Collimating Skywatcher Heritage 76p mini Dob

[Louis D]

1 hour ago, Alan64 said:

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.

With one caveat.  It does this only on axis.  Off axis, there is coma aberration necessitating a coma corrector for pinpoint focus across the field.  Refer to my post in another thread about spherical vs. parabolic mirrors and off axis coma in parabolic mirrors:

 

Edited January 15, 2020 by Louis D

[Barry-W-Fenner]

Morning all,

So lets start with the good.

I finished the flocking last night and reinstalled the mirror. had a collimation with the laser and everything seemed ok.

Now the not so good.

I still have the half circle / egg shape when defocused. All visual targets are very poor.  Stars are barely visible and are blurry at best. I have attached a pic of Sirius defucused to show what shape it is showing. I have also attached a pic of the focuser position. I dont know if these pics are of any use. I assume the focuser is in some way blocking the light path but with limited experience in collomation I have no idea!

 

Hopefully the pros on here can shed some light on this for me.

 

Thanks in advance.

 

Baz

 

   

 

 

[Barry-W-Fenner]

Oh, I still need to tidy up a couple of pieces of the flocking! However I really want to get the scope working at the moment as my Nephew is keen to have a look.

 

Regards

[Peter Drew]

Oh dear, that looks about as far out as it gets!,  some further attention is needed.  The focuser tube also looks to be encroaching on the light path if the view is in focus at that setting.  I wold start from scratch by fixing crosshairs of white cotton accurately across the tube opening then stand back and see if their reflection overlaps, when it does you can be sure that the mirror is reflecting centrally up the tube.  Next, look through  the empty focuser and adjust the secondary until you can see the image of the primary mirror central in the secondary mirror.  The result of this won't be perfect but it will be a lot closer than you show, it will be a better base to do more precise adjustment.  Good luck!   🙂 

[Barry-W-Fenner]

2 hours ago, Peter Drew said:

Oh dear, that looks about as far out as it gets!,  some further attention is needed.  The focuser tube also looks to be encroaching on the light path if the view is in focus at that setting.  I wold start from scratch by fixing crosshairs of white cotton accurately across the tube opening then stand back and see if their reflection overlaps, when it does you can be sure that the mirror is reflecting centrally up the tube.  Next, look through  the empty focuser and adjust the secondary until you can see the image of the primary mirror central in the secondary mirror.  The result of this won't be perfect but it will be a lot closer than you show, it will be a better base to do more precise adjustment.  Good luck!   🙂 

🙂 Well you know what they say. If you are going to do a job do it properly. Might aswell make sure it is really out of line 🙂 I assumed it was going to be a mile off as when it was delivered the secondary mirror was loose and swinging around in the tube. I have tightened it back up and was trying to align with my laser but the beam is bouncing around in the tube all over the place!

Regarding the focuser, it was not in that position when i took the pic of Sirius defocused. I just wound it in to show the lowest position of the focus tube in the picture. Apologies I should have mentioned this.

So to Summaries. I need to attached a vertical and horizontal string centrally across the opening of the tube. Then tweak the primary so the reflection is inline or disappears behind the strings.  Then look down the empty focus tube and adjust so it only has the primary mirror reflecting back. Would this adjustment need to be made with just the collimation screws or will it be likely that I need to lower or raise the secondary mirror also?

After these stages are complete I can look to tweak with the laser.

Hopefully I am understanding the above correctly.

Thank you very much for the assistance Peter, Greatly appreciated. I new have a plan in my head of how to get this little scope back into line. I was totally stumped last night.

 

Kind Regards

Barry

 

 

 

 

Edited January 16, 2020 by Barry-W-Fenner

[happy-kat]

A related read on collimation

Astrobaby's guide

[DaveL59]

one question and tbh I've no idea what the right answer is for your scope. The secondary position/offset looks different in the pics where one shows much more of the  screw thread than the other. Is the secondary central in the OTA on these or slightly offset toward the focuser side?

Someone with one of these may be able to answer that one, as that may be affecting your results.

[Barry-W-Fenner]

1 hour ago, happy-kat said:

A related read on collimation

Astrobaby's guide

Thank you Happy kat. I will have a read and get to work. 🙂

[Barry-W-Fenner]

17 minutes ago, DaveL59 said:

one question and tbh I've no idea what the right answer is for your scope. The secondary position/offset looks different in the pics where one shows much more of the  screw thread than the other. Is the secondary central in the OTA on these or slightly offset toward the focuser side?

Someone with one of these may be able to answer that one, as that may be affecting your results.

Truth be told I think it is all out of line. Hence why I am struggling. Astrobaby's guide which Happy kat kindly provided should be a huge help. I will  slowly go though the collimation process step by step and identify the problems! 

