Collimating Skywatcher Heritage 76p mini Dob

[Barry-W-Fenner]

Hi all.

My 6 year old nephew has just picked up the above scope 2nd hand to get him going and is rather frustrated as he cant see anything. He has told me he waited forever for it to arrive and is so disappointed he can't see anything, Bless him 🙂

I located Venus and Vega and unfocused them to the point where you should see them as a wide circle but they only showed half a circle, The collimation is a long way out.  I aligned the secondary mirror all ok, Red dot centered on the primary nicely. However I seem to be having issues with the primary. When looking on the laser target the red dot is right on the outermost ring.  The primary doesn't seem to adjust as such, It has 3 screws holding the primary in place which seem to be all or nothing. If you loosen them then the mirror becomes very loose and moves about in the tube. If you over tighten there is no adjustment and the red dot is back in the same location as when i started. I understand with my 200p you would loosen one then tighten another to make small adjustments Should the 76p work on the same principle.

Any advise from users of this little dob would be appreciated. I currently have a grumpy nephew.

 

Thanks in advance.

 

Baz

[Barry-W-Fenner]

Just to mention. The scope itself is in great condition and looks like it will give some excellent views. I just need to align the mirrors up.

 

Thank you

[Alan64]

The entry-level Newtonian-Dobsons. like your nephew's, and this 100mm f/4 of my own...

...both have a primary-cell that cannot be collimated, as it arrives...

It's simply a plastic tray with the mirror installed with three pieces of double-sided foam-tape...

My own is the same as the "Heritage" 100P.  Your nephew's is not a 76P, rather a 76mm with a spherical primary-mirror.  A 76mm f/4 like that one should have a parabolic primary-mirror, but it doesn't.

Some enlarge the three holes to allow some adjustment.  Some even slot the holes somewhat to allow for even more adjustment.  But that's about it, as it comes.

The secondary-assembly, however, is collimatable, but I'm unsure as to the extent.

It may certainly be tilted, at least.  I would concentrate on adjusting that aspect.  Is your laser-collimator tool collimated itself?  Many do arrive mis-collimated, particularly the entry-level units.  I have one myself, but I prefer to use passive-tools to collimate my Newtonians: a collimation-cap, and a Cheshire with cross-hairs; tried and true.

[NGC 1502]

Perhaps someone in a local astronomy club could help ?   Worth a try ? 

My club in Essex is probably too far, but we often get folk along asking for advice for similar issues......

Hope you sort it, Ed.

[Barry-W-Fenner]

7 hours ago, Alan64 said:

The entry-level Newtonian-Dobsons. like your nephew's, and this 100mm f/4 of my own...

...both have a primary-cell that cannot be collimated, as it arrives...

It's simply a plastic tray with the mirror installed with three pieces of double-sided foam-tape...

My own is the same as the "Heritage" 100P.  Your nephew's is not a 76P, rather a 76mm with a spherical primary-mirror.  A 76mm f/4 like that one should have a parabolic primary-mirror, but it doesn't.

Some enlarge the three holes to allow some adjustment.  Some even slot the holes somewhat to allow for even more adjustment.  But that's about it, as it comes.

The secondary-assembly, however, is collimatable, but I'm unsure as to the extent.

It may certainly be tilted, at least.  I would concentrate on adjusting that aspect.  Is your laser-collimator tool collimated itself?  Many do arrive mis-collimated, particularly the entry-level units.  I have one myself, but I prefer to use passive-tools to collimate my Newtonians: a collimation-cap, and a Cheshire with cross-hairs; tried and true.

Hi Alan,

Thank you for confirming the what can and cant be collimated.  This is a big help. I will now look to enlarge the holes slightly so i can adjust the primary. by hand.

 

Regards

 

 

[Barry-W-Fenner]

Oh, And just for the record my laser is providing a consistently straight dot against the wall. I did check it for alignment and it was mis-collimated.

 

Attached is my make shift laser collimation device, Any excuse to play with the kids lego 🙂

 

Regards

[Alan64]

The slotting occurs on the optical-tube...

Don't go up to and beyond the edge of the tube with the slot...

I eventually took that further with my own...

[DaveL59]

the NatGeographic 76/350 is the same setup with the primary fixed and no collimation screws, as Alan suggests one way is to extend the fixing screw holes and tweak that way. With time and a lot of patience you might be able to shim the primary till the alignment is better and then secure it in position, so avoiding extending the screw holes. Have you tried refitting the mirror after rotating the carrier and see if that corrects things, woth trying it in each orientation before starting any real surgery on the OTA etc.

You can also find that the secondary isn't aligned well with the focuser as in too far in/out of the OTA. Once you have it as good as you can get it, the tiny scope performs pretty well really.

[Alan64]

You do want to exhaust the possibilities and solutions before any modifications.

[Barry-W-Fenner]

Morning all,

Thanks for the advice gents, It is very useful! I picked the scope up this morning so will have a play tonight.  As advised I will remove the mirror and check the orientation 1st. Hopefully this and maybe a bit of shimming will get the mirror inline. In the event that I need to increase the hole sizes I will use the above guide. (Cheers Alan64)

So far viewing the moon has improved greatly by adjusting the secondary mirror. However stars only produce about 2/3 of a full circle when I unfocuse them.  I look forward to getting the scope collimated and showing my nephew some great objects.

