Bresser Dob-254 wisdom request - are my secondary knobs too short?

[cwis]

Hi all,

I THINK I'm right here, but please correct me if you see an error... I finally got round to trying to collimate my Bresser 10inch dob today. I've had it a few months (been cloudy for most of it!) and although I didn't really see anything wrong with the views I was having back in the distant past before the snows, I thought it was time to familiarise myself with it.

Secondary first, as is customary and I saw that it was not central to the focus tube  - it needed to move towards the primary mirror a few mm.  So I loosened the central screw and wound in the outers.  However before the mirror was central, the knobs bottomed out on the secondary holder:

I had longer spare screws, so ended up here. I'm not finished but it's in the ballpark central now:

This left the secondary mirror base this far away from the centre of the spider:

Unfortunately the supplied screws are only this long:

I notice olderDob-254 had screws instead of the knobs on mine. It looks like the spider was designed with recessed screws in mind:

That well is 5mm deep, so a 30mm screw headed adjuster would work fine. Has no one else experienced this with their 10 inch Bresser? I thought quite a few have been sold recently - am I the only one to shift the secondary?

I' will be replacing my 40mm spare screws with 30mm ones for tidiness btw in case they offend anyone's OCD...

Edited February 9, 2021 by cwis
Double image

[cwis]

Ended up here. Camera is slightly off centre but the eyeball shows a concentric white circle within the doughnut, with the Cheshire viewing hole and my eyeball in the centre. The Cheshire crosshairs are in line too showing the secondary is fine, and the three mirror clips are all slightly out of view but with a collimation cap are visible and equally spaced on the mirror. 

I replaced the longer screws with 30mm ones and as predicted they recess nicely without bottoming out:

 

I really can't see anything wrong with this setup - I wonder if an accountant changed the spec of the adjustment screws from "30mm cross head screw" to "30mm knurled thumbscrew" without realising the implications? 

 

[johninderby]

I prefer thumbscrews so replaced the stock ones with black stainless 35mm ones. 

Yes I had to move the secondary down a bit. Used a Concenter as that is the best way to get the secondary setup spot on. 

[cwis]

56 minutes ago, johninderby said:

I prefer thumbscrews so replaced the stock ones with black stainless 35mm ones. 

Yes I had to move the secondary down a bit. Used a Concenter as that is the best way to get the secondary setup spot on. 

 

Thanks for the confirmation John!  I did wonder whether I was missing something fundamental.  I just need a clear sky now to see what difference (if any!) it's made.

I keep looking at Concenters because although I'm a complete newby it looks like I like newts and I'm picky about collimation.  You wait - this time next year I'll be saying in an adenoidal voice "Yes well I think the flint element is a little pinched in its holder in this example"...

[johninderby]

The Concentre is one of those bits of kit I thought I’d try out a while ago. Now consider it an essential for setting up a newt (dob). No other method is as accurate. 

Recently replaced my TS Concenter with a Concenter Visus which is an improved version.

If only they were cheaper. Wish FLO would stock them.
 

 

[faulksy]

On 09/02/2021 at 17:52, cwis said:

Hi all,

I THINK I'm right here, but please correct me if you see an error... I finally got round to trying to collimate my Bresser 10inch dob today. I've had it a few months (been cloudy for most of it!) and although I didn't really see anything wrong with the views I was having back in the distant past before the snows, I thought it was time to familiarise myself with it.

Secondary first, as is customary and I saw that it was not central to the focus tube  - it needed to move towards the primary mirror a few mm.  So I loosened the central screw and wound in the outers.  However before the mirror was central, the knobs bottomed out on the secondary holder:

I had longer spare screws, so ended up here. I'm not finished but it's in the ballpark central now:

This left the secondary mirror base this far away from the centre of the spider:

Unfortunately the supplied screws are only this long:

I notice olderDob-254 had screws instead of the knobs on mine. It looks like the spider was designed with recessed screws in mind:

That well is 5mm deep, so a 30mm screw headed adjuster would work fine. Has no one else experienced this with their 10 inch Bresser? I thought quite a few have been sold recently - am I the only one to shift the secondary?

I' will be replacing my 40mm spare screws with 30mm ones for tidiness btw in case they offend anyone's OCD...

i ha the same issue with my wifes 10" had to get longer screws. very strange

[Louis D]

Makes me glad my secondary is mounted on a separate screw that adjusts for height, leaving the tilt adjusters free to do only that job.

[faulksy]

On 05/04/2021 at 23:00, Louis D said:

Makes me glad my secondary is mounted on a separate screw that adjusts for height, leaving the tilt adjusters free to do only that job.

my 20" is like that. 1 centre bolt and 4 tilt screws