15 replies to this topic

[NorthScope]

I just bought a Sharpstar 130HNT from Telescop Service (TS) in Germany. I first tried a small ASI676 (7.1mm square sensor). It worked pretty okay, with some triangled stars, but pixinsight fixed those.

 

Then I tried on my ASI6200MM , redused to APs-C format with EFW. Now the stars became like forks. I tried to do collimination based on the center star, but I notice that out in the edges, they are squeezed. Not sure where to start. I suspect the backfocus might be wrong? I have 54.5mm , but tough I should add some for the filter used. The backfocus should be 55mm. Not sure how exact, or the photos below can tell what is weong or where I should start.

 

please advice if anyone have an idea? 😅

 

 

[DeepSky Di]

Topic moved to imaging forum. Looks like a collimation issue. 

[jonnybravo0311]

These scopes have extremely tight tolerances for collimation. Further, because of the fast focal ratio of f/2.8, the critical focus zone is very narrow. This means any tilt in your imaging train is very noticeable. The further out from center you go (i.e. with a larger sensor) the bigger the impact you'll see. I have the 150mm version of this scope, and I still struggle fighting tilt.

 

If you can get it dialed in, the scope performs very well.

[EastTexasAstronomer]

I have this same telescope. Have you tried loosening the primary mirror clips? You mention previously you had triangular stars, which for me usually indicates I have the primary mirror clips too tight. There are several online resources. Just use your favorite web browser and search for how to fix pinched options on a Newtonian telescope. I would fix or rule out the pinched optics, re collimate and check the back focus with some software tool such as aberration inspector in N.I.N.A.

 

I agree that this is a great imaging scope once you get it dialed in.

Edited by EastTexasAstronomer, 16 March 2025 - 08:32 PM.

[mayhem13]

That’s a hot mess is what that is……sorry nothing to add to help other than to return the scope for a refund.

[michael8554]

Are those the whole image or just some edges ?

 

The second image just looks out of focus.

 

Or is that an out-of focus colli shot ?

 

You should say what you have posted.

[NorthScope]

Thank you for all responses.

 

The two images I took erarlier where one with focused on the fork stars, the other was an attemt to defocus for collimination.

After watching alot of videos again of issues by collimination, I found an interesting video explaining that the laser might had to be colliminated. I then made my own colliminating jig, and voila.. I figured out that the laser was off with a couple of centimeters on a 1,5 meter distance. 

 

After adjusting the laser, I laser aligned the Sharpstar again, and then the stars got immediately better. I pointed my scope to Capella, and did some shots:

This is mfull APS-C image within asiair;

 

 

And here is one zoomed in on the stars down to the right of the image.

 

 

I have not tried to defocus and adjust more on the stars, but I am satisfied with this result for tonight. 
There are probably more that can be done, so feel free to give me some hints 😊

 

thanks again.

 

Martin

[unimatrix0]

Just to save you some future headaches-  most of these Newtonians with the 2" focuser and small secondary mirror do not correct well for APS-C or larger. So if you go very much into the details and unable to get a better correction around the borders, it's because they just don't or better word - cannot correct that area.  If you get the center and around to show round stars - you can try to get the camera sensor distance dialed in a little better, just don't expect perfection. 

[DeepSky Di]

When I use ASIAIR for collimation I turn on the bullseye overlay and make the defocused donut as big as possible. It's important for it to be centered precisely in the display. The collimation movemements will drive it off center and you will have to recenter it after every movement.

 

After getting this as good as possible, there's a refinement technique of going from one side of focus to the other on a bright star and looking for diffraction rings.

 

Please check out section 11 of the MMPI FAQ where it covers collimation. There was also another recent post with good collimation links.

 

https://www.cloudyni...d-january-2025/

[NorthScope]

Thank you for the links. They ware good advice.

 

I tought I dialled in the collimination at the end this night, but I encountered another issue that I got a bit puzzeled over;

 

When collimating with the focuser fully retracted, I achieve good alignment. However, the collimation noticeably shifts when I extend the focuser significantly—for example, when focusing my camera (ASI6200MM with only a filterdrawe) or using narrowband filters such as my Sulfur filter.

 

Attached are two images demonstrating the difference clearly.

 

This is when the focuser is in (The ASIAIR focuser number is not right)

 

This is when the focuser is out:

 

 

I also found this thread on Cloudy Nights, where others have encountered similar tilt issues with the focuser on the Askar 13028HNT: 

 

link

 

Has anyone had success with simple focuser adjustments without significant disassembly? Any advice would be greatly appreciated! Not sure where to start now...

 

Thanks for any tips!

Edited by NorthScope, 19 March 2025 - 10:33 AM.

