A Black Spot !!

[Stu1smartcookie]

Hi everyone , I've just bought a Skywatcher 130 pds .. i  have an az-gti mount which ive upgraded the alluminium tripod for a steel one for stability . I've also added a skywatcher wedge and upgraded the firmware of the mount to take advantage of using the set up in EQ mode . So far So good ...but , when i set up this scope for the first time i used the accompanying 28mm (2") eye piece which achieved focus but i have an image of the small black ring (which sits in the middle of the primary mirror and is used for collimation) I don't get this problem when i use my  1 1/4" eyepieces . Its really annoying as it interferes with the view. Maybe someone can advise nme what i have got wrong here . I had to collimate the telescope when i first set it up ...( i collimated my laser collimater  first ) . The scope was quite a way off collimation but im sure its good now as ive had good views of jupiter ( albeit rather small of course ) . 

Edited July 20, 2019 by Stu1smartcookie
spelling

[paul mc c]

It could be whats called " kidney beaning"where you get this effect on wide field eyepieces if your eye is to close to the eyepiece.,or you may need an extension tube to bring the eyepiece to focus..

[Astro-Geek]

I had a similar worry with my 200PDS, a "blurry" dark blob in the middle of the view with certain eyepieces.

It only manifested itself with bright, daylight viewing conditions when testing the scope terrestrially,  (is that when you get the problem ?)

I Googled it, and was relieved to find it's a perfectly normal phenomenon with Newtonian telescopes, as it is the obstruction of the secondary mirror in the field of view, (not the dot on the primary).

The reason it occurs is because with low power (high mm) eyepieces, the light gathering of the scope in daylight allows your eye's iris to close down to a minimum, so that it is smaller than the central obstruction caused by the secondary mirror.

When you view dimmer objects ( like anything at night  !), your iris opens up much more, and it's "exit pupil" is then bigger than the cone of the shadow of that central obstruction.

The blurry blob also disappears when you use higher magnification eyepieces (smaller mm) even in daylight, because they give a darker image, and again, you iris opens up more.....

[Stu1smartcookie]

First of all , many thanks for the answers . And Astro you have completely se my mind at rest with your explanation . 

Now , another problem ( i do seem to create problems for myself ) . i bought a laser collimater and i thought is was collimated ..but it turns out that its far from collimated and , of course , i "uncollimated " my scope . To the point where ive  adjusted just about every conceivable screw available !! I think ive got it back to somewhere like normal , but only night will tell. Ive looked at getting a cheshire collimator now as i dont think i will EVER trust one of these laser collimaters again . Is using a cheshire the best way forward here?

[Ricochet]

51 minutes ago, Stu1smartcookie said:

Is using a cheshire the best way forward here?

For collimating the secondary mirror, yes. In fact the sight tube part of modern "cheshires" is vital. 

For the primary mirror the barlowed laser method is a great way to collimate if you cannot reach the collimation bolts whilst looking through a Cheshire and does not rely on the laser being properly collimated itself. 

[Stu1smartcookie]

Thank you Ricochet... so , i suppose a cheshire is my next purchase . Surely the laser if not collimated properly will give a false reading on the primary , wouldn't it ? sorry if that question is dumb , but it just seemed a bit logical to me . When i used the laser to collimate both mirrors i got the red spot in the doughnut of the secondry and the primary was almost as easy . Of course now i know that the laser was lying to me which makes me so wary of those collimaters

[Ricochet]

If you use the barlowed laser method you effectively turn the laser into a red torch that illuminates the central area of the primary mirror. You can then collimate using the shadow of the centre doughnut rather than the laser itself so if it is slightly out it will not matter. 

[Astro-Geek]

I use a laser and a long tube Cheshire (one after the other), because I find they both have advantages.  The laser gets me close and is very easy to use with a long tube Newtonian because I can adjust the secondary whilst watching the dot in the collimator target, but with the Cheshire I need to keep moving back and forth, as my arms aren't long enough.

Neither the Laser nor the Cheshire can be perfectly centred in the eyepiece holder tube because of the standard way that the gripping works.  The thumbscrews and the brass band both push them off-centre slightly when tightened.  (Maybe a Baader clicklock holder would be better, but I've never had one of those yet 🤓)

My own workaround therefore is to check the spot centering with each collimator rotated in three different radial positions. I can therefore judge and adjust to the exact centre by seeing the tiny offset as the dot moves around it with each 120 degree rotation.

I do this with both the initial step of centering the laser dot in the secondary mirror and also the final step of centering the dot on the lasers target disc, (effectively moving it around the "edge" of the centre hole as the collimator is rotated.

I get nice concentric Airy discs when viewing stars, so it works pretty well for me.