Sharpstar 15028HNT-AL

I do love a newtonian..I so was close to buying one of these this week albeit the 130-AL but I bought a mount instead. Interested to see how easy you find it to work with this second generation.Are you fitting an EAF? I read somewhere they are a tight fit because of the focuser orientation and not being able to rotate the focuser on this new version?!

As I'll be using a Player One Poseidon-C colour camera (IMX571) I don't really need any additional filters. Maybe the Askar C1 and C2 at some point, but for now the UV/IR and Ha+Oiii dual narrowband filters I allready have will do nicely. And yes, the UV/IR filter is needed as Player One doesn't have them preinstalled on their cameras

I used to have a player one camera so I'm 100% aware that you need a uv/ir cut. Narrowband filters tend to only work properly with f/3.5~f/4. Otherwise you suffer bandpass shift. The f/2.8 scopes really need specific narrowband filters designed for f/3 and faster scopes... the c1 will be OK because it's bandpass is so wide. My narrowband filters are 3.5nm, and if i used the Ha filter with a f/2.8 telescope, it would not capture Ha, but rather something 2.5~3nm away...

I'd need a new set of narrowband filters to do narrowband imaging with this scope.

Alex Nicholas:
I used to have a player one camera so I'm 100% aware that you need a uv/ir cut. Narrowband filters tend to only work properly with f/3.5~f/4. Otherwise you suffer bandpass shift. The f/2.8 scopes really need specific narrowband filters designed for f/3 and faster scopes... the c1 will be OK because it's bandpass is so wide. My narrowband filters are 3.5nm, and if i used the Ha filter with a f/2.8 telescope, it would not capture Ha, but rather something 2.5~3nm away...

I'd need a new set of narrowband filters to do narrowband imaging with this scope.

Band shift occurs at any angle other than null, and it will be showing at f/4 compared to an f/6-f/8 system. CO also would make things rather worse than for an unobstructed telescope. Finally, at f/2.8 even wider bandpass will show shifting and you may run short of Ha in some case (where the Ha signal is of the same level of the Hb). Fast narrowband filters are the answer in these cases (ALP-T).

andrea tasselli:

Alex Nicholas:
I used to have a player one camera so I'm 100% aware that you need a uv/ir cut. Narrowband filters tend to only work properly with f/3.5~f/4. Otherwise you suffer bandpass shift. The f/2.8 scopes really need specific narrowband filters designed for f/3 and faster scopes... the c1 will be OK because it's bandpass is so wide. My narrowband filters are 3.5nm, and if i used the Ha filter with a f/2.8 telescope, it would not capture Ha, but rather something 2.5~3nm away...

I'd need a new set of narrowband filters to do narrowband imaging with this scope.

Band shift occurs at any angle other than null, and it will be showing at f/4 compared to an f/6-f/8 system. CO also would make things rather worse than for an unobstructed telescope. Finally, at f/2.8 even wider bandpass will show shifting and you may run short of Ha in some case (where the Ha signal is of the same level of the Hb). Fast narrowband filters are the answer in these cases (ALP-T).

Yes, this is what I mean. Any faster than F/4 or so, with a 3 or even 6nm filter will start to be problematic. As I'll mentioned, the Askar c1 filter being very wide at 15nm Ha, and 35nm OIII MIGHT be OK at f/2.8... however, with my 3.5nm filters, or even the askar D1 I used to have, you will run into trouble.

So, as mentioned before I flock the inside of the tube of this telescope.

The built-quality of this telescope becomes apparent, if you start to disassmble it. There's completely blackened M3 hexscrews used everywhere, but the focuser which is attached with blackened M4 screws. And these are not painted screws, but chemically blackened.

First I removed the spider with the secondary mirror holder. The secondary mirror is in pristine condition and exactly 70mm x 100mm. Unfortunately the edges are not blackened, but I'll get to that later.



Next I removed the primary mirror cell, which is attached with 6 of those M3 hexscrews, instead of just four like the parts in the front of the scope. I marked it down woth painters tape and a sharpie, to know which way to reinstall it. The fit of the parts don't allow for much tilt or rotation however. All the wholes and threads line up with 0.5mm clearences. The primary mirror mask has a clearence within the tube of about 1mm.

The primary mirror cell is improved compared to the older versions of this telescope. The aperture mask has springs to hold the mirror in place (see 1) and the fit of the mirror in the cell is snug using felt liners on the inside of the three "pillars" the aperture mask gets attached to.

The counterscrews to fix the mirrorcell in place after collimation are not threaded into the aluminum, but have a stainless steel insert, which allows it to be tightened well without getting loose do to the difference in materials of screw and thread (see 2).

The mirror touches the mirror-cell itself in only three tiny spots. Sharpstar continued to use copperpins here (see 3).



Measuring the inner circumference of the tube by using a long strip of paper nd marking down where it overlaps. No calculations or guessing needed.

Draw a centerline down the tube by connecting the holes that are used to attach the front-ring and the primary mirror cell. The tube itself is 410mm long, and the front of the aperture mask within the tube is at 350mm, where I marked it all round the tube to better align the flocking paper later. Having cut the selfadhesive velour to size I did a quick testfit.



Next I removed the remaining parts from the tube. The frontring, which actually connects the tube to the spider. The finder shoe is matching the radius of the tube to perfection.

Last but not least the focuser, which has a tiltmechanism built in (see the three red circled threads). The inner tube of the focuser is baffeled, which is a nice touch.

And we're left with a completely stripped aluminum tube.



The selfadhesive velour - I use ProtoStar flocking purchased form FLO - is peeled back about a centimetre at first and carefully lined up with the centerline and the front of the tube. We then peel back the backingpaper little by little and press the flocking down onto the inside of the tube using a 40cm ruler in my case to avoid bubbles from forming. Do this slow and in little increments and the result will be a nicely flocked tube without any bubbles or wrinkles.

The difference of reflectivity to the paint of the tube becomes apparent right away.

Now we use an exactoknife to cut out the hole for the focuser and the screws and start reattaching all the parts. The focuser I installed first, which I feared would be difficult to realign. However this was very easy, as the inside of the outer tube of the focuser is exactly the same size as the hole in the tube so it was very easy to lign up the two parts again.



Some additional shots to show the difference in reflectivity before we attach the findershoe, the frontring and the primary mirror cell in it's previous position we marked down with painterstape and a sharpie line.



Before installing the spider and secondary mirror assembly we adress the edges of the secondary mirror by flocking it with the same selfadhesive velour. I simply rolled the mirror onto a piece of the flocking material which I cut to size first (220x100mm). Then I trimmed the overhanging material with an exactoknife and reinstalled it all.

I attached the dewshield once more to make a before and after comparison of the inside of the tube. Bottom middle is with the flocking material installed and the bottom right was from a few days ago without it. I think the result was absolutely worth the effort.



And the scope is back together again, ready to be stored away until September when it gets dark enough again.