[erictheastrojunkie]

Sounds like your focuser is not square with the light path, that should be step one, ensure your focuser is square prior to collimation. Some people put a collimation laser in their focuser, remove the secondary mirror/stalk, put in a pencil or screwdriver with a long enough shaft and see if the laser hits the pencil/shaft. Then rotate the focuser all the way in and out and see if the laser remains in the same spot. If the laser moves off then your focuser needs to be adjusted for tilt. That of course relies on your laser being well collimated itself. You can also use a cheshire site tube, which is what I do, put it in the focuser and rack it to a position where the site tube just barely has the secondary mirror edges centered, then rack it in and see if the secondary mirror becomes off-center, rack it out and see if the same thing happens. That will tell you if the focuser is square as well and doesn't rely on laser being collimated. The downside to the site tube is you have to rely on your perception of the space/centering of the secondary mirror. 

 

After you ensure your focuser is square and not changing while focusing, then you need to do the regular full collimation. Make sure the secondary mirror is centered and not twisted, reflections line up in concentric circles, etc. I finish my collimation with a tri-bahtinov mask that I 3D printed to make sure all sets of diffraction spikes are equal. 

[j21blackjack]

I tried the 13028hnt-al and also had serious issues with the focuser, it's a different focuser entirely though if you have the carbon fiber version. My focuser had a huge shift right when it hit the stop all the way out, but it seemed fairly straight other than full in or fully out. My biggest issue was that any amount of rotating the focuser caused large changes in the collimation, and there's really no way to adjust focuser tilt on the metal version. Between the EAF clearance issues and focuser screwing up any hope of collimation, I ended up sending mine back. It was nothing but frustrations with a fairly expensive wide field scope, too much for my limited clear nights per year.

[NorthScope]

 remove the secondary mirror/stalk, put in a pencil or screwdriver with a long enough shaft and see if the laser hits the pencil/shaft.

 

I finish my collimation with a tri-bahtinov mask that I 3D printed to make sure all sets of diffraction spikes are equal. 

I'm a bit hesitant about starting to disassemble the mirrors within the scope, so I first attempted just rotating the focuser slightly and then tightening all the bolts. It appeared that one of the bolts was somewhat loose. After tightening it, the collimation laser maintained its position during focusing, with only minimal movement compared to before. I'll use ASTAP next to investigate the tilt issue further, although I'm not particularly experienced with this aspect.

 

Following your suggestion, I created a tri-Bahtinov mask. It took some effort to make it fit precisely to my telescope, but I think it turned out well. I'll upload the design to Makerworld so others can easily download and print it without having to construct it themselves.

 

 

The mask produced the following result on the star Capella, which I then used to collimate the telescope to what I consider near perfection:

 

 

Thanks for the help and suggestions.

[NorthScope]

I tried the 13028hnt-al and also had serious issues with the focuser, it's a different focuser entirely though if you have the carbon fiber version. 

I have the carbon fiber version. Initially, I considered the metal version, but I was concerned that the significant temperature fluctuations here (ranging from 7 to 10 degrees in a single night) would require frequent refocusing throughout the night. I suspect Askar moved to aluminum primarily to reduce production costs, but that's a different discussion altogether. ​ 

[NorthScope]

Here, I've used ASTAP to examine the tilt issue. Below are three images generated by ASTAP, indicating a tilt issue of around 20%. However, I suspect there might also be an issue with backfocus (In or out ?)

 

As always, I'd appreciate any interpretations or advice on this 

 

This is taken with ASI6200MM with APS-C setting;

 

 

 

 

 

[erictheastrojunkie]

Using a full frame sensor on an f2.8 newtonian is going to be a challenge no matter how you slice it, every single aspect of the imaging train has to be perfect. Throw in the fact that you're using a filter drawer and the 6200mm being a bigger/heavier camera and you are almost certainly going to have tilt. Looking at the distribution of numbers in the ASTAP plot it definitely looks like tilt, I'd try adjusting the tilt plate on the camera and see if that resolves the problem there, then once you have tilt sorted do an aberration inspector test with a perfectly focused star in the middle of the frame, make sure backfocus is as perfect as you can get it. If you make adjustments to backfocus, then re-run the tilt inspector. If the tilt still looks good, move on to doing your collimation, if not, then redo the tilt plate adjustments. Collimation should always be last. Tilt -> backfocus -> Collimation, all have to perfect at that sensor size and focal ratio. 

 

FWIW I would not rely on the laser collimation at all, in fact I'd toss the laser in a closet and forget it ever exists. A cheshire site tube is a good piece of equipment to have, with an f2.8 Newt I'd probably be looking at getting an OCAL, that's how precise you need to be (literally microns off can mean noticeable effects with a full frame sensor).