Again, Thanks all for your  input, I will be telling my Nephew how a great bunch of observers from all over have helped me sort out the problem 🙂I will let you know the results asap! 

 

Baz

 

 

[banjaxed]

Good luck Baz, we all have to learn that this hobby is not as straightforward as it appears and can be very frustrating to say the least.

[DaveL59]

as a thought, the focuser isn't down in the light path is it? as that'll add an obstruction that might be contributing to the 2/3 defocused circle.

[Barry-W-Fenner]

3 hours ago, banjaxed said:

Good luck Baz, we all have to learn that this hobby is not as straightforward as it appears and can be very frustrating to say the least.

Very true! I am often left frustrated with this hobby, However when you do have an observing night where everything goes right it certainly outweighs the frustration 🙂

[Barry-W-Fenner]

1 hour ago, DaveL59 said:

as a thought, the focuser isn't down in the light path is it? as that'll add an obstruction that might be contributing to the 2/3 defocused circle.

Hi DaveL59

Thanks for the suggestion, I will certainly check this tonight also, I hadn't  thought of that so that could be the potential cause.

 

Baz

[Louis D]

I played with one of these at an October star party, and the image is mostly unusable outside the inner 30% of a Plossl's field of view due to severe spherical aberration, so keep your subject centered as best you can.  I was able to make out Jupiter's disk and four main moons, but I don't recall if I could discern any banding on Jupiter.

[Barry-W-Fenner]

Just a quick update, I managed to gently bore the 3 holes out slightly last night. This has given enough adjustment to line the primary up properly.  Before putting the mirror back on I went through my box of astro bits and had some spare flocking from when I lined my 200p. I spent some time lining the tube. Admittedly its not the best job as I was using random off cuts I have left over but I have had done approximately 90% - The tube is much darker now and I am hoping this will help improve the  contrast. It all helps i guess! 

I will put the mirror back tonight and re-align

Thanks all.

Baz

Edited January 15, 2020 by Barry-W-Fenner

[Barry-W-Fenner]

Oh I forgot to mention. I did try to change the orientation of the mirror before surgery. However due to the seam in the tube the mirror only installed in one position. 

 

Regards

[happy-kat]

How did the focuser tube look, was it extending into the telescope tube?

[Barry-W-Fenner]

3 hours ago, happy-kat said:

How did the focuser tube look, was it extending into the telescope tube?

To the best of my limited knowledge it looked ok. It wasn't extending into the path of the secondary mirror, Assuming that is what I was looking for. To be honest only one of the mirror holes was what seemed to be keeping the mirror from lining up in the center of my collimator. I made them all slightly bigger for a bit of extra adjustment but hopefully tonight I should be able to get the scope put back together and  collimated properly!

I will keep you all posted.

Thanks everyone for your contributions and help,  Alan64's plonker proof guide with pics has been a great help! 🙂 

 

Baz

[DaveL59]

ah, you're looking for it extending down into the tube hence affecting the light path to the primary, rather than obstructing the secondary  

[Alan64]

4 hours ago, Barry-W-Fenner said:

Oh I forgot to mention. I did try to change the orientation of the mirror before surgery. However due to the seam in the tube the mirror only installed in one position. 

The odd thing about it all is that we must assume that the back of the tube was cut precisely and perpendicularly to the length of same at the factory...

...and square to one another.  Then, the plastic cell is practically perfectly flat, we hope; as is the mirror, and again we hope.  Then, we assume that the three pieces of double-sided foam-tape used to install the mirror into the cell are of the same thickness.

Lastly, as long as the cell seats fully onto the end of the tube, then there should be no need to adjust it.  Does the cell fully seat onto the rim of the tube, in fact?

There is one other thing.  Elsewhere it had been opined that there's no need to center-spot a spherical primary-mirror.  If the mirror is off-center, no matter, according to those individuals.  The reason being is that the surface of the mirror conforms to a perfectly round sphere, and no matter where you place it round the sphere...

Spherical-mirrors cannot focus all of the rays of incoming light from an object in the sky to a single focal-point, and as a result of its simple shape.  But a parabolic-mirror can.  The narrow end of an egg, a parabola...

You cannot move the mirror round and about in that event, as its surface would not conform to the odd and uneven curvature of the egg.  Therefore, you must center-spot a primary-mirror, and for a precise collimation.  Again, a parabolic mirror can focus all of the incoming rays of light to a single focal-point, via its exotic curvature, which then results in the sharpest and most pleasing images possible.

Still, I don't think that it's good practice in suggesting that a spherical primary-mirror does not need to be center-spotted, as that can lead to a beginner in not center-spotting their mirror at all, regardless of its curvature.  A spherical-mirror is round, it has a center, therefore center-spot it...

https://garyseronik.com/centre-dotting-your-scopes-primary-mirror/

This primary-mirror, from my "Bird Jones", is a spherical, therefore I center-spotted it...

