Trouble getting a GSO 8" f4 reflector to focus

Boothie (Sean) · #1 13-05-2016, 10:27 PM

So I finally got my new scope, pick it up from a member on here. Well my father did, and it came down to Canberra yesterday with a family member. Well surprise new kit, clouds roll in. Luck for me the clouds have moved on and I had a chance to set up the scope.

I was having real trouble getting it to focus when I was tweaking the finder scope this afternoon and soon realised tonight that it wasn't just an issue with things on earth. I had to dismantle one of my barlows to get the eyepiece back far enough to focus.

Am I missing something? Its running GSO 2" crayford style 10:1 focuser, with reducer to 1.25. tube length is 800mm with a 200mm primary. The eyepieces I was trying to use have been fine in my other loaner scope. Is this related to it being a 'Fast' scope?

Any help/guidance would be great.

Sean

raymo · #2 13-05-2016, 11:13 PM

I suspect that it was bought, and the owner tried to attach a DSLR, and
found it wouldn't focus, like most dobs[Newtonians]. If I'm right he probably
moved the primary mirror further up inside the tube to enable it to focus
with a camera. This would mean you would have to use an extension tube
to focus with an eyepiece.
Your eyepieces will work o.k. with a fast Newt.
If I'm right, then no problem; If I'm wrong we''ll have to look elsewhere for the solution to your problem.
raymo

rmuhlack (Richard) · #3 13-05-2016, 11:59 PM

Unlike most dobs where there isn't enough focuser inward travel to reach focus with a camera, there isn't enough *outward* travel on my GSO 200mm f4 to reach focus with a camera without using an extension tube. This means that a LONG extension tube is mandatory for visual use, which may be your problem.

raymo · #4 14-05-2016, 12:17 AM

I hadn't heard of that, Richard; maybe all GSO f/4 scopes are the same.
raymo

barx1963 (Malcolm) · #5 14-05-2016, 02:01 AM

First off, I do not think the scope is a dob. Certainly GSO do not do f4 dobs, so most likely it is an imaging newtonian, just like this one http://www.bintel.com.au/Telescopes/...oductview.aspx in which case an extension tube i certainly going to be necessary. They are optimised to place the focal plane where a sensor would be ie. outside the focuser while en eyepiece would be looking for a focal plane inside.

Malcolm

Boothie (Sean) · #6 14-05-2016, 09:11 AM

Yeah its a Newt. on a Eq3 mount (Maybe 2) and I was thinking it might be set for imaging, due to the short f ratio. Extension tube hunt shall begin but will make do with the dismantled barlow for the short term.

Is there any other qwerks I might come up against with this scope? Currently my experience is limited to a 100mm f10 Royal for the 80's. I am mainly going to be using the scope for visual observing, but looks like if I have inadvertently allowed the opportunity to open Pandora's box if I so desire (Loose my self-control, which has been know to happen!)

From brief skim reading I have managed to get in since receiving the scope, am I right in thinking that collimation of these faster OTA are less forgiving?

If this is the case should I look at getting a collimation tool? With a young family the amount of time I have to tinker is limited, and a more straight forward and expedient process would be appreciated.

Shiraz (Ray) · #7 14-05-2016, 08:44 PM

Quote:

Originally Posted by Boothie

Yeah its a Newt. on a Eq3 mount (Maybe 2) and I was thinking it might be set for imaging, due to the short f ratio. Extension tube hunt shall begin but will make do with the dismantled barlow for the short term.

Is there any other qwerks I might come up against with this scope? Currently my experience is limited to a 100mm f10 Royal for the 80's. I am mainly going to be using the scope for visual observing, but looks like if I have inadvertently allowed the opportunity to open Pandora's box if I so desire (Loose my self-control, which has been know to happen!)

From brief skim reading I have managed to get in since receiving the scope, am I right in thinking that collimation of these faster OTA are less forgiving?

If this is the case should I look at getting a collimation tool? With a young family the amount of time I have to tinker is limited, and a more straight forward and expedient process would be appreciated.

Sean, the f4 Newtonians are quite tricky to set up perfectly for imaging, but for visual use, a simple laser collimator is very easy to understand/use and should do the job well enough. It might need a bit of collimating itself and Alexander put up an excellent thread on how to do that. http://www.iceinspace.com.au/forum/s...ad.php?t=75601 Unless aiming for perfection, you will probably not have to mark the secondary mirror in your scope, as Alexander did

janoskiss (Steve H) · #8 14-05-2016, 09:03 PM

As long as the primary mirror is accurately centre spotted, you can collimate the scope quite precisely using a Cheshire style visual collimating tool. The shorter FL does make the optics less tolerant to imprecise collimation, but at the same time the shorter tube makes it easier to do be precise because you can reach the collimation screws while looking through the focusser. Laser collimators have their pitfalls and unless you get a fancy expensive one a Cheshire is better IMO.

Boothie (Sean) · #9 14-05-2016, 11:36 PM

Thanks for all the guidance, I will look into a simple Cheshire type collimator I think.

But with the extension tube that I will need, where should I aim for the focus point to be? Is the middle of the focuser's travel be where I'm aiming for say planets (Jupiter, Saturn) with a 6mm eyepiece? Or should it be closer to the fully wound in or out point?

Based on the barlow with the element removed which is my current working solution, which is around 25mm once inserted has the focuser around 2/3 of its travel.

Bintel sell a 35mm or 50mm 2" extension, the 35 would place it around middle while the 50 would have the focuser wound in most of the way.

I'm leaning towards the 35 as this keeps the focuser tube clear of the tube during observing. Is my logic/understanding missing something with this approach?

Cheers,
Sean

janoskiss (Steve H) · #10 15-05-2016, 12:25 AM

If your focusser has more than 50mm travel, then go with the 50mm extension. Otherwise get the 35mm. The idea is to not have a "gap" in the range of accessible locations for the EP: fully retracted focusser with extension tube should overlap (at least marginally) with fully extended focusser without extension tube. In other words: you want to avoid the possibility of the focusser alone not having enough outward travel and at the same time the focusser + extension tube not having enough in-travel. Hope this makes sense.