Do I need a collimator?

[Apollo_95]

Ive had my Skywatcher 130p Heritage Dobsonian for abt 4-5 years now so not really a beginners question but I think this is a common issue when some people get a new scope. Everything has worked perfectly since I got the scope until recently. I had it out the other night for the first time since a house move last year. The problem is that the red dot finder is now no longer in sync with the eyepiece. I was initially trying to see Venus but the red dot was way off the mark. I also tried to vuew the moon & had the same issue. I know there are adjustment dials on the red dot finder but when I try to get it aligned the red dot is still too high for the object I have in the centre of the eyepiece. This is even when its on its lowest setting ie I cant make the red dot go any lower. The scope is well protected in its original box & I am not aware of it receiving any bumps. I suspect though that the mirrors are now slightly out of alignment, hence why I am asking about getting a collimator. Ive read reviews on collimators & watched the process on youtube for laser collimators but Collimators seem to be very hit & miss. Some seem to work for some but not for others. from what Ive read some collimators need to be collimated first. Ive called the camera shop I bought the scope from to see if they would collimate it for me but they dont & Im struggling to find somewhere that would do it for me as I would prefer a professional take a look at it before I start tinkering. Has anyone else had this issue? Any suggestions or assistance would be very much appreciated. Thanks Andrew

[almcl]

Newtonians do tend to need collimation from time to time, and no 'one-size-fits-all' solution seems to find universal acclaim. 

So perhaps the first step would be to find out if yours needs adjusting? 

Using your highest powered eyepiece on a bright star in the exact centre of the field of view, defocus slightly and see how central the black 'doughnut' is in the bright ring.  If they are concentric, like this:

the mirrors don't need adjusting.

If the star looks like this:

then collimation might be worthwhile.

But you may have two choices - leap in and start learning how to use a collimation tool OR try and adjust the red dot finder's position so that it lines up with the eyepiece.  A small piece of cardboard or similar placed under one end of the red dot's finder shoe while the mounting screws are loose may be all that's needed to restore alignment and, if the views through the eyepiece are satisfactory, retighten the mounting screws and that may be all that's needed?

 

[Jiggy 67]

Almost certainly after that amount of time, in fact, definitely certainly you will need to collimate. It sounds scarier than it is . Depending on your budget some lasers don’t need collimating such as the HoTec laser which is reall good but expensive. You need a collimating cap to get the secondary aligned with the focus tube and a Cheshire or a laser for everything else. Google Astro Baby’s guide for the best guide

i use the HoTec. Once you’ve collimator the secondary with the focus tube with a collimating cap (which you will only need to do once) using the HoTec means you can finish the collimating of the primary and secondary in minutes

Edited February 10, 2020 by Jiggy 67

[Buzzard75]

Just to be clear, collimation of the telescope has absolutely no effect on the alignment of the red dot finder to the telescope. If you're having trouble aligning the finder to the scope then it's possible your finder has been damaged in some way.

As for collimators, it's a good idea to have a collimation cap at a minimum. It will allow you to make whatever adjustments you need. A Cheshire would also work, but they are more expensive than a cap. Lasers are nice and can be useful if you have a center spotted mirror, but if you don't, you'll still need a cap or a Cheshire to align the secondary mirror correctly. I tend to use both a cap and a laser. I will check the rotation of the secondary with the cap (which is never usually a problem), collimate the secondary and the primary using the laser, and then do a final confirmation check with the cap again. I have a truss type dob and have to check my collimation every time I take it out. I usually only have to make minor adjustments. If you have a solid tube dob, you shouldn't have to adjust it anywhere near as often.

[CraigT82]

22 minutes ago, Buzzard75 said:

Just to be clear, collimation of the telescope has absolutely no effect on the alignment of the red dot finder to the telescope.

Actually... it does. If you tweak the primary collimation screws whilst looking through the EP the image will move around. If you tweak them sufficiently the image will move enough so that it is no longer in agreement with the image in the finder 

[Louis D]

I would start with the RDF's mount and stalk.  See if they are bent or loose.  The foot may have loosened on the tube.  It may be possible to loosen things up and shim the mounts until it can be aligned with the internal adjustments.