The telescope was then collimated, and I saw glory.

Incidentally, all primary-mirrors are first figured into a spherical there at the factory, and then some of those are parabolised for other, larger, and more costly Newtonians.  I can't help but wonder, if the optician is having a good day, if some mirrors that are destined to be shipped out as spherical are parabolised after all.  Stranger things have happened, and all the more reason to center-spot a spherical primary-mirror.

[Barry-W-Fenner]

33 minutes ago, Alan64 said:

The odd thing about it all is that we must assume that the back of the tube was cut precisely and perpendicularly to the length of same at the factory...

...and square to one another.  Then, the plastic cell is practically perfectly flat, we hope; as is the mirror, and again we hope.  Then, we assume that the three pieces of double-sided foam-tape used to install the mirror into the cell are of the same thickness.

Lastly, as long as the cell seats fully onto the end of the tube, then there should be no need to adjust it.  Does the cell fully seat onto the rim of the tube, in fact?

There is one other thing.  Elsewhere it had been opined that there's no need to center-spot a spherical primary-mirror.  If the mirror is off-center, no matter, according to those individuals.  The reason being is that the surface of the mirror conforms to a perfectly round sphere, and no matter where you place it round the sphere...

Spherical-mirrors cannot focus all of the rays of incoming light from an object in the sky to a single focal-point, and as a result of its simple shape.  But a parabolic-mirror can.  The narrow end of an egg, a parabola...

You cannot move the mirror round and about in that event, as its surface would not conform to the odd and uneven curvature of the egg.  Therefore, you must center-spot a primary-mirror, and for a precise collimation.  Again, a parabolic mirror can focus all of the incoming rays of light to a single focal-point, via its exotic curvature, which then results in the sharpest and most pleasing images possible.

Still, I don't think that it's good practice in suggesting that a spherical primary-mirror does not need to be center-spotted, as that can lead to a beginner in not center-spotting their mirror at all, regardless of its curvature.  A spherical-mirror is round, it has a center, therefore center-spot it...

https://garyseronik.com/centre-dotting-your-scopes-primary-mirror/

This primary-mirror, from my "Bird Jones", is a spherical, therefore I center-spotted it...

The telescope was then collimated, and I saw glory.

Incidentally, all primary-mirrors are first figured into a spherical there at the factory, and then some of those are parabolised for other, larger, and more costly Newtonians.  I can't help but wonder, if the optician is having a good day, if some mirrors that are destined to be shipped out as spherical are parabolised after all.  Stranger things have happened, and all the more reason to center-spot a spherical primary-mirror.

Hi Alan,

Thanks for the above, Very informative. Although I dont fully understand all of it 🙂

The cell did fit onto the tube perfectly before I modified the tube holes. However my collimator then showed the red dot well off center of the bullseye. This lead me to believe that the primary was out of line. After I made the holes larger in the tube I am now able to line up the red dot in the center of the bullseye, Doing this makes the cell sit roughly 1-2mm further back than it should and not butt up flush with the tube,However I assume this would now make the mirror correctly collimated?

Reading the above am I right in saying the Primary mirror on this scope theoretically shouldn't need to be collimated as it should be lined up correctly from factory and conforms to a perfectly round sphere thus not requiring collimation? However in your experience it is better to center the mirror and collimate any way? I will collimate when i reassemble the scope tonight for peace of mind. It will be interesting to see if this resolves the issue of objects only defocusing to a half circle. However as other members have pointed out this might be down to something entirely different such as the light path being obstructed.

Regards

 

Baz

 

[Alan64]

15 minutes ago, Barry-W-Fenner said:

Hi Alan,

Thanks for the above, Very informative. Although I dont fully understand all of it 🙂

The cell did fit onto the tube perfectly before I modified the tube holes. However my collimator then showed the red dot well off center of the bullseye. This lead me to believe that the primary was out of line. After I made the holes larger in the tube I am now able to line up the red dot in the center of the bullseye, Doing this makes the cell sit roughly 1-2mm further back than it should and not butt up flush with the tube,However I assume this would now make the mirror correctly collimated?

Reading the above am I right in saying the Primary mirror on this scope theoretically shouldn't need to be collimated as it should be lined up correctly from factory and conforms to a perfectly round sphere thus not requiring collimation? However in your experience it is better to center the mirror and collimate any way? I will collimate when i reassemble the scope tonight for peace of mind. It will be interesting to see if this resolves the issue of objects only defocusing to a half circle. However as other members have pointed out this might be down to something entirely different such as the light path being obstructed.

Regards

 

Baz

 

That's okay, as I didn't understand it when I first came across it, and several weeks ago.  It sounds to me that that mirror is parabolic, if the mirror is directing the laser-beam when tilting the mirror.

[Barry-W-Fenner]

3 minutes ago, Alan64 said:

That's okay, as I didn't understand it when I first came across it, and several weeks ago.  It sounds to me that that mirror is parabolic, if the mirror is directing the laser-beam when tilting the mirror.

correct, It does in fact move the laser beam when tilted slightly. So I assume I will collimate when installed.

 

Thank you

[Alan64]

However, don't get too excited as to the possibility.