ST 80 focus issue

[PH-R]

Hi all,

I dusted off my equipment the other night to ready myself for the upcoming winter. I aligned main scope, finderscope & guide scope starting with 20mm down to 5mm and everything was great. When I attached my webcam to the guidescope and started PHD to practice guiding I could not see the target star, I then remembered about this problem before and attached the diagonal, which straight away showed me the star albeit out of focus. I then focused the star(the tube was entering the scope for direction) until I could no more because the diagonal came into contact with the scope. I'm then left with a star this shape >< (not quite focused).

What puzzles me is the following: if I cannot achieve proper focus with the diagonal because the focuser is turned all the way in, then logically if I remove the diagonal the cam would then gain say 2 inches more focus but does not even see the star. Am I missing something obvious?

I have tried to explain this the best I can but if you feel you maybe able to help and need further info. please don't hesitate to ask.

Regards,

Paul

[steppenwolf]

How much travel is there in your focus tube? If there is less travel in the focus tube than their is light path in your diagonal then it is possible that the focus point for the camera is in between the two distances.

With this in mind, have you got an old cheapo Barlow lens?. If so, unscrew the lens and use the rest as an extension tube.

For what it is worth, I think you actually need more outwards focus than you are getting with the diagonal in place so I reckon you will need the diagonal and the lens-less Barlow tube.

[PH-R]

Hi Steve

Thank you for the helpful reply. The next clear night I get I shall give the barlow solution ago.

:icon_scratch:I still don't understand how adding more length will help as it seems like if I could push the tube further into the scope by say a half inch I would achieve focus.

Best wishes,

Paul

[steppenwolf]

I still don't understand how adding more length will help as it seems like if I could push the tube further into the scope by say a half inch I would achieve focus.

Perhaps the image below will help, this is my own ST80 in focus with a guide camera in place - the red line is approximately the position of the focal plane - look how far out from the optical tube it is!

[PH-R]

Hi Steve

Thank you again. You have inspired me with the pic so I just took one with the position exactly as it was the other night, not quite achieving focus. I hope you can understand more seeing the pic than I was able to describe in words.

Best wishes,

Paul

[Photosbykev]

If you remove the diagonal and fit an extension tube about 25mm long i.e. an empty barlow tube you will get focus with the focus tube 3/4 of the way out.

[PH-R]

Hi Kev

Thank you for the response. Next clear skys I get I will be out with the barlow

BTW I love/agree with the quote in your sig!!!

[steppenwolf]

Hi Paul,

I still think that you need to add, not subtract light path length to your particular system to achieve focus and suspect that your 'probable focus' position is incorrect.

[ollypenrice]

Forgetting diagonals and prisms, the distance from the back of the black focuser on the ST80 (the point at which the drawtube emerges) to the chip needs to be about 13.5 centimeters.

On one of my guide cams a single de-lensed Barlow gets me that distance. On another I needed two delensed Barlows to get there. I Araldited one into the other to avoid flexure risk.

I'm sure this will sort out your issues.

Olly

[PH-R]

Managed to get out last night and tried the barlow solution and I'm happy to report it worked!! So thank you all for your advice on this issue.

Best wishes

Paul