 

Regards

 

Barry

[Peter Drew]

2 hours ago, Barry-W-Fenner said:

Truth be told I think it is all out of line. Hence why I am struggling. Astrobaby's guide which Happy kat kindly provided should be a huge help. I will  slowly go though the collimation process step by step and identify the problems! 

 

Regards

 

Barry

I think you are now on the right track.  The good news is that it can eventually be sorted out by patient adjustment. Nothing is broken!    🙂

[Barry-W-Fenner]

I will get there eventually thanks to your and happy kats advice Its all part of the learning.

 

Cheers Peter

Baz

 

[Louis D]

3 hours ago, Barry-W-Fenner said:

I have tightened it back up and was trying to align with my laser but the beam is bouncing around in the tube all over the place!

First, crank the focuser all the way down when using the laser to avoid focuser tube wobble.  If the focuser tube is wobbling all over the place as it likely is with these cheap focusers, the laser spot is meaningless.

Second, adjust the secondary so the laser spot hits the center of the primary.  Even if it's not center spotted, you should be able to guesstimate the center pretty well.

Third, adjust the primary so the laser spot returns to the secondary's center.

With your tube so far out of alignment, sweep your hand across the open end of the tube before looking down it to avoid being blinded by the laser.  You need know where the spot is before looking down the tube.  If it isn't hitting your hand, it's either hitting the secondary (good!) or the tube wall (bad!).  If it's the latter, you'll be able to see it obviously hitting the wall.  Carefully move the primary to bring it back on center to hit the secondary while avoiding blinding yourself in the process.

Fourth, continue with fine primary adjustments to bring the laser spot back to the laser output port.  If it's at least returning back up the focuser tube, you're really close.

You'll still need a sight tube to verify the secondary is centered under the focuser tube, although there may be very little you can do to adjust that.

[Barry-W-Fenner]

14 minutes ago, Louis D said:

First, crank the focuser all the way down when using the laser to avoid focuser tube wobble.  If the focuser tube is wobbling all over the place as it likely is with these cheap focusers, the laser spot is meaningless.

Second, adjust the secondary so the laser spot hits the center of the primary.  Even if it's not center spotted, you should be able to guesstimate the center pretty well.

Third, adjust the primary so the laser spot returns to the secondary's center.

With your tube so far out of alignment, sweep your hand across the open end of the tube before looking down it to avoid being blinded by the laser.  You need know where the spot is before looking down the tube.  If it isn't hitting your hand, it's either hitting the secondary (good!) or the tube wall (bad!).  If it's the latter, you'll be able to see it obviously hitting the wall.  Carefully move the primary to bring it back on center to hit the secondary while avoiding blinding yourself in the process.

Fourth, continue with fine primary adjustments to bring the laser spot back to the laser output port.  If it's at least returning back up the focuser tube, you're really close.

You'll still need a sight tube to verify the secondary is centered under the focuser tube, although there may be very little you can do to adjust that.

Hi Louis D

Thank you for the advice. I will use this also to assist me as I do use a laser.

I have various solutions now and am confident I will get the scope collimated, I will be happy with just getting it back inline tonight with the aim of finer tuning later. 

Regards

Barry

[happy-kat]

moonshane has an excellent colimnation guide which is lurking on posts on this site. I linked to the one I did as it shows what it looks like in a fast scope with I think the 76 is.

[DaveL59]

now I'm home and looking at the EP view you posted, I wonder if your secondary is tilted correctly so that it is giving a full view of the primary. Or perhaps its not far enough under the focuser/too far. Tho how you'd adjust the lateral position of the secondary in your case isn't clear as on my mini 76/350 it has 4 screws as per the pic. The outer 3 set the tilt and the central one locks the setting, but the combo can be adjusted to move the secondary further in/out of the OTA. I replaced the original screws in mine so I could get the secondary placed better, seems a common issue on the NG and some others.

Ideally you'd want to:

1. Centre (or offset as appropriate) the secondary in the OTA aperture
2. Centre the secondary under the focuser tube
3. Adjust secondary tilt so you can see the full primary (and clips)
4. Adjust primary 

I use a cheshire to fiddle with mine, don't have a collimation cap and never got round to buying a laser unit but I've managed fine so far. It's fiddly and requires patience for sure especially the first few times. I usually do adjustments in the day then check against a suitable target and later if the sky is ever clear enough will verify and tweak if needed.

The focuser tube shouldn't obstruct the view of the primary even when racked all the way in (when looking down the focuser tube), it'll only obstruct the light path to the primary from the target and likely wouldn't be too far into the OTA when focused with a regular eyepiece.

Edited January 16, 2020 by DaveL59

[Alan64]

When you look down into the empty focusser you should see that the secondary-mirror is centered directly underneath the focusser, and that it appears as a perfect circle...

