star shapes

[msh1]

hi all whats causing the star shape its st80 and a qhy8l i got the scope from flo a couple of weeks ago first chance to use it last night and this is what i got and its in focus ?? this is from the  centre of image 

[Alien 13]

It looks like coma but this is normally found arount the edges with all the "tails" radiating away from the center the ones in your image look to be all pointing in the same direction which to me would suggest that the focusser/camera is not square on to the objective lens.

Alan

[ollypenrice]

The ST80, though it has its virtues, is about as unsuitable as it gets for imaging. It is an achromat, so not all colours come to the same focus. Worse, it is a fast achromat so this is amplified, as is coma. However, the main issue here is the distortion, as you say. The most likely candidate is that the front cell is miscollimated - so is not pointing the beam down the central axis of the tube. I have seen this on an expensive apochromat after it had been subjected to a fall from its tripod and I was able to rectify it because the cell was adjustable. On the ST80 I don't think it is. However, I remember scope guru Ed Ting saying that with these budget achros from China you can try standing them vertically on their dewshelds and them tapping the tube with your fingers for a good few minutes. This, it seems, can resettle the elements.

The focuser is a primitive affair on these scopes so the other things to check are

1) that the entire rear casting is tight in the main steel tube (three radial screws hold it).

2) that the drwtube is not sagging so as to hold the chip at an angle to the lightpath. This is also a decent possibility.

Olly

[msh1]

thanks guys i was going to use the st80 as a guide scope but the pic looked odd took  this image just to check and its not good 

olly do i loosen the retaining  ring completely then tap it to recenter then nip it back up 

mark

[Alien 13]

This thread might give you a few pointers regarding re seating the lenses.

http://stargazerslounge.com/topic/209030-what-to-do-when-it-is-cloudy-again/?hl=%2Bst80+%2Bflocking#entry2265924

Alan

[lensman57]

thanks guys i was going to use the st80 as a guide scope but the pic looked odd took  this image just to check and its not good 

olly do i loosen the retaining  ring completely then tap it to recenter then nip it back up 

mark

Make sure that the sensor of your CCD is not tilted, is there anywhere in the  image that the stars look focufine? This could also be caused with the camera not being orthogonal in the basic focuser.I also echo Olly's thoughts on a miscollimated front cell.

A.G

[ollypenrice]

I don't think the tapping trick involves loosening anything.

Olly

[msh1]

Make sure that the sensor of your CCD is not tilted, is there anywhere in the  image that the stars look focufine? This could also be caused with the camera not being orthogonal in the basic focuser.I also echo Olly's thoughts on a miscollimated front cell.

A.G

there is no perfect pinpoint stars in the image  i have stripped it down and tapped away  

[Martin-Devon]

Hope you get this sorted soon - if there was a defect though in the original assembly I'd go back to FLO and have a discussion. Just to add that I've used an ST80 as a guide scope for the last 3 years, linked up to a Lodestar CCD, and it's been absolutely superb. You don't say the length of sub for the download that is shown, but for guiding purposes I use 3 second subs on a continuous loop (to Maxim) and the stars are perfectly round, no problem.

Martin 

[Viewmaster]

The ST80, though it has its virtues, is about as unsuitable as it gets for imaging. It is an achromat, so not all colours come to the same focus. Worse, it is a fast achromat so this is amplified, as is coma.

Anyone tried NB imaging with a ST80 ?

[msh1]

i think i have found the problem ,the focuser tube had so much slop in it i could move the camera up and down, i tightened up the 2 little grub screws seems ok now just need a clear night now 

[lensman57]

Anyone tried NB imaging with a ST80 ?

The only filter with half a chance is the Ha, even Apos have got problem with an Oiii and the objects with appreciable Sii line are not exactly abundant. My only concern is why use a filter that costs a lot more than your scope to image with.

A.G

[msh1]

its lots better not perfect but for guiding it should work

[MikeODay]

I have a similar problem with my 10" Skywatcher Quattro f4 Newtonian.

The stars are round at the bottom left of the image and progressively change to look something like the ones above on the top right.  I take it from the above that it could be caused by the camera sensor not being square to the tube (for various reasons), is this right or could it be a secondary mirror collimation issue? 

thanks

Mike