8 inch Bresser Messier dobsonian question - mirror holders

[Altair8389]

Hi,

I have just taken delivery of a 8 inch Bresser Messier Dobsonian.

I was testing it today, trying to focus without success on the aerial of the house at the end of my garden. After inserting an extension to the hexafocus I managed to get the Moon, Jupiter and the stars in quite good focus.

I did a rough collimation with a Cheshire and laser ....I thnk my laser is a bit uncertain about how well centered it is. Anyhow Jupiter with its multiple belts and the 3 satellites were well seen even without a Barlow.

What is wrong with this mirror cell ? Only 2 can be seen.

[Stu1smartcookie]

The aerial is too close to achieve focus , try an object that is further away . I assume it’s to line up the finder with the scope why you are trying this . 

[Ricochet]

Is the "top" mirror clip in your second picture touching the mirror? From the picture it looks like the mirror has slipped down between the lower two clips. Is that what it looks like when you look down the tube or is it just the photo angle? 

If it is like that you will have to remove the mirror cell and then dismantle it to shift the "posts" inward so that they all just touch the mirror when it is centred. You might prefer to just it back and get a replacement. 

[alacant]

2 hours ago, Altair8389 said:

What is wrong with this mirror cell

It's hard to tell from the photo. Our guess is that they've put the 203mm mirror in an older 208mm cell.

Take it off and have a look/measure?  It's very easy on the Bresser..

Cheers

Edited November 3, 2022 by alacant

[Altair8389]

9 hours ago, Stu1smartcookie said:

The aerial is too close to achieve focus , try an object that is further away . I assume it’s to line up the finder with the scope why you are trying this . 

Thanks, I thought that may be the case. I managed to get the moon and jupiter and stars in focus later that evening. I had to insert the 2 inch extension adaptor to get focus.

 

[Altair8389]

8 hours ago, alacant said:

It's hard to tell from the photo. Our guess is that they've put the 203mm mirror in an older 208mm cell.

Take it off and have a look/measure?  It's very easy on the Bresser..

Cheers

I think you could be right. I have contacted BresserUK, and await their advice.

 

[Altair8389]

9 hours ago, Ricochet said:

Is the "top" mirror clip in your second picture touching the mirror? From the picture it looks like the mirror has slipped down between the lower two clips. Is that what it looks like when you look down the tube or is it just the photo angle? 

If it is like that you will have to remove the mirror cell and then dismantle it to shift the "posts" inward so that they all just touch the mirror when it is centred. You might prefer to just it back and get a replacement. 

I am not sure if I want to do this with a "new OTA". I am awaiting the reply from Bresser UK. I am only going to use it for visual until I can master collimation (if needed) and the ins and outs of a Dobsonian. I might switch to astrophotography, perhaps mounting the Dobsonian base on an equatorial platform...

 

[Mr Spock]

With any Newtonian you will need to master collimation  It does get easier with practise though.

[SthBohemia]

52 minutes ago, Spile said:

However your primary donut is not centred so I would definitely check and if necessary, fix that. Details on my blog below.

Excellent article on collimating a Newtonian. It should be highlighted as a feature within SGL as questions, re-collimation keep appearing.

[Spile]

22 hours ago, SthBohemia said:

Excellent article on collimating a Newtonian. It should be highlighted as a feature within SGL as questions, re-collimation keep appearing.

Thank you. 

[Altair8389]

On 03/11/2022 at 20:57, Ricochet said:

Is the "top" mirror clip in your second picture touching the mirror? From the picture it looks like the mirror has slipped down between the lower two clips. Is that what it looks like when you look down the tube or is it just the photo angle? 

If it is like that you will have to remove the mirror cell and then dismantle it to shift the "posts" inward so that they all just touch the mirror when it is centred. You might prefer to just it back and get a replacement. 

I took the mirror cell out and adjusted the top mirror clip so it just touched the edge of the mirror cell like the other 2. In my haste, I did not measure the positions and size of the mirror cell, but I think it is quite well centered and the 3 mirror clips are showing up in the focuser tube and in the Cheshire. I will have a go at collimation. If there are problems with collimation, I will measure it again and ask Bresser to sort it out or get a new OTA.

I think the doughnut is centered on the mirror, the photo is probably at an angle as I have to avoid the secondary mirror assesmbly. I am not 100 per cent sure as I have not done precise measurements yet on the mirror/mirror cell and mirror clips.

[Astronomist]

I have the 10" version and have noted the same thing, on closer inspection it seems that the invisible mirror clip overhangs the mirror by slightly less than the other 2 .

I will try to upload a photo but I am new to the forum. . . .

the second photo should show the invisible clip.

[Astronomist]

oops , sorry if those photos are too big.

 

[Astronomist]

Update: having disassembled my scope and measured the cell it is apparent that the disappearing clip is caused by the fact that the mirror has 1-1.5mm lateral movement space around it to avoid pinching the  optics- the mirror settles into whichever side of the scope faces down, causing the upper mirror clip to overlap the mirror by less. I certainatly don't  find it causes problems with collimation.

 

I hope this is helpful

 

 

Edited November 12, 2022 by stargazer_3000

[Ricochet]

4 hours ago, stargazer_3000 said:

the mirror has 1-1.5mm lateral movement space around it to avoid pinching the  optics-

This is significantly more movement than I have ever had on mine. I adjusted mine so that there is a gap the thickness of a piece of paper between my clips and mirror edge, the same as often suggested for between the clips and mirror face. 

[Spile]

Deal with with the offset and rotation error and get the secondary centred under the cross hairs. Then you can use the mirror clips as a valid check. Otherwise you risk doing the collimation hokey-cokey