Another ST-80 First Light

[Pixies]

I've joined the club, and tonight was the first chance to use the SW Startravel 80 in anger. Clear Outside had been promising clear skies all day and at 11pm the clouds vanished and I saw stars for the first time in weeks! I had the new scope and the dob out cooling for an hour already.

Firstly, a star test on Polaris. Hmm - very odd shaped stars. Having watched Astro-Biscuit's latest video, I recognised pinched optics and tried to loosen the lens cell. Jammed tight! I had fiddled with it earlier in the week, when I had heard the lenses moving when I tilted the scope - tightening the plastic cover just enough to stop the movement. Now the scope had cooled down, it was very tight. I left it inside for 30 minutes and it eventually freed-up. I gently took up the slack, replaced the dew shield and tried again. Now the stars were sort of round, but at higher magnification I could see it was out of collimation and exhibiting coma. I'll need to look at this later.

Anyway , seeing was good and transparency very good. I could see all 7 stars in Ursa Minor - which is very rare here. Clear skies, good seeing, good transparency and a new moon. What more could I ask for? Well, for the gusty westerly wind to stop, actually! It was going to get chilly.

Started with Cancer, a bit later than I had hoped.

M44 Beehive - Just like my binocular view, the 30mm Vixen plossl giving a nice wide field. My star-hopping EP for tonight.

M67 cluster - Much fainter than expected. Hardly visible and small. Better with 8mm EP but little star resolution. Of course, this is only an 80mm scope.

Iota Cancri - Easily split even at low power. Main star yellow/white and secondary looked blue. Very pretty.

 

Now on to Leo

Algieba - Split with 4mm ortho (x100 mag). Both airy disks clear and good seeing with little disturbance. Coma very obvious, with conical diffraction patterns.  

Leo Triplet - Yep. This surprised me. Only the 2 main Messier components visible with averted vision, though.

Vesta - easy to locate in finder. A new one for me.

 

Now some targets near to the zenith. The sky is very transparent. Forgot the pain of straight-finders and high objects. My knees are killing me.

M81 and M82. I could actually see them in my 10x50 bins. Easy to see in scope and both perfectly framed with 12mm BST (x33 magnification)

M51 - faint but I could make out both cores of the Whirlpool galaxy with averted vision.

Two other galaxies - M94 (bright and easy to find, like a planetary nebula), M63 (the Sunflower galaxy, faint but visible with AV)

 

It was getting late and I was getting cold, so decided to call it a night with the ST80 and give the dob a chance. I had wanted to check out Markarian's chain, but it was much later than expected and Leo was far south now, and into a brighter part of the sky. Still - worth a shot. Back to the 60mm RACI finder, phew! Easy to hop to the location, but the sky through the scope was bright and I couldn't make out much.  M86 and M84 were obvious with AV, but any other galaxies in the chain were just fleetingly noticable. Nearby M87 (Virgo A) was bright and visible with direct vision.

Now I really was cold and started packing up. Only to notice Hercules appearing over the easterly roof. I couldn't miss the opportunity to check out the Great Cluster. Bright and clear in the bins. A larger hazy patch with the ST80 but it took the larger aperture of the dob to resolve stars, my 8mm Vixen LVW showing cascades of yellow and white stars across the dense cluster. Lovely.

 

[Epick Crom]

Nice report Pixies, I was observing the Leo Triplet as well last night, beautiful sight. You sure put your scope to good use 👍

[John]

Nice report !

One trick that can help the star image quality with these scopes can be done indoors in daylight. Point the scope upwards and keep it that way though this process. Remove the lens cap and dew shield. Loosen the lens retaining ring a couple of turns so that the lens elements are loose. Gently "slap" the objective cell all around it's sides. This gives the lens elements a chance to settle into their positions. Then very gently tighten the lens retaining ring again just enough to hold them in position.

If the comatic stars continue to be an issue, you will need to think about the collimation of the scope. Having a laser collimator is helpful to check if the focuser is aligned with the central axis of the objective. With the diagonal off, shine the laser down the tube and note where the laser exits the objective - it should be pretty much bang in the centre. If it is not, tilting the focuser to get the laser central is required and then some form of shims to hold that tilt when the 3 screws that hold the focuser to the tube are tightened. Once you are confindent that the focuser optical axis is aligned with the objective optical axis, you can use a cheshire collimator (again without a diagonal in place) to see if there is objective tilt:

The challenge with these simple refractors is that there is no focuser or objective tilt adjustment facility built in which is why shims are needed to apply any necessary tilt.

