Focusing problem, need some help.

[lockyerlad (Brian)]

Hi all, need some sound advise. I have a GSO 16 trussed dob that has a problem where I cannot rack the focuser in far enough to focus planets, moon or stars with the higher powered 1.25 ep's (6,9,15 and 20), the 25 and 32 work OK. The 2 inch (25 and 50) work fine. I have tried moving the primary and secondary up and down the tube but no success. All the ep's are GSO plossl. Today I focused on a tree some 500 mt away and all ep,s worked fine even with a barlows. I have collimated the scope with the eye and then a laser collimater to check. Can someone help please.

[brian nordstrom (As avatar)]

Basically it sounds like the primary and secondary are set to far apart , if the focus is happening at. 1/2 km you need to bring then closer together. , by a good 50mm or so judging the fairly long fl of the 16 inch GSO,s .

How much in focus do you have left at 1/2km ? , also check the focuser draw tube for a removable extension tube first before moving anything as they sometimes come with one pre-attached that just needs removing for visual.

Also what type of 1 1/4 - 2 inch adapter did the scope come with. ? The longer type would allow the 2 inch and longer 1 1/4 ,s to focus but not the shorter 1 1/4 eyepieces , try just holding the shorter eyepieces in the 2 inch hole (carefully) without the adaptor and see if they focus to infinity .


Photos of the focuser would help .

Brian.

[ausastronomer (John Bambury)]

Originally Posted by brian nordstrom

Basically it sounds like the primary and secondary are set to far apart

Brian.

That's incorrect.

Cheers,
John B

[Stonius (Markus)]

Originally Posted by ausastronomer

That's incorrect.

Cheers,
John B

Just a suggestion; you may want to elaborate a little... :-)

[ausastronomer (John Bambury)]

Hi Markus,

The relative distances between the primary and secondary mirror have "ZERO" to do with determining the focus position in a newtonian telescope. With a newtonian telescope the focal position is determined by the position of the primary mirror relative to the focal plane of the eyepiece. Incorrect positioning of the secondary mirror can cause miscollimation or vignetting but it doesn't change the focus position. Consequently to fix the problem you need to change the distance between the primary mirror and the eyepiece. As these mass produced telescopes are all cut on cnc machines and the mechanicals shouldn't vary, I would guess that the primary mirror has been ground with a focal length that is a fraction too short (maybe by only 1 or 2 cm) for the telescope structure. I would be taking it back. Outside of that the primary mirror needs to be collimated as high as is physically possible in the tube and it may reach collimation.

Cheers,
John B

[brian nordstrom (As avatar)]

What ?? , the relative position of the 2 mirrors is the focal length , am I right ? the secondary does not magnify only turns the light at 90 degrees so it can be seen outside the tube ( otherwise your head will block the incoming light ) . The focal plane is at a point outside the tube ( best place for it I think ) so an eyepiece can magnify the image being produced by the primary , am I right ???

Yes the position of the secondary ( distance from primary along the tube ) moves the focal plane towards or away from the best position for looking through an eyepiece ( comfortably ) .

Lets for arguments sake we move the hole the focuser is mounted over by , say 50mm away from the primary , ok ? ( up the tube ) , now the image is still seen fully luminated but the secondary is not optimum size for this position , OK ? but to large for this position , But it will still produce an image .

Now you will find all your eyepieces wont come to focus as the focal point is inside the focuser tube or worst in the tube its self .

Geez , do a ray trace I am not talking about the focal length of the primary but the relationship between the secondary and the focal length of the primary to put the focal plane in the best position possible so all eyepieces come to focus .

Sorry for hijacking but I don't like being told that I am wrong when I have built over 10 Newtonions in my time and the position of the focuser in relation to the secondary and primary is paramount ,. I have made basic mistakes along the way and learned from them .

The best mistake was setting the position of the focuser ( cutting the hole in the tube ) using an object only 500 metres away , when I looked at the moon there was no way it would focus and I had to block the 1st hole up and hole saw another about 50mm further back so my eyepieces would come to focus ( this didn't change the focal length ) , bummer , worked perfectly as a terrestrial scope in its 1st setting but a 10 inch Newt aint made for that .

Brian.

Originally Posted by ausastronomer

That's incorrect.

