So I bought my Sharpstar 15028HNT-AL back in September last year, based off a few users brilliant images, and the spec-sheet alone… Lets face it, it’s hard to argue with F/2.8 when you’re primarily a narrowband deep sky imager.
Having owned a few newtonians in the past, albeit, F/4, F/5 and F/6 newtonians, I knew that collimation would be required regularly, I knew the process and I knew that an F/2.8 newt was going to be significantly more sensitive to error than even the F/4… What I didn’t know, however, was the meaning of the word ‘significantly’ in this instance…
On first receiving the scope, and after the obligitory rain that ensued for a couple of weeks, I was able to make my first test image. For this, I used simply the collimation cap and a defocused star to get it looking ‘OK’… and overall, it wasn’t bad - however I did notice that the flats showed that the ‘center spot’ of the light cone was a LONG way from the center of my camera sensor…
📷 Sharpstar 15028HNT First Light test.
https://app.astrobin.com/i/wjaafu/
After this experience, I learned that the scope was incredibly sensitive to off-axis light, dew and that the collimation that looked ‘ok’ was nowhere near good enough… I looked at the OCAL, but at its extreme price, I had to try a laser first. I bought one, had another go at the collimation (which was actually looking relatively close even with the laser) and then made this test…
📷 Second test of the Sharpstar 15028HNT
https://app.astrobin.com/i/rarr6l/
You’ll notice this is cropped quite heavily - why? because outside of this field of view, the stars were MUD! despite the laser telling me everything was dead on - there was MASSIVE tilt in the optical train (ASTAP reporting 39% tilt in fact…)
The thought crept into my mind at this point… “Have I made a mistake selling my beautiful refractor to purchase something as sensitive as an f/2.8 newtonian?”. But as I said those words in my head, I heard “f/2.8”, and remembered that IF I could tame this monster, I would have an EXTREME imaging tool!
A third, fourth and fifth attempt was made at collimation… The Sharpstar documentation in hand, the collimation cap and the Baader Laser Colli MkIII at my disposal.. Again, I had things looking close. What I could notice is, that despite having the secondary mirror holder spaced at 5mm from the back of the spider as per Sharpstar’s documentation, the secondary did not look how I would have expected… It didn’t look like a circle, rather, quite oblong, and while I was able to get the primary mirror completely in the secondary, and get the laser return beam to go exactly back into the laser emitter, it just felt like something was still wrong… but the tools said it was right, so I did another test image…
📷 SMC Nebulae
https://app.astrobin.com/i/wvua6p/
Again - this doesn’t look too bad after processing, but ASTAP was still reporting between 35 and 40% tilt in the optics, the flats were still very uneven, with the center of the light cone significantly off-center, almost 50% of the distance between the center of the image and the left hand edge… My mind kept focusing on that detail… The flats are off-center, and the secondary looks wrong based on what I know about newtonian collimation… But also, I’d read that a laser collimator was also grossly inadequate for an f/2.8 optical system, and I’d need something more accurate, that could factor in more than just ‘does the laser hit the center of the primary and come back into the laser diode’.
But was this collimation? or did I actually have tilt in my optical train. I ran ASTAP over a number of images from my Askar 120APO, and they were dead flat, next to 0% tilt. So I felt that the camera was not introducing tilt..
So what? Was the focuser of my 15028HNT mis-aligned. Did I have sag in the focuser? was my primary mirror loose and flopping about?? Enough was enough, something had to be done.
I took the plunge and bought an OCAL 4 Pro… It confirmed to me what I had been thinking I could see…
1. The secondary mirror was not in the center of the focuser tube, in fact, it was a long way off.
2. The secondary mirror was not even remotely circular.
I got it as close as I could, while adhereing to the documentation still, and made my fourth test image.
Now, to say that I was mad about this would be a lie… There was still 35% (Extreme) tilt, stars in the bottom corner appeared as small donuts, BUT, I’ve wanted to shoot this target for nearly 20 years, but I’ve never had equipment that made it possible from my bortle 5 sky… I was blown away when the first 600s sub rolled in, so despite the flaws, I let it run anyway, and collected ~8hrs of data…
📷 LDN 1622 - The Boogeyman Nebula (Ha false-colour)
https://app.astrobin.com/i/3qfxqz/
I was so happy to have got this image, but still very dissatisfied with the stars in the bottom left corner, and the unusal looking flares from the bright stars.. Collimation was clearly wrong, and at this felt I’d done everything except one…
So discarding the Sharpstar manuals information regarding the 5mm spacing between the secondary holder and the back of the spider, I got to work on the scope, manually moving the secondary until it was central in the OCAL’s field of view. I then manually (by hand) adjusted the tilt and rotation of the mirror until it was perfectly circular and concentric with the focuser tube.. This step took me almost 1h of fiddling, but once done, I screwed in the adjustment bolts so it would hold, then measured the gap between the secondary and the spider, 8.5mm… 3.5mm further from the spider than Sharpstar’s documentation, BUT! it looked how I expected a secondary mirror should look. I removed the primary from the tube, and leveled it in the mirror cell so it was ‘neutral’ with no tilt applied, and ensured that the retaining ring was tight enough that the mirror couldn’t flop, but not so tight that it could possibly pinch the optics..
