Collinmation breading paranoia

[Stu1smartcookie]

Hi 

I am a bit confused about what i am seeing when i check collimation of my skywatcher 200p Dob.  I use a laser collimator which seems to be "on point" as i have had really nice views of Jupiter and saturn Recently and when i do an out of focus star test the inner circle seems pretty much centered . However when i look down the focuser without an EP  the circle sits to one side and the actual outer "circle" is elipitical rather than round ,  . This is bothering me , more so the fact that i am able to focus really well on stars and planets but looking down that focuser really doesnt inspire any confidence that the mirrors are  aligned properly . Its at this point that i have to bring into the conversation the "Stu" factor ... yes , i've been dabbling with an allen key and a screwdriver , so i've probably done something wrong . I've never noticed this on any other newt i've owned .. If everything was out of focus then i could understand it . 

Any ideas most welcome .

[Pixies]

can you take a pic?

[Pixies]

Also - is your star test like this:

or more like this:

or this:

[Stu1smartcookie]

Hi Pixies , im at work at the mo , so i cant show you a pic until later ... regarding the images above the first one matches what i see . But its almost as if i see a non circular image behind it , like an oval . 

[Ricochet]

If you are seeing something like the first image then you are defocusing way too far and not doing a star test. The amount of defocus you need is probably measured in micrometres and will produce a set of rings hopefully like the second image. 

Having said that, if you've used a laser then this is likely the cause of your problem. If you use a laser for the primary it should be with the barlowed laser method or you will just be doubling any error from the secondary alignment. 

[Pixies]

My guess is that the secondary is rotated, so you are seeing the primary aligned but it is being clipped by the fact that the secondary is not showing it's full face towards the focuser.

A laser won't account for that, and a still show it being collimated correctly. A bit like this:

 

Do you have a Cheshire or a collimation cap?

[Starwatcher2001]

At f/5.1, the scope is pretty fast, so the the secondary looking oval is quite normal and nothing to worry about. If your views are good and the star test is giving concentric circles, everything is good.

This is a good resource that explains more:  http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

Edited September 15, 2020 by Starwatcher2001

[Pixies]

15 minutes ago, Starwatcher2001 said:

At f/5.1, the scope is pretty fast, so the the secondary looking oval is quite normal and nothing to worry about. If your views are good and the star test is giving concentric circles, everything is good.

This is a good resource that explains more:  http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

The secondary should still present a circular shape to the focuser, though. The oval will appear in the primary reflection as a silhouette .

In the Astro-baby guide:

[John]

The 200p dob is F/5.9. A sort of medium focal ratio by today's standards.

 

[Stu1smartcookie]

Well i was shot to pieces regarding the star test .. lol ... but , thanks Ricochet , if you dont know , you dont know - as the saying goes . 

Pixies , i dont have a cheshire ( i was lulled into the techie laser cos it looks cool camp )but i will invest in one.  Come to think of it i did undo the secondary and it may well have been rotated , so thats a distinct possibility . 

Honestly , many thanks for your imput here , and thanks for the video ,Pixies . Starwatcher , the views are good but i reckon they could be better ... 

I am not sure how i will get on later but i will have another try .

[Stu1smartcookie]

11 minutes ago, Pixies said:

The secondary should still present a circular shape to the focuser, though. The oval will appear in the primary reflection as a silhouette .

In the Astro-baby guide:

I really think this is where my problem lies 

[jetstream]

Just look through the empty focuser, straight on and make the secondary look round. The gap between the focuser and round secondary should be the same. Contrary to some opinions f5.9 is not fast and is an accommodating f ratio. This will get you in the game.

Get a cheap cheshire and all will be simple IMHO.

[Pixies]

You can also make a simple collimation cap yourself. Lots of videos on youtube.

[Pixies]

19 minutes ago, jetstream said:

Just look through the empty focuser, straight on and make the secondary look round. The gap between the focuser and round secondary should be the same. Contrary to some opinions f5.9 is not fast and is an accommodating f ratio. This will get you in the game.

Get a cheap cheshire and all will be simple IMHO.