[Apollo_95]

Many thanks for all the advice. Upon closer inspection the RDF had in fact been bumped off its runner. Its now been re-positioned & screws tightened. Problem solved!!

[Louis D]

3 hours ago, Apollo_95 said:

Many thanks for all the advice. Upon closer inspection the RDF had in fact been bumped off its runner. Its now been re-positioned & screws tightened. Problem solved!!

Good to hear that.  Sometimes it's the most basic things that need checked.  I once put my 12mm Nagler T4 in the focuser and freaked out that I couldn't achieve focus on anything.  I eventually pulled the eyepiece out of the focuser and noticed I hadn't removed the 1.25" barrel's translucent cap.  I was using the 2" skirt with an extension that makes it impossible to see or even easily remove that cap. ☺️

[Alan64]

On 10/02/2020 at 10:38, Apollo_95 said:

Ive had my Skywatcher 130p Heritage Dobsonian for abt 4-5 years now so not really a beginners question but I think this is a common issue when some people get a new scope. Everything has worked perfectly since I got the scope until recently. I had it out the other night for the first time since a house move last year. The problem is that the red dot finder is now no longer in sync with the eyepiece. I was initially trying to see Venus but the red dot was way off the mark. I also tried to vuew the moon & had the same issue. I know there are adjustment dials on the red dot finder but when I try to get it aligned the red dot is still too high for the object I have in the centre of the eyepiece. This is even when its on its lowest setting ie I cant make the red dot go any lower. The scope is well protected in its original box & I am not aware of it receiving any bumps. I suspect though that the mirrors are now slightly out of alignment, hence why I am asking about getting a collimator. Ive read reviews on collimators & watched the process on youtube for laser collimators but Collimators seem to be very hit & miss. Some seem to work for some but not for others. from what Ive read some collimators need to be collimated first. Ive called the camera shop I bought the scope from to see if they would collimate it for me but they dont & Im struggling to find somewhere that would do it for me as I would prefer a professional take a look at it before I start tinkering. Has anyone else had this issue? Any suggestions or assistance would be very much appreciated. Thanks Andrew

Laser-collimators are generally used for larger and longer Newtonians on Dobson mounts.  The cheaper lasers usualy have to be collimated first, and can be more trouble than they're worth.  I have one myself, and it is difficult to collimate, and before I can use it for a telescope.  In any event, I don't use one to collimate my smaller, shorter Newtonians, like your own.  Instead, I use a Cheshire and a collimation-cap, both, and during a single procedure.  

Cheshire... https://www.firstlightoptics.com/other-collimation-tools/premium-cheshire-collimating-eyepiece.html

Beware of cheaper Cheshires, as the cross-hairs may not be aligned correctly, and cannot be corrected.

Collimation-cap... https://www.firstlightoptics.com/other-collimation-tools/rigel-aline-collimation-cap.html

In the case of Newtonians, the peep-hole and cross-hairs of a Cheshire act as those of a sight-tube, and aid in centring the secondary-mirror directly under the draw-tube of the focusser...

Then, adjusting both mirrors, you direct their centres towards each other, back and forth until they are aligned...

When the cross-hairs of the Cheshire on the outside are aligned with the mirror-image of same in the centre, and both along with the primary-mirror's centre-spot, you're golden.  I then use a collimation-cap to verify, and tweak further if necessary...

When tightening down the primary's lock-bolts after adjustment, the cap allows you to keep an eye on the alignment to ensure that nothing shifts out of position when tightening.

Note how the lighter circle is not centred within the larger black circle.  It's askew, off-centre.  That is normal for a short Newtonian(f/4, f/5).  It is known as the secondary off-setting, and it occurs during a normal collimation procedure.  There's nothing you have to do to accomplish the off-setting.

Newtonian collimation tutorials...

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

https://garyseronik.com/a-beginners-guide-to-collimation/