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SW MN190 - How to remove the corrector lens


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I'm looking to replace the focuser on my Skywatcher Mak-Newt MN190. This will require removing the front corrector lens to access the focuser attachment screws.

I've seen a few posts from people who have done this but I'd like to be a bit clearer about just the basic steps involved in detaching the corrector lens - which screws, in what order etc.

Would appreciate further advice from anyone who's done this.

Thanks

Adrian

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  • 2 weeks later...

Hi Adrian - I'm sorry I missed this too...

I thought I was going to have to do this myself a while back (to check that the fitted moonlite focuser was flush with the tube)

I got a LOT of input (both in thread and PM's) on how to do it and even put together a checklist on what I was going to do... However, the Hotech laser collimator I bought (primarily to enable the reseating task) highlighted my problem as poor collimation at stage one so I didn't need to follow the rest :hello2:

Here's a link to that thread (here), but I can also send you my checklist if you like?

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You don't need to remove the front corrector to replace the focuser, the four screws around the edge of the adaptor plate can be removed, the nuts on the back are soldered to the tube.

Be careful not to over tighten when refitting as you will break the solder making it more difficult.

A problem on some MN190's is that the focuser is tilted in respect to the secondary, and although they look perfectly collimated with a laser, are in fact not, stars will be a selection of oval shapes. This is easy to fix with some packing under the edge of the focuser plate, and using a hotech or similar the laser beam should be centralised on the secondary, but make sure you have collimated the mirrors as best as you can prior to this. The little marking on the secondary is helpfl in aligning the focuser with the laser.

HTH

Tim

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You don't need to remove the front corrector to replace the focuser, the four screws around the edge of the adaptor plate can be removed, the nuts on the back are soldered to the tube.

Be careful not to over tighten when refitting as you will break the solder making it more difficult.

A problem on some MN190's is that the focuser is tilted in respect to the secondary, and although they look perfectly collimated with a laser, are in fact not, stars will be a selection of oval shapes. This is easy to fix with some packing under the edge of the focuser plate, and using a hotech or similar the laser beam should be centralised on the secondary, but make sure you have collimated the mirrors as best as you can prior to this. The little marking on the secondary is helpfl in aligning the focuser with the laser.

HTH

Tim

Hi Tim

I have suspected a focuser misalignment with my MN190 after unsatisfactor collimation.

How do you see the secondary mark has centralised with focuser using the laser.

Many thanks.

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Thanks guys. I got helpful advice from Ian (Martin20) who has done this job. I'm just waiting for the Moonlite now.

Thanks also for the advice on alignment issues. The Hotech collimator looks good .... but expensive. How do you rate it?

AndyUK .... yes please, Id like to see your checklist - have PMd you.

Tim (TJ), you mentioned that I don't have to remove the corrector. I only wish it was so! Removing the SW focuser is OK but the replacement Moonlite screws attach from the inside and go into threaded holes in the curved base from below!

Thanks for the replies, folks.

Adrian

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Hi Adrian,

The Hotech collimator looks good .... but expensive. How do you rate it?
Expensive, yes, but possibly one of the best astro items I've personally ever bought (equal with the MN190 anyway!). The Hotech and bobs knobs has taken ALL the grief out of collimation and it takes me all of 2 mins to collimate the scope - It's now part of my setup procedure...

(I'll send on the checklist via pm)

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<<... and Bob's knobs are a perfect partner>>

Yep, got those too Andy and will fit at the same time I'm doing everything else!

Btw, when you were checking out focuser alignment/ collimation, did you find the secondary mirror square-on? I know it's possible sometimes in a Newt that the secondary needs to be rotated slightly (around the main optical axis) in order to present an exactly circular profile at the eyepiece. I haven't actually checked this carefully yet on my scope but I was wondering how on earth you would make such an adjustment once the corrector is back on! Not like you can just reach in and give it a tweak like on a native Newt.

Adrian

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Btw, when you were checking out focuser alignment/ collimation, did you find the secondary mirror square-on?
... Ah... Don't forget mine turned out to be purely a primary/secondary collimation issue, not a focuser alignment issue - I didn't need to get that far (thankfully!). As I understand it, that's part of the reason why it's important to put the corrector plate back on in exactly the same place as when you to took it off... although to be honest, I hadn't thought about mitigating that particular risk other than to ensure that all the marks are accurately aligned... (sorry - I can't help you on that one...)
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<<... and Bob's knobs are a perfect partner>>

Yep, got those too Andy and will fit at the same time I'm doing everything else!

Btw, when you were checking out focuser alignment/ collimation, did you find the secondary mirror square-on? I know it's possible sometimes in a Newt that the secondary needs to be rotated slightly (around the main optical axis) in order to present an exactly circular profile at the eyepiece. I haven't actually checked this carefully yet on my scope but I was wondering how on earth you would make such an adjustment once the corrector is back on! Not like you can just reach in and give it a tweak like on a native Newt.

Adrian

Behind the collimation cover on the front of the collector plate is a large knurled nut that needs to be slackened slightly. You then put your first two fingers into where the collimating screws are a thumb on the outside and very gentle rotate.

Unless you really have to I'd leave well alone.:D I'm having alignment issues with mine at present and with the kind and very patient help from AndyUK :icon_salut:slowly getting it sorted.

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Thanks Tony. Sounds like something to steer well clear of if possible! What sort of alignment problems have you had?

The reason I asked about rotation was that I'm getting what looks like a centering problem: I have vignetting in the two bottom corners of the frame but none at the top. This is after a first attempt at collimation with a cheshire.

Several possible causes of course, including focuser not being square-on, or collimation still being off. But if I have my geometry right, it could also be caused by slight rotation of the secondary that would deflect the light cone in a direction at 90 degrees to the OTA axis - i.e. the up-down direction in the camera frame.

Anyhow, first things first: fit the new focuser when it arrives and check mechanical alignment, then do a new collimation with the Hotech, and try again .... before looking for anything more complicated.

Adrian

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Hi Adrian. I'm getting comma shaped stars in one corner. I with the help of AndyUK have been trying to eliminate possible causes. We are getting closer to solving it, but it's a question of trial and error. Very frustrating.

Edited by Freff
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This thread gives me flashbacks of collimation last summer. I now just use the Barlowed laser method, and so far it's proved adequate for my level of imaging. I know I'll need to get more involved in this process if/when my images are of a decent enough quality to discern any problem.

I have noticed similar problems to Freff when using my 190 visually, and I expected my full frame sensor to notice it too, but so far my field seems fairly flat through the camera. Most odd.

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