New CR2 focuser and collimation SW250

[davhei]

Yesterday I changed the stock rack and pinion focuser of my SW 250 solid tube dob for a Moonlite CR2. Went fairly smoothly but when I took the scope out in the evening (clear skies the very same night, imagine that!) to check focus distances and collimation I noticed something I had not seen before.

When star testing collimation on arcturus the star image was appearing cut off, which became more apparent when I moved it further out of focus. See picture where you can see the shadow of the secondary.

If I moved my eye slightly to the side in the eyepiece it became normal again, showing the full circle.

The usual star test looked decent though with concentric circles collapsing to a point. When collimating earlier I did only minor adjustments to the mirror alignments prior to the focuser change, just like any night. Used a cheshire combo tool and barlowed laser.

Snapped another picture through a collimation cap with colored paper behind the secondary.

My guess is the main issue is that the secondary mirror is not centered in the focuser and needs to be moved in the direction of the opening, i.e. towards the spider, by manipulating the center screw.

Would this misalignment give the error I saw? I would like to think that the focuser is ok, understand moonlite has good collimation quality control and fiddling with the focuser collimation screws would be last on my list.

I have not done any adjustments to move the secondary before so am a bit apprehensive to start messing with it if the issue is elsewhere.

Appreciate your thoughts.

Edited March 27, 2020 by davhei

[johninderby]

Have you checked the focuser for being square to the tube.

For checking the secondary position there is nothing better than a Concenter. Really usefull colimation tool and makes it easy to perfectly centre the secondary. It’s the first tool I use when setting up a newt.

https://www.teleskop-express.de/shop/product_info.php/info/p5506_TS-Optics-Concenter-2--Collimation-Eyepiece-for-Newtonian-Telescopes.html

Edited March 27, 2020 by johninderby

[davhei]

3 hours ago, johninderby said:

Have you checked the focuser for being square to the tube.

For checking the secondary position there is nothing better than a Concenter. Really usefull colimation tool and makes it easy to perfectly centre the secondary. It’s the first tool I use when setting up a newt.

https://www.teleskop-express.de/shop/product_info.php/info/p5506_TS-Optics-Concenter-2--Collimation-Eyepiece-for-Newtonian-Telescopes.html

I haven’t squared the focuser. Most people seem to have a smooth transition between focusers when using the moonlite installation kits with SW dobs, but perhaps that is it.

I was reading astrobabys collimation guide but couldn’t find that particular part covered. Do you have a preference for how to do it? Read that some take out the whole secondary mirror, measure on the opposite tube wall from the focuser and use a laser to adjust the focuser. Was hoping for something easier than removing the whole mirror assembly.

Edited March 27, 2020 by davhei

[johninderby]

The focuser doesn’t have to be perfectly square and would have to be badly out of square to make a real differance. 

I’d recommend getting a Concenter and then adjusting the secondaty. No other tool such as a laser or cheshire eyepiece isn’t  very accurate at setting up the secondary mirror. Found that after adjusting the secondary with the concenter the scope was much easier to collimate as you are not compensatingnfor an out of postion secondary. 

Edited March 27, 2020 by johninderby

[davhei]

Tweaked the secondary a bit while installing bob’s knobs and two milk jug washers. Checked the secondary position with the cheshire combo tool and collimation cap and got the below result. Seems decent, can’t imagine it would cause the problem I described, or?

I’ll test again this evening, unbelieavably it looks like yet another clear night. Let’s see if it persists.

 

Edited March 27, 2020 by davhei

[davhei]

Tried the scope against a star last night and unfortunately the same issue. In low magnifications, the out of focus image of the star has an eye shape instead of a round shape.

Moving my eye position at the EP tranformed the image into a normal round one, but when centering my eye for normal observing it ceases to be round. This causes a loss of light, and a clear darkening of the image. Vignetting?

Rotating the EP in the focuser did nothing to change it.

Does anyone recognise this? Is it a matter of squaring the focuser?

Some people (for instance a reply I saw from skywatcher that someone posted earlier) seem to insist that as long as the focuser axis is aligned with the center of the secondary it doesn’t matter that much if the focuser is not centered in the OTA. Don’t know what to think, hmm.

