Washer size for the secondary on a gso dob

[DAT]

I have a Stellalyra 8” dob and I would like to put a washer on the secondary for more accurate adjustments, does anyone that has done this mod on this dob know what size I need?

I have measured the holder to be 44mm and have found an m6 x 45 matt black washer. I think m6 should be large enough to accomodate the central screw. Would the extra mm be visible?

I have seen there are various approaches to this, including just using milk jugs.

After watching a couple of videos previously suggested, my idea is to have a steel washer with a cut out milk jug washer glued on. I plan on having the plastic washer between the adjustment screws and the steel washer to get a better grip and avoid scratches and to ensure a nice snug fit around the central screw. Is my thinking correct? 
 

I could figure this out once I remove the secondary but would love to have the washer at home before I do so.
Any thoughts would be much appreciated 

[PeterStudz]

For my 8” Dob I just used two milk bottle washers cut by hand. As suggested in the article below. Worked out the size after I got the secondary out. Making and installing was then just a five to ten minute job.

I thought about using a metal washer but the two milk bottle washers have worked great. And since I installed them about 2 yrs ago they’ve worked perfectly. 

Any reason why you any to do it? Is there a problem? My Dob was a partial rebuild and required work on the secondary. 
 

https://stargazerslounge.com/applications/core/interface/file/attachment.php?id=136801

[DAT]

29 minutes ago, PeterStudz said:

For my 8” Dob I just used two milk bottle washers cut by hand. As suggested in the article below. Worked out the size after I got the secondary out. Making and installing was then just a five to ten minute job.

I thought about using a metal washer but the two milk bottle washers have worked great. And since I installed them about 2 yrs ago they’ve worked perfectly. 

Any reason why you any to do it? Is there a problem? My Dob was a partial rebuild and required work on the secondary. 
 

https://stargazerslounge.com/applications/core/interface/file/attachment.php?id=136801 392.83 kB · 454 downloads

I think collimation is looking good but this has been suggested to be by a few people and does not seem too complicated, collimation at the moment is limited (apparently) by the fact that the adjustment screws fit into indentations. I was going to replace the original screws ( with thumb screws) and thought might as well put the washers on as they should enable me to be more precise. Perhaps not necessary but it seems like a fun task to do and learn about the scope

[bosun21]

I always carry out this modification to every dobsonian I've owned. Last time I just bought ten plain washers from eBay and sprayed them matt black. This mod eliminates any stiction when positioning the secondary mirror holder and has worked great every time.

 

 

 

 

[PeterStudz]

20 minutes ago, DAT said:

I think collimation is looking good but this has been suggested to be by a few people and does not seem too complicated, collimation at the moment is limited (apparently) by the fact that the adjustment screws fit into indentations. I was going to replace the original screws ( with thumb screws) and thought might as well put the washers on as they should enable me to be more precise. Perhaps not necessary but it seems like a fun task to do and learn about the scope

I must admit that I only really understood collimation when I had the secondary in my hand and played around with the adjustment screws. Seeing it move closeup made it obvious. With my secondary the indentations were created by the adjustment screws digging into the holder over time. They weren’t manufactured that way. 

I hand my secondary out several times and by the end I was confident that I could get it all back together, lined up and collimated in a short space of time and without the hassle I had to begin with! So in that respect it’s worth doing. 

[DAT]

2 hours ago, bosun21 said:

I always carry out this modification to every dobsonian I've owned. Last time I just bought ten plain washers from eBay and sprayed them matt black. This mod eliminates any stiction when positioning the secondary mirror holder and has worked great every time.

 

 

 

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Do you by any chance know the size of the washers you used?, both the outside diameter (I think it should be 43mm--44mm) and the inside (5mm-6mm?). I am struggling to find and exact fit but maybe that is not so important...

[bosun21]

10 hours ago, DAT said:

Do you by any chance know the size of the washers you used?, both the outside diameter (I think it should be 43mm--44mm) and the inside (5mm-6mm?). I am struggling to find and exact fit but maybe that is not so important...

The scope that I bought the washers for unfortunately is a Skywatcher 10" Flextube Synscan dobsonian which has a completely different secondary mirror holder. I have carried out the mod to two Stella Lyra dobsonians as well. One was an 8" and the other a 12". As long as the washer is smaller than the diameter of the secondary mirror and that the centre hole is large enough for the central screw.

[PeterC65]

I'm about to make the same mod to my 6" and 8" Sky-Watcher Newtonians. I plan to remove the secondary to measure it before ordering the parts, but my understanding is that the central thread size is M5 and the secondary stork is a little over 30mm so I'm currently planning on buying M5 x 30mm washers (this one). This washer is quite thin which I think might be important for maintaining the adjustment range, and it is black. Accu are a great supplier for nuts and bolts and the like by the way.

 

[PeterStudz]

In my Skywatcher 8” Dob I found the secondary holder to be slightly smaller in diameter than part that contains the adjustment screws. If the washer is the same size (or even just a bit smaller) than the secondary holder it’s too small - the screws are very close to the washer edge. 

I found it best to make the washer the same size, or slightly wider, than the holder that contains the adjustment screws. Easy for me as I cut milk bottle washers using a craft knife. Doesn’t matter if it’s a bit wider - you’ve got the back of the mirror in the way and as long as it’s not wider than that it’s not going to make any difference.

And while you are at it - you can always blacked the back and sides of the secondary 😀

Edited May 28 by PeterStudz

[DAT]

Seems like it’s best to take the  secondary out and measure everything then. I will also make sure the washer is about the same size as the holder as the screws are close to the edge. My only other question is does the central whole need to be big enough for the screw and the spring or just the screw? Looking at all the pictures I can’t see why either way would not be ok.

It’s interesting though as I found a CN thread saying most recent gso dons do not require this mod. It’s definitely more a “challenge” than a necessity for me, hopefully it will make the collimation process easier to understand .

Thanks for all the replies

[bosun21]

1 minute ago, DAT said:

Seems like it’s best to take the  secondary out and measure everything then. I will also make sure the washer is about the same size as the holder as the screws are close to the edge. My only other question is does the central whole need to be big enough for the screw and the spring or just the screw? Looking at all the pictures I can’t see why either way would not be ok.

It’s interesting though as I found a CN thread saying most recent gso dons do not require this mod. It’s definitely more a “challenge” than a necessity for me, hopefully it will make the collimation process easier to understand .

Thanks for all the replies

Just the screw 👍.

[PeterStudz]

I find (I’m just using a couple of milk bottle washers - no metal) that you really notice the difference in fine adjustment - at that point the washers give finer control without any little stick/slip which I had without the washers in. And something I haven’t heard anyone mention… with the washer mod the adjustment screws have a nice feedback, eg you can progressively tell when you are nearing a limit. Without & before this mod the screws would come to a dead stop with little or no warning. 

[bosun21]

5 hours ago, bosun21 said:

It’s interesting though as I found a CN thread saying most recent gso dons do not require this mod

The newer GSO dobsonians definitely do require the mod if you want to have fine adjustments available for the secondary mirror holder. In fact the GSO (Stella Lyra) dobsonians require the mod even more so than the Skywatcher scopes. The former having much deeper indentations on the holder which prevents fine adjustments.

[Spile]

I’ve never found the need to add compression washers to the outer screws of my secondary although I did replace the Allen bolts with M4 30mm ones purely for convenience.