 2s through my luminance filter
- I had to increase the autofocus backlash from 450 to 600 steps. I should probably redo all my filter offsets, too, if only to see if they have changed. The autofocus step size was unchanged.
New Image
I've imaged the two nebulae (IC 59 and IC 63) near gamma Cas before, and it was time to revisit that to see how I have progressed.
Here is my 2009 attempt
This poor image was tortured with wild stretching and clipping, then oversaturated to show some color.
In 2022 things are looking better: better mount, better camera, better telescope, better processing. Here is the full frame
And here is the nebular part of the image at full scale
Acquisition details are at AstroBin.
What really surprised me about this image was that it looks so good for having so little data. It's based on about 82 minutes total exposure spread across the LRGB channels. And short exposures, too: only 90s each!
The optical performance of the FSQ is--at least to me--breathtaking.
