GSO 10" DOB Focus

[Tandum]

Does anyone know if you can achieve focus with a camera on a new GSO 10" DOB without modification?

[Davros]

I have a 10 inch GSO dob that has been tube shortened to allow focus with a DSLR/ So i guess you will need to trim your tube.

[Tandum]

Cheers Mick, how much has to go, I'm wondering if a low profile moonlight will do it?

[AlexN]

This I can help with mate... I am a metal worker.. If you need a tube shortened, you let me know! :)

A low profile moonlite MAY get you to focus, but if it doesnt, its still a very nice focuser, and a lot better than the standard GSO one.

[Tandum]

Cheers alex, I pm'ed you.

[Calibos]

Wowza!! I can't believe the first thing people suggest is cutting the metal tube of the dob without knowing what kind of camera we are talking about, without knowing if its a lack of infocus or outfocus and without first suggesting, recollimating the primary mirror further back or forward or suggesting lowprofile focusers or drawtube extensions.

[Davros]

A low profile focuser and adjusting the primary mirror to an extreme of its movement may get it there. Mine was shortened 35mm by the PO for focus with a DSLR.

[Tandum]

Thanks Mick, I'll check out some focusers :)

[Tandum]

To be honest keith, most cameras we use as pretty similar. Most have sensors that are 17-40mm from the coupling with most being around 20mm.

[Calibos]

Ah, so the gun wasn't being jumped as much as I thought :ashamed: Davros saw the cameras in your sig and knew which direction your focus defiency lay and the quickest cheapest way to sort it? My Bad. Sorry for doubting you guys. :ashamed:

[Tandum]

Yeah, his DSLR is about 40mm away, so he cut 35mm off the tube. Mine are about 25mm away so I might get away with the moonlight focuser but I need to modify it so a dslr will focus and I can sell it down the track :)

[astro744]

GSO make a low profile focuser, M-MLP230, http://www.gs-telescope.com/content.asp?id=139

and it should fit your existing mounting holes. Most GSO dealers should carry it.

Also make sure you use a low profile T-adaptor such as FF145, http://www.gs-telescope.com/content.asp?id=137

Also loosen the primary collimation screws first to the max and then re-collimate by tightening two.

If all that is still insufficient, move the mirror up the tube if possible. I'm not sure how the mirror cell is mounted but it's usually three screws at 120deg. just drill 3 new mounting holes provided you can slide the mirror cell up the tube. No need to cut the tube but you may need a slight re-balance. Of course if the mirror cell must go at the end then you may have to cut the tube but only after all other options have been tried first.

Note moving the mirror up by sliding the cell or cutting the tube will have an impact on the light cone and you may need a larger secondary mirror unless it is already oversized. (Although you may not notice the difference).

Note for visual use you will then need a 2" extension for your focuser, again GSO have three lengths to choose from and the shortest should suffice.

[gjelke]

Hi Robin,
sounds you have the same problem I have with the 12" Dob. The way I have solved the infocus problem for the moment is to use a 2X Barlow. This gets a lens further down the tube and I am able to acheive focus.

The other way I have tried and had it work is to use an 35mm extension tube and no barlow and that works also.

I have only tested this so far on the Dob mount and at large targets such as the Moon, Saturn and Jupiter.

Cheers

Greg

[toryglen-boy]

use a Baader MPCC, i use one and it moves the focus right in.


;)

[Tandum]

Just to round this off, I got the tube and some rings and I can get focus with the QHY5, the DSI2 and the QHY8, as it is. The starlight with the wheel could not. This is a 10" GSO dob with a cheap as R&P focuser. I think the lock screw is plastic :)

I measured 10mm spare infocus when the QHY8 was plugged in and it's sensor is 22mm from the coupling so another 10 is needed for DSLR focus.

I'm guessing the moonlight focuser I ordered will solve these issues and I won't need to trim the tube.

[gazza83]

Hey Tandum,

How did you go with the moonlight? I'm thinking of one of these scopes with the future possibility of EQ mounting.

Cheers

[Tandum]

Gary,
Yep I got focus with all my cameras with the moonlight installed. I don't have a canon any more but next time I drag it to a star party I'll see if a canon can focus. It may just squeek it in :) It's a pretty heavy tube and can get a bit of a wobble going in a breeze on an eq6. I'm considering replacing the tube with something lighter.

[gazza83]

Hi Tandum,

That's interesting. Some people are using a 12" on the EQ6. Are you thinking of going back to an 8" newt, or a different type of scope?

Cheers

[Grahame]

Hi Guys,

I have a 10" GSO Dob, and had the same problem with a canon 40D. Since then I have installed the GSO low profile focuser and using an MPCC have about 2mm of spare focus if that - This was also achieved with moving the mirror up slightly on its collimation screws also.

Its a close call but it can be done without modifying the tube :)

EDIT: Im also running this scope on an EQ6 with guide scope and have had pretty good success with it.

[Tandum]

No, I'm going to continue with the 10". When I said 'change the tube', I meant change the metal tube to a compound tube to save weight, but I don't have any spare cash to throw at it at the moment. I did get a price on a set of custom rings which came in at around $260 but we needed a new fridge instead :) The scope is not going anywhere and there's no rush ...

[AlexN]

Sure would be funny if the newt up and grew legs then walked out the door... :D hahaha.. A CF tube would not only drop the weight by over half, but it would look mighty swanky too.. :) I would make a suggestion though.. Make the new tube 10" longer than it has to be.. That way you've got yourself a free dew sheild built into the scope... It'll add contrast, slow/stop the formation of dew on the primary and secondary and even with that extra tube length, you'll still be down at least 10kgs on the original aluminium tube..

[Tandum]

I was looking at these (http://www.fpi-protostar.com/blite.htm). They come in 48" (1220mm) lenghts and the current tube is 1205mm. They say tube weight is 3.28Kgs so I was thinking of stripping this dob to weigh all the bits.

The curved secondary mounts might be worth a look also.

[AlexN]

Bah, curved secondaries are good for visual work if you really despise diffraction spikes... for imaging, you'll still get spikes, just they will be weird curvy spikes... I reckon the weight of your 10" tube would have to come in at something like 9~12kgs..