Aplologies if you know all this !

 

 

 

[Pixies]

Many thanks, @John. I had read up about it but it's always good to hear from someone you know had the experience to recommend the method.

It was a change to see the diffraction patterns so clear and stable, last night. Even if they were showing coma. 

[SuburbanMak]

1 hour ago, Pixies said:

Many thanks, @John. I had read up about it but it's always good to hear from someone you know had the experience to recommend the method.

It was a change to see the diffraction patterns so clear and stable, last night. Even if they were showing coma. 

Great report thank you!

 After I stripped my ST80 down last week I noticed some up/down play in the focuser - I was able to eradicate this by tightening the two tiny hex screws on the outside of the barrel. 
they push down in on a little plastic bush about 2 1/2 inches long housed in the casing above the silvery barrel. Seems this gives some control of the level of friction & play, I guess if you tightened them differentially they would fine adjust the tilt of the focus tube. Worth a go maybe. 

Edited March 14, 2021 by SuburbanMak

[John]

55 minutes ago, SuburbanMak said:

Great report thank you!

 After I stripped my ST80 down last week I noticed some up/down play in the focuser - I was able to eradicate this by tightening the two tiny hex screws on the outside of the barrel. 
they push down in on a little plastic bush about 2 1/2 inches long housed in the casing above the silvery barrel. Seems this gives some control of the level of friction & play, I guess if you tightened them differentially they would fine adjust the tilt of the focus tube. Worth a go maybe. 

Good advice

Its worth checking that before doing the stuff that I mentioned earlier in the thread. A floppy focuser is not going to help the star test !

With a bit of adjustment and maybe a re-grease these Skywatcher rack and pinion focusers can be made to work quite well.

 

Edited March 14, 2021 by John

[Tiny Clanger]

5 hours ago, John said:

Nice report !

One trick that can help the star image quality with these scopes can be done indoors in daylight. Point the scope upwards and keep it that way though this process. Remove the lens cap and dew shield. Loosen the lens retaining ring a couple of turns so that the lens elements are loose. Gently "slap" the objective cell all around it's sides. This gives the lens elements a chance to settle into their positions. Then very gently tighten the lens retaining ring again just enough to hold them in position.

If the comatic stars continue to be an issue, you will need to think about the collimation of the scope. Having a laser collimator is helpful to check if the focuser is aligned with the central axis of the objective. With the diagonal off, shine the laser down the tube and note where the laser exits the objective - it should be pretty much bang in the centre. If it is not, tilting the focuser to get the laser central is required and then some form of shims to hold that tilt when the 3 screws that hold the focuser to the tube are tightened. Once you are confindent that the focuser optical axis is aligned with the objective optical axis, you can use a cheshire collimator (again without a diagonal in place) to see if there is objective tilt:

The challenge with these simple refractors is that there is no focuser or objective tilt adjustment facility built in which is why shims are needed to apply any necessary tilt.

Aplologies if you know all this !

 

 

 

Funnily enough I've been having a PM discussion with someone who purchased as their first 'scope one of these ST80s as that rare beast ...  a brand new setup (must have been hiding at the back of a warehouse ) . At first light a few nights ago they saw 'V' shaped stars, and wondering if the lenses were not properly bedded I suggested the partial unscrew & shake a bit while upright as a possible cure, and it worked, proper stars on second light. Phew. 

The grub screws tightening the internal plastic strip against the focus tube works really well to tune the slop out of the mechanism, I do like simple effective and easily understood solutions like this, it's  fun to fiddle with when there is zero chance of actually using the 'scope !

Heather

[Pixies]

Thanks all. The scope is fairly new. Only used a few times before I received it and the focuser actually feels spot-on.

I bought it with the intention of fettling, so no concerns there. Considering the pinched optics (my fault), it won't surprise me if it's just a case of shoogling the lenses to sort out the collimation.

[Oat]

I have just ordered one of these yesterday - i managed to find a UK supplier who had a couple in stock with the basic AZ mount. Looking forward to it arriving during the week.

[SuburbanMak]

2 minutes ago, Pixies said:

Thanks all. The scope is fairly new. Only used a few times before I received it and the focuser actually feels spot-on.

I bought it with the intention of fettling, so no concerns there. Considering the pinched optics (my fault), it won't surprise me if it's just a case of shoogling the lenses to sort out the collimation.