I could tell within one of your images that the secondary-mirror appears rotated off to one side.  The mirror is actually oval in shape, yet tilted at a 45° angle...

This is true for all Newtonians.  It therefore appears as a circle, as you see it through the focusser, and as the primary-mirror "sees" it from the back...

Be careful when rotating the secondary-mirror, as we do not yet know how that is effected.  You will probably need to loosen the three tilting-screws round the hub in order to free it up in order to rotate it.

[Barry-W-Fenner]

Hi all,

I managed to get the scope collimated to a reasonable fashion last night, the secondary mirror needed quite a bit of adjustment to line my laser centrally on the primary. from there I went through the usual motions and lined up the primary and checked the red spot in the bullseye area of the laser.

I took the scope outside and managed to locate the brightest star possible  between all the cloud. The star was clear and crisp, The only thing that seemed to be happening was it had a slight point coming off the top of it. I assume this will be the defraction from the single vein? I unfocused the star and it produced a near perfect circle. It wasnt quite spherical all the way, One side still needs a little work, However I am pleased I managed to actually view through the scope successfully.

I was really hoping to see the moon and to get an idea of how good the viewing is now but had no chance with all the cloud, Maybe tonight. From the above I pics provided I can see that my secondary mirror still needs a small amount of adjustment to make it completely spherical in the focuser. I might have a look at this tonight. Or as it is Friday I might take the night off and sink a cold one!

With a dob of this size, Realistically what should I be able to see through the stock 10 & 25mm eye pieces? My only reference I have is my 200p which is not an ideal comparison. Would I be right in saying that objects in this scope should be as sharp as in the 200p but less detailed?

Thank you to everyone who has contributed above, With the various guides above I now have a much better understanding of collimation and it has been a great learning experience. 

Regards

 

Barry

 

[Alan64]

Yes, that slight point is due to the secondary-stalk.  Hopefully, once you get it as precisely collimated as you might, the images should be sharper than they were before.  Resolution, the level of detail, is dependent upon aperture; the more the merrier.

Do keep in mind that that's the smallest Newtonian on the planet, yet larger than what Newton himself had at his disposal.

The general, static range of eyepieces, from 4mm to 40mm, works differently with this telescope and that.  A 10mm can be a high-power eyepiece for one telescope, and a low-power for another.  In this case it's a low-power somewhat, 30x.

In theory, that being a 3" aperture, it is capable of up to 150x, and a bit more even when aimed at the Moon.  Let's see what 100x would take...

300mm ÷ 100x = a 3mm eyepiece.  3mm eyepieces aren't particularly plentiful.  I would combine a 9mm eyepiece with a 3x-barlow, and for a effective, simulated 3mm.  A 3x-barlow for that one, if you want to max it out.  A 2x can be useful as well.

[Barry-W-Fenner]

22 minutes ago, Alan64 said:

Yes, that slight point is due to the secondary-stalk.  Hopefully, once you get it as precisely collimated as you might, the images should be sharper than they were before.  Resolution, the level of detail, is dependent upon aperture; the more the merrier.

Do keep in mind that that's the smallest Newtonian on the planet, yet larger than what Newton himself had at his disposal.

The general, static range of eyepieces, from 4mm to 40mm, works differently with this telescope and that.  A 10mm can be a high-power eyepiece for one telescope, and a low-power for another.  In this case it's a low-power somewhat, 30x.

In theory, that being a 3" aperture, it is capable of up to 150x, and a bit more even when aimed at the Moon.  Let's see what 100x would take...

300mm ÷ 100x = a 3mm eyepiece.  3mm eyepieces aren't particularly plentiful.  I would combine a 9mm eyepiece with a 3x-barlow, and for a effective, simulated 3mm.  A 3x-barlow for that one, if you want to max it out.  A 2x can be useful as well.

Thanks Alan,

I have most of the BST eye piece range. I can try the following combinations.

5.5mm plus x2 barlow

8mm plus x2 barlow

The 5.5mm with barlow should achieve around 110 mag. Hopefully I will be able to view the moon tonight and see how the scope performs. It looks like I will need to pick my Nephew up a 3mm or 5mm eye piece to enjoy the views.

The statement about Newtons dob being smaller than the 76 really puts things into perspective. You sometimes forget what he (and others) achieved at the time with there equipment.  On the plus side though they must have had the great advantage of far less light polluted sky's!  I managed to aquire the Isaac Newton collectable 50p coin along with the Stephen Hawking black hole 50p here. They are something to keep away from the kids, I dont want them spent on sweets / candy.

 

Thank you

 

Barry

 

 

 

[Barry-W-Fenner]

Any recommendations for a cheapish  3 or 5mm eye piece for this little dob.