After refitting the mirror, I tweaked the secondary adjustments to point it directly at the primary - and then the primary to center everything up in the OCAL’s field of view… This took a few iterations, as every adjustment of the primary necessitated a slight tweak on the secondary, but finally… the adjustments converged.
It finally looked GOOD! not ‘ok’… but GOOD.
Weather here has been a nightmare, but I got a 15min window of clear sky, so I took the opportunity and I put it outside with the intention of taking a single test frame just to see the field… No polar alignment, no guiding… didnt even turn on the cameras cooler… I just wanted a test sub to confirm the collimation…
I ended up taking 3× 30s subs in Ha… This was the result.
📷 EtaCar_Ha_90s_Half-Size.jpg
Not bad for 90s of total integration with a 3.5nm Ha filter… and ASTAP reports 5% tilt - which it deems as negligable… This made me VERY happy. I packed up, before the cloud and ran began…
A week later, I got an hour of clear sky to test properly…
This is 25×180s subs.
📷 Vela_Ha_75_half-size.jpg
This one was obviously with an accurate polar alignment and guiding in play, camera at -10c… And this is the first time that I’ve started to feel like this scope was the right choice!
There is still a long way to go - but I’m seeing the promise this scope has!
Clearly, collimation has been the focus thus far… I do however have some other thoughts on the scope…
1 - Reflections in the OTA.
The carbon fiber model was flocked from the factory, however the AL version is not - and there were a number of reflections going on inside the tube.
Resolution: I have flocked the OTA internals, and the edge of the secondary mirror.
2 - Stray light in the OTA.
The neighbors porch light, street lights 300m away.. Street lights 500m away that were reflected off the red-brick wall of my house, even the IR light from my back yard security cameras… if it emitted, or reflected light, it ended up in the OTA, and washed out contrast dramatically, sometimes to the point of making the target invisible in the frame entirely.
Resolution: I designed and printed a 24cm long light shield for the scope. Once printed, this was then flocked to match the OTA internals.
3 - Dew on the primary and secondary mirrors on humid nights.
The usual weather here of a night time is very humid, but especially October through to May…. > 80% humidity is average here… Coupled with the 35°C days, promptly dropping to 19~21°C after dark, dew is common… and the primary mirror in this scope LOVED to catch it all! not only that, but the secondary would condensate, then drip water droplets onto the primary… Good times!! Something had to be done!
Resolution: The aforementioned light shield is also a dew shield. This has made some pretty dramatic improvements. Further to this, I have added a dew heater around the rear of the OTA where the primary mirror is held. This has now resolved the issue, such that the scope lasted 6h under the stars in 80~95% humidity, and rapidly dropping temperatures without suffering any interference from dew.
4 - OAG pick off prism causing shadow in the light path.
The light cone on an F/2.8 optical system is pretty steep! I was unable to retract the pick off prism far enough to remove it from the light cone, while still being able to see stars…
Resolution: As I couldn’t retract it any further, my only option was to move from the OAG to a William Optics Uniguide 32mm f/4 guide scope..
5 - Rotating the camera via moving the whole focuser is annoying.
Rotating the camera is inevitable between nights due to having to remove the camera to perform collimation checks, and the only way to adjust this rotation is to rotate the entire focuser assemly, which can be difficult due to the position of the EAF…
Resolution: Due to the change from the OAG to the external guide scope, I have enough available backfocus now to add a rotator into my imaging train, which I will do as soon as I can… This will remove the issue as I will lock down the focuser in the orientation that is most sensible to me, then rotate the camera or collimation tools electronically.
Issue conclusion:
So, while the scope is complicated, results are this in a nutshell.
1 - It suffers from internal reflections - flock EVERYTHING you can.
2 - It suffers from dew and stray light - get/make a dew shield, run a heater band around the primary.
3 - Rotating the whole focuser is annoying - if you have room in your backfocus calc, get a rotator for this scope.
4 - Collimation is EXTREMELY sensitive - Get the correct tools and take your time. Ignore what the manual says about mirror spacing and just collimate it so it looks perfect via the tools you are using.
What a mission this scope is turning out to be - I LOVE how fast it builds SNR on even very dim targets, but getting it properly adjusted and running perfectly is incredibly difficult.
Is it worth it? Well that’s a purely personal decision I think.
I know I need to tweak the collimation a little more still, and I know that I need to buy high-speed narrowband filters to get the most out of these optics… But given the extremely limited time I get to astro-image, having a scope that is building signal as quickly as this one does, that is as small and portable as this one is… well, the value to me is immeasurable. I’m also a sucker for big diffraction spikes, and this scope will certainly produce them when pointed at a bright star!
It’s been a MASSIVE P.I.T.A!
But I love it, and would buy it again in a heartbeat!