Yep - concentric is what you are looking for. A Cheshire makes this easier to get precise. It or a collimation cap will make sure your eye is centred in the focuser, too.

Edited September 15, 2020 by Pixies

[Stu1smartcookie]

I've had scopes for a few years (newts and all ) but ive never fully understood collimation , a sort of , it looks ok so lets get on with it attitude .. I am really glad that i started this thread . 

Ricochet , i've never even heard of the barlowed laser method ! 

[Starwatcher2001]

I've built newts, and I'm still not sure I fully understand collimation 🤣

I was going on this section, but I guess it depends what Stu is actually seeing through the focuser.

[Starwatcher2001]

1 hour ago, John said:

The 200p dob is F/5.9.

 

I stand corrected, thanks John. I took the time to look it up on FLO, but god knows where I must have been looking.

[Pixies]

53 minutes ago, Starwatcher2001 said:

I've built newts, and I'm still not sure I fully understand collimation 🤣

I was going on this section, but I guess it depends what Stu is actually seeing through the focuser.

Yeah - it's the image of the secondary that reveals the offset (red).

There are some pictures in the A-B guide that confuse matters too. In this one, it shows the vanes (blue arrows) as offset, too. But this is rarely the case with modern scopes

[Pixies]

48 minutes ago, Starwatcher2001 said:

I stand corrected, thanks John. I took the time to look it up on FLO, but god knows where I must have been looking.

The Explorer 200p is f5. 

[jetstream]

3 hours ago, Starwatcher2001 said:

so the the secondary looking oval is quite normal and nothing to worry about.

Yes truly understanding collimation is a pursuit IMHO. However we don't need to have a high level of understanding to collimate a newt. Khadder is one of the most knowledgeable people with regard to collimation.

His excellent image shows the effect of speed on how the collimated image looks- but it doesn't matter-just use a sight tube (cheshire/sight tube combo) and it automatically sets itself. The OP's f5.9 will look more like the left picture than the right.

Btw- there are 2 offsets...- sometimes. My 15" f4.8 is not offset (on stalk) but my 24" f4.1 is- doesn't matter as long as the sec is big enough, to me anyway.

Edited September 15, 2020 by jetstream

[markse68]

4 hours ago, Pixies said:

Also - is your star test like this:

or more like this:

or this:

Hi Pixies, out of interest what does the third image indicate?  I have had near perfect intra/extra focus symmetry before but very rarely- usually the distribution of energy in the rings is quite asymmetrical but i reckon that maybe down to mirror not quite being at ambient perfectly- it takes quite a long time i think to perfectly stabilise. Also , how much does seeing quality affect star testing? I guess if it’s poor you’re not ever going to see perfect crisp rings like the middle image?

[Nigella Bryant]

My f4 newt has an offset and looking down the focuser tube it's not centered, worried me for ages before I found it was right. As posted before. 

Edited September 15, 2020 by Nigella Bryant

[Pixies]

5 minutes ago, markse68 said:

Hi Pixies, out of interest what does the third image indicate?  I have had near perfect intra/extra focus symmetry before but very rarely- usually the distribution of energy in the rings is quite asymmetrical but i reckon that maybe down to mirror not quite being at ambient perfectly- it takes quite a long time i think to perfectly stabilise. Also , how much does seeing quality affect star testing? I guess if it’s poor you’re not ever going to see perfect crisp rings like the middle image?

I put the third in as the second is pretty hard to get unless you have very good seeing.

Really - I was wanting to check that the OP's star-test wasn't just the out-of-focus image of the secondary. It appears to be, but the fact that this is oval is a good pointer that the secondary might be rotated and clipping the image of illuminated primary.

Edited September 15, 2020 by Pixies

[markse68]

6 minutes ago, Pixies said:

I put the third in as the second is pretty hard to get unless you have very good seeing.

Really - I was wanting to check that the OP's star-test wasn't just the out-of-focus image of the secondary. It appears to be, but the fact that this is oval is a good pointer that the secondary might be rotated and clipping the image of illuminated primary.

Ah ok- you had me a bit worried there 😅 

Mark