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cyborg421

Removing primary mirror from Skywatcher Dob

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Hi All,

Does anyone know of an idiots guide to removing a skywatcher 250 dob primary mirror, preferably with photos?

Thanks in anticipation. :rolleyes:

Simon

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Based on a 200P but I think they are the same.

6 philips screws around the base of the tube - remove these with the tube lying horizontally.

Once they are all out, slowly and carefully back the cell assembly (black bit) out of the OTA.

How far do you want to go with this?

Next up you could remove the mirror clip screws (3 lots of 2) and carefully remove the clips without touching the mirror.

Mirror is then free to come out of the cell.

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Actually, I'd go with having the scope vertical. Remove all the screw and then lift the tube off. Doing it horizontally means

1. gravity will pull on the remaining screws as you progressively pull them out, making your job harder

2. mean you have to roll the ota (!) to reach all of them

3. when you take the last one out, the cell may fall out, damaging something.

Edited by Odd Dob

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Actually, I'd go with having the scope vertical. Remove all the screw and then lift the tube off. Doing it horizontally means

1. gravity will pull on the remaining screws as you progressively pull them out, making your job harder

2. mean you have to roll the ota (!) to reach all of them

3. when you take the last one out, the cell may fall out, damaging something.

Fair points, but I had Bobs Knobs so I couldn't put it vertically :rolleyes:

The cell is really jammed in, I'd be suprised if it could just fall out - I did have a hand behind the mirror though just in case ;)

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With the smaller scopes, such as the 8", a duvet on the floor, OTA lying on it and cell removed. Job done.

However, it depends on the size- I've done it horizonatally and vertically (ooh, matron) and the easiest way is vertically with a larger scope - remove those screws one by one and support the mirror with one hand. The OTA doesn't seem to deform as much either but you have to watch the weight of the secondary at the other end...

In both cases, when putting the mirror cell back onto the OTA, you can carefully rotate the cell until the screw holes in the cell match up with the OTA and then Bob's your father's bro. :rolleyes:

Edited by Beulah
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I've done it both ways, and had a heart stopping "thunk" noise doing it your way. Nothing damaged, but I could do with out the scare :rolleyes:

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Thanks guys,

A couple of further q's

1) Does removing the 6 screws mean the "cell" will drop out?

2) Is it easy to put back?

3) Am I likely to regret doing this at any point?

I would like to flock the lower tube of my flextube but would rather have a functioning dob without flocking rather than do anything harmful to the primary!

Thanks

Simon

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1) Does removing the 6 screws mean the "cell" will drop out?

Yes

2) Is it easy to put back?

The smaller the telescope, the easier it is (6-8inch). For example, the Skywatcher manual suggests two people assist in the installation of the primary of a 12 inch dob, I managed on my own, although it was a little tricky.

3) Am I likely to regret doing this at any point?

Not if you are careful. I have removed mirrors from telescopes from 6 to 12 inches of aperture to flock the OTA or wash the mirror at sometime.

Edited by Beulah

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I would suggest marking the cell and tube with bits of tape, so that you can put back the screws exactly in the holes they came out from. It might not be needed, but over the years I have always marked things so it stops problems occurring that weren't there to start with.

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I would suggest marking the cell and tube with bits of tape, so that you can put back the screws exactly in the holes they came out from. It might not be needed, but over the years I have always marked things so it stops problems occurring that weren't there to start with.

If memory serves, the cell only goes back in one way as it has a notch in it to fit the seam in the OTA

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As always, useful advice.

Thanks to all who replied.

One last question, does anyone know the mass of the primary cell in kg for a 10" Skywatcher?

Or even approximately?

SImon

Edited by cyborg421

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Sorry Old thread i know.

I need to remove mine so i can add a handle to my 8" dob!

Removed the 6 scews, but it's so tight i just cant move it.

so will i now have to remove the screws that hold and adjust the mirror?

Any ideas will be great thanks :D

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With screws removed, grab hold of the black base of the mirror unit and see whether you can twist it, one way then the other. You'll find that may loosen it before you can carefully pull the mirror unit from the tube.

Easier to do this on the floor with the tube lying on a duvet or rug - get somebody to hold the secondary mirror end of the tube.

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Hi thanks for that.

It's solid lol it just wont budge...:D

Cheers anyway. now where's that hammer :)

P.s shall i remove the other screw to get more grip.. it's just wont come off!

Cheers :p

Edited by up2nogood

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Hi,

All sorted. i had to unscrew the mirror screws then pull it very careful.

The mirrors very dirty.

What's the best way to clean the mirror

thanks :D

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I've been looking at a cleaning thread on here which gave this link. I don't know if it's good advice or not! Someone said you must rinse with distilled water.

Alexxx

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I removed and cleaned my 200mm mirror last night after it had been stored in the loft for a few years, even though the scope was stored with all the caps and covers on the mirror was filthy. I used some cheap unscented kitchen roll and reverse osmosis, de-ionised water.

I used unscented kitchen roll to ensure no chemicals were left behind on the mirror and the RO/DI water (essentially empty water) dries without leaving any streaks or marks. You can buy this from any fish shop that sells live marine fish. (Not the local chippy)

Ps, I am no expert and have never done it before but was very pleased with the results.

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I use water from my dehumidifer to rinse off the mirrors. Doesn't cost anything because it's a by-product of stopping my olde dampe house from being sponge-like! :D

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taking mirrors out of scopes is a learning curve the more you do it the easier it gets, take your time be well prepped and everything will go smoothly and dont hesitate. id go with lifting the tube off verticaly. im about to clean my 16" mirror this week ready for galaxy season :D ave fun!!!  

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