Edited March 28, 2020 by davhei

[Barry-W-Fenner]

Afternoon all,

 

I have just been searching threads on collimation and came across this one. Did you manage to resolve the issue? I ask as I have recently fitted a dual speed focuser to my 200p I to have a similar thing happening to me when I de-focus a star. It inst anywhere near as bad as yours to be fair, I am just losing a small slice on one edge, However it is the same shape as your 2nd pic.

Recently my viewing has been fine unless I go upto and over x150 Mag, Then things are getting very blurry. I thought it was down to bad seeing however I am not totally convinced it is. If i use high power on Venus or the Moon x240 they both produce excellent views. it Seems to be stars that are blurry and have 1 of the 4 defraction spikes is much more noticable almost pulling the start tear drop shaped.

I have collimated my mirrors with a calibrated laser and they seem ok, However if this is due to an issue with the focuser install then its all going to be out of line I assume.

John- Where did you purchase your concenter from? I would like one of these as they seem an excellent solution for collimation of a newt. They are a bit on the £££ side though 🙂

Davhei - I hope you managed to resolve your problem. (No pun intended) 🙂

Any advice appreciated.

 

Baz

 

 

 

[davhei]

Hey Baz,

I haven’t unfortunately. What I’ll do probably tomorrow is take out the secondary and make sure the focuser axis is squared, remount the secondary and fine tune the alignment as good as I can get it. Essentially a complete adjustment. Never done that before but kind of look forward to fiddling with the scope a bit.

Next clear night I’ll check and see if it is sorted. Fingers crossed.

If not, well.. let’s cross that bridge then..

If you find a solution I’d be thankful if you please post a follow up in this thread. I have a similar thread on CN in the reflectors forum, got some good input there that might help you out.

 

/Dave

[banjaxed]

I am no expert but if you are both experiencing similar problems after replacing your focusers then IMHO this is where the problem lies. Could you loosen the focuser screws slightly so it can be moved a little while you are observing to see if it makes any difference?

[davhei]

1 hour ago, banjaxed said:

I am no expert but if you are both experiencing similar problems after replacing your focusers then IMHO this is where the problem lies. Could you loosen the focuser screws slightly so it can be moved a little while you are observing to see if it makes any difference?

Certainly makes sense. If the issue persists after I give aligning the focuser a go then I will do that to check what happens.

[davhei]

On 01/04/2020 at 17:30, Barry-W-Fenner said:

Afternoon all,

 

I have just been searching threads on collimation and came across this one. Did you manage to resolve the issue? I ask as I have recently fitted a dual speed focuser to my 200p I to have a similar thing happening to me when I de-focus a star. It inst anywhere near as bad as yours to be fair, I am just losing a small slice on one edge, However it is the same shape as your 2nd pic.

Recently my viewing has been fine unless I go upto and over x150 Mag, Then things are getting very blurry. I thought it was down to bad seeing however I am not totally convinced it is. If i use high power on Venus or the Moon x240 they both produce excellent views. it Seems to be stars that are blurry and have 1 of the 4 defraction spikes is much more noticable almost pulling the start tear drop shaped.

I have collimated my mirrors with a calibrated laser and they seem ok, However if this is due to an issue with the focuser install then its all going to be out of line I assume.

John- Where did you purchase your concenter from? I would like one of these as they seem an excellent solution for collimation of a newt. They are a bit on the £££ side though 🙂

Davhei - I hope you managed to resolve your problem. (No pun intended) 🙂

Any advice appreciated.

 

Baz

 

 

 

Hey,

 

Thought I’d follow up on this. No real quick fix as such, but I spent some time positioning the secondary and getting collimation as good as I could. Perhaps not dramatically different to the way it was in my first post but certainly better. Learnt a lot in the process. Also noticed that my eye placement at the EP made a bit of difference and it was easier to see that when properly dark adapted, which I wasn’t in my first post.

All in all, I thought the star images from my dark site yesterday was as good as any I had before the focuser change, if not better. If it was secondary placement, my eye placement at the ep or just me being a bit less neurotic. Not sure. I’m quite happy though and feel comfortable with the scope now.