John Tucker · Feb 28, 2026, 12:11 PM
Show us the image!
📷 EtaCar_Ha_90s_Half-Size.jpg
As mentioned - don’t judge this too closely.. This is 3 * 30s subs.
Sharpstar 15028HNT-AL.
ToupTek ATR2600M (Cooler off.. I was just testing) + ToupTek 3.5nm Ha filter.
Emcan Astro EM31 Pro mount.
Collimation:
Previously, I would get focus with an average HFR of 1.8px with the Ha filter, which indicates its pretty sharp, however the HFR SD would be wild… With this collimation, the HFR came out at 1.59px.
Also, the center of the brightness was usually about 30% in from the eastern side of the sensor, as if the light cone was centralised NOT in the center of the focuser.
As it turns out, the Sharpstar documentation says the secondary mirror holder should be 5mm back from the rear edge of the spider. My secondary is now 8.5mm back from the back of the spider, and my light cone is almost 100% dead center…
Prior to adjusting this, I had to tilt the secondary considerably in order to get the whole primary mirror in the reflection, and thus, ASTAP was reporting a 38% tilt across the sensor, resulting in muddy stars in top left and bottom right corner. The subs in this image measure at 6% tilt and ASTAP says ‘don’t touch a thing’!
The tools?
I first tried the collimation cap that came with the scope.. Lovely machined little tool, absolutely useless when collimating a super fast newt… I haven’t used it in AGES!
Next, I attempted the Baader Laser Colli Mk III, I could get that thing to say EVERYTHING IS PERFECT and still be WILDLY mis-collimated… Largely because the secondary can be WAY out of wack, but as long as you tilt it such that the beam hits the center of the primary, then adjust the primary to return the beam, it says your collimated when you may well be aligned in the light path, but the tilts on each of the mirrors is WILDLY inaccurate!
Finally - I took the plunge and bought the OCAL 4 Pro.
Initially, I was a little confused with it, and found that with the specified mirror spacing, I could never get the secondary to be concentric with the focuser, nor could I make it appear round… I attempted it a few times and got results that OTHERWISE looked ok, but again, massive tilt in the images… It took 4 weeks of attempting and re-attempting before I said ‘You know what.. The manual is WRONG… I put the secondary where it needed to be to be central in the focuser, then adjusted it until it was perfectly round. After this, I tweaked it to make sure the OCAL said it was pointing directly at the primary center spot, then moved to the primary and tilted it such that the reflection of the OCAL was placed exactly in the center of the ocal image. I had to do two iterations to fine tune it until the final adjustment on the primary did not affect the alignment of the secondary… Finally - it looked how it should!
Polar Alignment: None.
I put the mount down ROUGHLY where it usually is.. probably out by 1° give or take…
Guiding: None.
I just swapped out my OAG for an external guide scope on this scope as the light cone is so steep that I couldnt get the prism at a point where it didn’t cast a shadow on the sensor and could still see stars.
I only put the scope outside as I had a 45 minute window before it was forecast to rain, but I wanted to see what this attempt at collimation had produced… After the first test shot, the result looked VERY promising… I thought, why not?, slew to a nearby target and fire off a few test subs.. I pointed at Eta Carina as its very large and bright, and I didn’t need to use go-to (remember… no polar alignment at all) as I know exactly where it is…
This was the result…
A grand total of 90s in 3.5nm Ha light..
If this can be achieved in 1.5min, imagine 15h…