“Shoogling” - nice. 
 

[Pixies]

10 minutes ago, SuburbanMak said:

“Shoogling” - nice. 
 

https://www.collinsdictionary.com/dictionary/english/shoogle

[Pixies]

As @John said (and others), tapping the objective cell when loose and reseating the lenses help fix the coma. It's still slightly there, though, and when checking the collimation, it is slightly off.

Nothing major, but something I'll deal with over the next few days.

Thanks everyone

[Basementboy]

I'm a beginner and just bought an ST80 as my first telescope - and was very disappointed to see "V"-shaped stars. 

But (with a little advice from some of the sound folk in this very thread), by holding the tube vertically, unscrewing the ring and giving the lenses a little shakedown, it did indeed seem to fix the problem, as advertised! 

They still feel/sound loose to me (there's a slight clacking movement if I very gently shake the OTA, as if something is moving between two fixed positions) but the stars are now stars. And I split Castor! Very exciting. (For me)

Now to figure out how to wring the best out of this not-immediately-wonderful AZ3 mount

[Pixies]

1 minute ago, Basementboy said:

I'm a beginner and just bought an ST80 as my first telescope - and was very disappointed to see "V"-shaped stars. 

But (with a little advice from some of the sound folk in this very thread), by holding the tube vertically, unscrewing the ring and giving the lenses a little shakedown, it did indeed seem to fix the problem, as advertised! 

They still feel/sound loose to me (there's a slight clacking movement if I very gently shake the OTA, as if something is moving between two fixed positions) but the stars are now stars. And I split Castor! Very exciting. (For me)

Now to figure out how to wring the best out of this not-immediately-wonderful AZ3 mount

I admit, the clacking sound is a little disheartening. I assume it will go as things tighten up as it cools down. But it seems to me that if it is moving about, then at times is it not going to be aligned correctly. I did the lens-shoogling thing several times and the results were about 50/50 OK/coma.

I was playing around with the collimation last night, too. I can get it better than it was but still not perfect. I reckon that if one could file the focuser attachment screw slots in the OTA into a more elongated shape, then add a very thin rubber o-ring between the focuser and OTA, it would provide some range of focuser tilt adjustment.

As for the AZ3 mount. I can say that the AZ5 is great!

[John]

There may be a limit to how good you can get the collimation and star test with these F/5 achromats. A lot of them have an optical issue called spherical aberration (SA) in the form of either under or over correction to some degree. Many of the slower achromats from the same source have the same.

It's worth keeping at it to get it as good as it can be but I feel there is likely to be a limit beyond which you would need to experiment with rotating one of the objective elements or even more drastic work to gain improvements.

I have a 90mm F/11.1 achromat from the same manufacturer (Synta) and I've been fiddling with that for about 4 months now but it is still showing a little flaring of the star test despite my efforts. The next stage, when I have time, will be to experiment with the relative rotation of the lens elements to see if there is some "wedge" in one of them. It's a time consuming process though and we have not had enough clear nights to justify spending time on that.

 

 

Edited March 15, 2021 by John

[Pixies]

Thanks @John. 

I've been using an artificial star indoors, so far. The best I can get it just now, still shows some miss-collimation. When I take it to one side of focus, the CA shows a bright red central 'dot' inside a blue halo, but the red dot is not central to the halo.

Also, a laser projects a dot about 2-3mm off-centre on the objective lens, and the reflections in the Cheshire are not concentric.

I know what you mean about chasing perfection in a budget scope, but having the optics aligned should be achievable. In fact, I think it should be expected, even in a budget scope. I'm not complaining though - and I don't want Stu thinking I'm regretting buying this from him. I love fettling and I bought this scope with the intention of getting my hands dirty with it.

As for the internal reflectivity. The coating on the inside of the tube doesn't look too bad. It's a relatively new scope, so perhaps things have improved. Does this look typical?:

 

[John]

That looks quite typical for a Synta scope. Their internal blackening is OK but nothing special in my experience.

I hear what you say regarding the optics but I have to confess that I've owned two or three scopes over the years that I never managed to get a really satisfactory star test from

I'm not suggesting that yours is like that I hasten to add.

My 90mm F/11.1 produces pretty decent performance visually in terms of splitting double stars, detail on the moon and planets etc even if it's star test is not that great.

 

Edited March 15, 2021 by John