GSO 12" - moving up the primary mirror

[jsmoraes]

Goal: to reduce the distance between primary and secondary mirror so that the focal point is compatible with camera Cannon and SPC 880 in direct focus.

Description:

After several attempts, I finalized by installing long screws and strong springs. The adjustment screws now are 8 centimeters long and locking screws are 7 centimeters. The spring has 8.5 cm long in a rest state.

Thick washers were necessary for the adjustment screws to avoid physical contact and damage to mirror

The distance between the mirror cell and OTA rear plate was 6.4 cm; 6.3 cm and 6.1 cm

The photos demonstrate how it was done.

spring and adjustment screw:

http://multiply.com/mu/jsmoraes/image/KyDHfesmweEhq8IAGuZ8yg/photos/1M/300x300/761/detalhemola.jpg?et=ZnKaJE837ivpVFlw kdBCqQ&nmid=0

locking screw:

http://multiply.com/mu/jsmoraes/image/jLEeb2U9-AgocSbvsnev-A/photos/1M/300x300/762/detalhetrava.jpg?et=%2BE1XKM9DqKr8N okEbGvjag&nmid=0

both screws:

http://multiply.com/mu/jsmoraes/image/PMNfq0pi00cupXevylN9aQ/photos/1M/300x300/763/detalhesparafuso1.jpg?et=9sCYw50HyO hu78SXoPEpzA&nmid=0

the original and new screws:

http://multiply.com/mu/jsmoraes/image/R0a90DmKCLQFAdPoiR94jg/photos/1M/300x300/764/comparaParafusos.jpg?et=q2oXVg32a1L gjgYw1uKcdQ&nmid=0

http://multiply.com/mu/jsmoraes/image/uC+HEl6mFhNPQsIUeUz7Uw/photos/1M/300x300/765/parafusooriginal.jpg?et=1qt89DQf0wR X77vTSJX3xA&nmid=0

Main problem:

The long screws behaved as stems, and due to the weight of the mirror tend to skew, decentralizing the primary mirror inside the tube and by consequence in relation to secondary.

After several tests, with different springs, I decided by a strong spring, allowing at least adjust the mirror with the OTA upright without the aid of locking screws.

Despite the strength of the spring, they were unable to prevent skewing of the screws when the OTA is horizontally positioned. To avoid this movement, it is necessary to hang the position with locking screws.

To avoid damage to cell by rubbing and scraping I needed polishing the locking screw end.

Result:

Positive:

Got success in the offset of the focal point. Didn't seem to have increased significantly the coma, typically present in a f/5. Although, I have the impression that there are an increase, mainly in the periphery of the field when used 25 mm and 30 mm eyepiece.

It was not observed changing the position of the mirror with the movement of the telescopio, over the mounting or in case of installation and uninstallation.

Negative:


The collimation is more complicated and laborious due to the possible offsets of the mirror with the OTA.

After various trial and error, I concludes that this must be done in three steps.

1) with the OTA upright and with all locking screws loose. Primary and secondary adjustment with laser. Some correction of secundary might be necessary, because the primary mirror can move horizontally. After this adjustment, hang the mirror, observing the collimation laser. Little by little, with alternating sequence of locking screws.

2) with the OTA in horizontal position, review the grips of locking screws, that have a significant influence on collimation, watching the laser.

3) star test. Here is complicated enough. Because you will need to work with the 6 screws: 3 to tuning and 3 to locking, simultaneously.
Release slightly one locking screw around , change slightly the position of the adjustment screw and back to hang with the locking screw. Very large movements can displace the primary mirror, and as punishment it will do a return to item 1 of this procedure.

What take 15 to 30 minutes, now it can take up to hours, as the 6 screws have influences on collimation. Even understanding its logic, as the locking screws are very far from the adjustment screws, they have significant influences on collimation. With a very complicated interaction with the adjustment screws.

Conclusion:

The procedure works, and avoids cutting mutilation of OTA. So an operation fully reversible.

However, due to complications with the collimation only venture to make it who really has an interest in astrophotography, and lots of patience to collimate, when necessary.

The use of laser collimator is almost mandatory to ensure adequate primary and secondary centralization.

That is all, folks !

[mswhin63]

Nice job,

Consider installing shaft or even long internal threaded rods that can be attached to the OTA to minimise the flexing. I too am considering this venture but as it seems I am in focus using the original bolts I haven't really put the effort in doing this project.

[Shiraz]

good going Jorge - the primary mountings on these need mods anyway and you appear to have come up with a usable system that also includes moving the mirror forward.
As a possible alternative, did you consider using blocks between the back ring and the mirror ring? if you glued some blocks on the back ring you could run the screws through them to give some lateral support to the screws - while using short springs. Might make collimation easier and if you glue them they could probably be removed if you want to return the scope to original condition. And it might be possible to do without those dreadful locking screws and all the complications that come with them.
anyway, thanks for an interesting post. regards ray

[jsmoraes]

thanks, Malcon and Ray.

Yes, I did, Ray. But, by the time, I did not find any block or tube to involve the screws and to immobilize them.
Despite of complication in collimation task, what I need test more accurately is the centralization of mirror inside the OTA, with the secondary mirror and with focuser.
I have not any better information up to now. The clouds do not let me do.

The collimation task might be a practice matter

[rjbruce]

Hi Jorge,

Another idea, though more expensive, would be to use something like an OCS unit from Denkmeier (www.denkmeier.com) to extend the focal point further out. Depending on the amount of inward focus you have available these types of unit will allow you to achieve focus on a camera that wouldn't normally focus.

I use the OCS unit (which is the "corrector" element from the front of the binoviewer) screwed straight into the 2" camera adapter and my scopes will reach focus with a Canon DSLR without moving (and somewhat de-stabilizing) the primary.

The OCS also has the benefit of coma correcting the field as well, but it does add a slight magnification factor to the focal length. Possibly a TV Paracorr or the like might also do the trick depending on your focusers available travel.

All that said, springs are cheaper though. :)

Regards,
Rob

[jsmoraes]

thanks Rob.

Despite of good performance, the problem with displacement of primary mirror due to long screws and so losing of collimation, I took the decision to cut off 4 cm of OTA. Now I am running well, and no problem with displacement of mirror and lost collimation.