 

Thank you

[happy-kat]

Bare in mind that 8* magnification using 42mm binoculars will show the 4 main moons around Jupiter, so this telescope will be usable at 30x. Think crisp and small I wouldn't chase large and blurry.

The Lunar 100 might be something to tick off using this telescope.

[Barry-W-Fenner]

14 minutes ago, happy-kat said:

Bare in mind that 8* magnification using 42mm binoculars will show the 4 main moons around Jupiter, so this telescope will be usable at 30x. Think crisp and small I wouldn't chase large and blurry.

The Lunar 100 might be something to tick off using this telescope.

Great advice,  Quality over quantity 🙂

[Alan64]

1 hour ago, Barry-W-Fenner said:

Thanks Alan,

I have most of the BST eye piece range. I can try the following combinations.

5.5mm plus x2 barlow

8mm plus x2 barlow

The 5.5mm with barlow should achieve around 110 mag. Hopefully I will be able to view the moon tonight and see how the scope performs. It looks like I will need to pick my Nephew up a 3mm or 5mm eye piece to enjoy the views.

The statement about Newtons dob being smaller than the 76 really puts things into perspective. You sometimes forget what he (and others) achieved at the time with there equipment.  On the plus side though they must have had the great advantage of far less light polluted sky's!  I managed to aquire the Isaac Newton collectable 50p coin along with the Stephen Hawking black hole 50p here. They are something to keep away from the kids, I dont want them spent on sweets / candy.

Thank you

Barry

I would think that you might want to get him at least a 2x-barlow of his own to putter round, with the original kit-eyepieces...

https://www.ebay.co.uk/itm/SVBONY1-25Barlow-2X-Lens-Multi-Coated-M42x0-75Thread-Camera-Connect-Interface/361950654736?hash=item5445f0b110:g:3hEAAOSwSKtaauef

...or... https://www.ebay.co.uk/itm/SVBONY-1-25-Achromatic-Barlow-Lens-2X-Metal-two-element-for-Telescope-Eyepiece/323934195326?hash=item4b6bfb8e7e:g:Q9EAAOSwaQ9dID84

Dedicated eyepieces at 5mm and, gracious, 3mm, among the inexpensive, standard ones anyway, have tiny eye-lenses through which to see, and poor eye-relief where you'd have to almost if not actually touch your eyeball to the eye-lens of the eyepiece to see the full field-of-view.  Rather, a wider-angle 6mm with a larger eye-lens, and combined with a 2x-barlow would realise a wide-angle 3mm...

https://www.ebay.co.uk/sch/m.html?_odkw=&_ssn=retekessofficialstore&_armrs=1&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.X6mm.TRS0&_nkw=6mm&_sacat=0

Edited January 17, 2020 by Alan64

[Alan64]

1 hour ago, Barry-W-Fenner said:

Great advice,  Quality over quantity 🙂

One thing that I've learned over time is that if you don't push the magnification up then you will never know how truly capable a telescope may be in fact, and in showing the user that which few people have ever seen.  Never say never.  That applies to all telescopes, including your 200P.

I recently got this refractive equivalent to your nephew's reflector...

 

...a 70mm f4.3 achromat, also with a 300mm focal-length.  I haven't pushed it to its limits quite yet, as I've got to blacken and flock it first, but I do have high hopes, high-powered hopes, and when I insert the eyepiece/barlow equivalent of a 1.5mm(200x) into it.

Edited January 17, 2020 by Alan64

[Barry-W-Fenner]

54 minutes ago, Alan64 said:

I would think that you might want to get him at least a 2x-barlow of his own to putter round, with the original kit-eyepieces...

https://www.ebay.co.uk/itm/SVBONY1-25Barlow-2X-Lens-Multi-Coated-M42x0-75Thread-Camera-Connect-Interface/361950654736?hash=item5445f0b110:g:3hEAAOSwSKtaauef

...or... https://www.ebay.co.uk/itm/SVBONY-1-25-Achromatic-Barlow-Lens-2X-Metal-two-element-for-Telescope-Eyepiece/323934195326?hash=item4b6bfb8e7e:g:Q9EAAOSwaQ9dID84

Dedicated eyepieces at 5mm and, gracious, 3mm, among the inexpensive, standard ones anyway, have tiny eye-lenses through which to see, and poor eye-relief where you'd have to almost if not actually touch your eyeball to the eye-lens of the eyepiece to see the full field-of-view.  Rather, a wider-angle 6mm with a larger eye-lens, and combined with a 2x-barlow would realise a wide-angle 3mm...

https://www.ebay.co.uk/sch/m.html?_odkw=&_ssn=retekessofficialstore&_armrs=1&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.X6mm.TRS0&_nkw=6mm&_sacat=0

Thank you Alan! I appreciate the advise and will look at one of the above to utilise with his existing eye pieces.

 

Regards