Cheers,
John B

[ausastronomer (John Bambury)]

If he moves the secondary down the tube he will also need to move the focuser the same distance down the tube. This will move the focal point out by the amount the secondary and focuser have moved down the tube. Whilst it might allow him to reach focus it will do a really good job of hacking up and uglying up his perfectly good telescope leaving an ugly gaping hole and 4 small holes where the focuser used to be. A far better way to do it would be to dock the bottom off the bottom tube and move the whole mirror cell up. This can be done without leaving any ugly scars.

However, if its a reasonably new scope I would be sending it back and getting one that reaches focus properly. You shouldn't need to modify a telescope you have paid good money for.

Cheers
John B

[Stonius (Markus)]

Being a truss scope, I wonder if shorter trusses would fix it?

[ausastronomer (John Bambury)]

Not with the types of poles / fittings used on that type of scope. You would need to cut them and re weld them. The poles have a flattened steel end that a bolt goes through. I docked 10mm off the poles on my 18" Obsession to push the focal point out a little but that was a simple job to just cut the aluminium poles with a docking saw as the poles seat by bottoming out in split blocks.

Cheers
John B

[glend (Glen)]

If it is a GSO 16" Dob it is actually a three strut system, with the strut ends having a steel fitting that bolts to the top and bottom sections in two places making ut a rigid connection. There are no poles with flattened end on that scope. I used to have one and the design has not changed. You can see it on the photos on the Andrews website. It is concerning that the original poster has been moving mirror without a good handle on the problem. I agree with John, that these are all made on the same machine and things like strut length do not vary. They do not need any adjustment to achieve focus, unless there is a manufacturing defect, which i have never heard of yet, or it was assembled or ham fisted adjusted out of spec. I lean towards the later.
Brian focusing on a tree is not the same as a star test, where the scope is effectively at infinity. I recall that my Televue Nagler 11mm ep always required alot of back focus on that scope and the extension tube was needed. I has a full set of GSO Superview EPs and they did not need extension. When you try to focus on a star and are racking the focuser inward is the star getting larger or smaller? If it is getting larger your going the wrong way

[lockyerlad (Brian)]

Thanks all for the advice, won't be cutting struts or any other major work though. I did contact Andrews and they gave me a link to the same problem that appeared in a 2013 IceInSpace forum. The guys replaced the primary mirror adjusting bolts and screws with slightly longer ones and this fixed the problem.
My scope is a few years old, just getting back into it and appreciate you giving the time to help, it is good to understand more how the focusing works.Sorry if I caused any friction.
It is a bit slack for GSO though, we should not even be talking about this problem. Here is the link if you are interested http://www.iceinspace.com.au/forum/showthread.php?t=114061
Once again guys, thanks for the help.

[ausastronomer (John Bambury)]

Originally Posted by lockyerlad

The guys replaced the primary mirror adjusting bolts and screws with slightly longer ones and this fixed the problem.

It is a bit slack for GSO though, we should not even be talking about this problem. Here is the link if you are interested http://www.iceinspace.com.au/forum/showthread.php?t=114061
Once again guys, thanks for the help.

Hi Brian,

That will give you up to about an extra 10mm of collimation adjustment, depending on which bolts you use. That is probably going to be enough to enable the scope to reach focus with all your eyepieces. Certainly the easiest option to try first. Just be aware that this "may" affect the scopes ability to hold accurate collimation as the scope is moved through its full motion range in altitude. With the design of the mirror cell used in these scopes the shorter these bolts are the more stable will be the scopes collimation as it is moved through its full motion range in altitude. You can check this by collimating the scope when it is pointed at an altitude of say about 45 degrees. Then point the scope to zenith and check the collimation again, then point the scope near the horizon and check the collimation again. It may or may not change.

Cheers,
John B

[Kunama]

Hi Brian,
Perhaps you should work out how much more back focus is needed. First have a look at the 2" to 1.25" adapter, some use up more back focus than others.
If your adapter uses minimal backfocus then point the scope at a planet and insert the suspect 1.25" eyepiece without the adapter and see how far into the focuser you need to push it to get focus.

Then you can work out whether longer bolts are needed or if a different adapter would suffice.

[lockyerlad (Brian)]

Thanks Mat and Brian. I have replaced the bolts with longer ones and have focus now on the range from 6mm to 50mm except the 15 that still does not quite get there. Can't work out why?
The collimation shift I will look at next time i'm out in the coldSo frustrating that the scope did not quite get there with the bits supplied. Anyway thanks again.