Usman

OK. Thanks so much for your advice, Piero 🙂

Hi Piero,

That sounds promising. Thanks!

I also have a question about the focuser drawtube protrusion into the OTA (which you mentioned in your second-last paragraph). By displacing the primary and thereby making the secondary appear larger, would I then be able to reduce the amount of this focuser drawtube extension into the optical tube (owing to the new position of the focal plane)? I'm asking because that would then also reduce the overall obstruction in the optical path and be an additional benefit of moving the position of the primary mirror.

Thanks, Mike.

11 hours ago, johninderby said:

That’s fine. You should only see the inner edges of the clips not the edge of the mirror.

OK. Thanks, John.

9 hours ago, Louis D said:

From what I was able to find online, your scope has a 24% central obstruction by diameter, so it's a good compromise between being oversized and undersized.  There will be some light falloff if you were to try astrophotography, but since it's considered a visual only scope, that's irrelevant.  Some folks prefer visual only scopes with no larger than a 20% CO to maximize contrast at the expense of full illumination.

Thank you for the info, Louis.

3 hours ago, Piero said:

Is your primary mirror pushed all the way in by the collimation screws / knobs or is it at about half way of the travel?

If it is all the way in, there are two potential downsides: 1) the primary mirror touches the bottom of the 3 clips - which can cause astigmatism, 2) not all the primary mirror is reflected by the secondary mirror - which is what you described.

Also, have you offset your secondary, in particular moved it forward to the primary?

Regarding your questions, the WHOLE primary mirror surface is supposed to be reflected by the secondary. Ideally, there should also be an additional margin. If not, the telescope is operating with an aperture stop. For instance, if the clip is 1/4” inside, and you just cover them, the effective aperture is not more than 7.5". This does not affect contrast, but is a minimal reduction of light grasp and therefore resolution. The other issue in this context is the secondary mirror. The edge of the secondary mirror is the weakest part - this is well known in high end secondary mirrors, so much that the provided interferometer analysis of these mirrors usually discards the edge automatically. If the secondary mirror does not reflect the whole primary mirror, it means that the secondary edge is used. This can affect the views at low power.

Before replacing the secondary mirror, I would check how the primary is reflected by the secondary mirror when the focuser is ranked out with your eyepieces, rather than fully in / fully out. Then, I would try to pull back the primary a little if possible. I would avoid offsetting the secondary toward the primary too.

These changes are all minor. For a commercial telescope, I wouldn't upgrade the secondary, unless damaged - it is not flat or the coatings are ruined. The former shows as tube-aligned astigmatism, the latter as light loss and scattering. The former is a quite common problem. It can be due to the mirror itself or by its support (including incorrect gluing).

Thank you for the detailed explanation, Piero.

If I've understood you correctly, it seems that the primary mirror is pushed all (or most of) the way in from the back (please see the attached photo of one collimation screw - the other two are very similar). Does that mean that, in this case, there's some room to push it further in towards the back of the tube, if I loosen each collimation screw by exactly the same amount?

Hi,

I tried to fine-tune the collimation on my new Skywatcher 200P f/6 Dob today and realized that I can't see the outer edge of the primary in the secondary mirror. The alignment looks fairly OK, though not perfect, and I think at this point the circumference of the primary should be visible in the secondary mirror. I am concerned that the reason might be the size of the secondary mirror, which might be too small for the light transmission from the primary mirror. Can you please let me know if that really is the case and whether I can swap the secondary mirror with a larger one? Also, would this undersized secondary cause a sgnificant loss in contrast, resolution, etc.? Is this also typical of this particular telescope model?

1st photo - Focuser racked all the way in. 

2nd photo - Focuser racked out.

(I had slipped a sheet of paper behind the secondary and was pointing the telescope at a shirt when taking these photos 🙂)

Thank you.

OK. Thanks, Dave.

OK. Thank you  🙂

Hi guys, 

I have been trying really hard to collimate my new Skywatcher 200/1200 Dobsonian and after trying for several days (:)), this is the best I have managed to achieve so far. I know that it's not perfect but could you please let me know if this is adequate or do you think I need to make some further adjustments (and, if so, please let me know which ones)?

The mirror configuration is the same in all three photos but it might look slightly different due to the parallax error.

Thanks! ;)

Hi,

I've recently received a pair of Bushnell 10x50 Legacy binoculars as a gift and while the image quality is generally very good, I've noticed an odd problem when looking at Venus or the Moon through the right eyepiece or through both the eyepieces together. In the case of Venus, there's a ray that emanates from it and goes at a certain, fixed angle. The ray moves in both directions depending on the movement of my binoculars and when I saw it the first time, I thought it was a shooting star. I tried rotating the binoculars and the ray's angle correspondingly moved with it; I looked at it through the right eyepiece but with my left eye this time but it was still the same. In the case of the Moon, it's a small white sphere that moves in a certain direction instead of a ray of light. Since this doesn't happen when I only look through the left eyepiece, I'm guessing there's a problem somewhere in the optics on the right-hand side of the binoculars.

Could you please help me figure out what might be causing this issue and how I can rectify it (if at all possible)?

Thank you for your help.

12 hours ago, Ricochet said:

This is not right. I believe that the stalk is threaded both ends and just screws into both the secondary holder and the side of the telescope tube. Check when the secondary support swivels if it is turning on the stalk or if the stalk itself is actually rotating as well so that you know which joint is faulty. Once you have determined that, turn the loose part to tighten it up. If you can get one full rotation you may be fine, but if it is always loose in the correct orientation then I would suggest going back to your retailer and see what solution they suggest (probably replacement). 

 

Edit: looking at photos down the barrel of the H130p it appears there is a nut where the stalk enters the tube side wall. If this nut is not fixed to the wall of the tube as the mounting point then it is a lock nut and you should tighten the nut to stop the stalk from turning. 

Thanks for your reply. At first I thought I could tighten the nut on the tube wall but the stalk itself isn't rotating even a little bit - it's just the holder which is swiveling from one position (as shown in the first photo) to the one shown in the second photo (so it's not making a complete rotation otherwise I would have tried to tighten it that way). I tried checking if there was a nut on the holder which could fix its position relative to the stalk but there isn't. As far as I can tell, the only way to completely fix the position of the holder is to use steel epoxy to 'weld' it to the stalk but I don't want to do that on a brand new telescope

Hi,

I was trying to collimate my new Skywatcher Heritage 130P for the first time yesterday and after I got my secondary mirror centered under my focuser, I tried to use the three alignment screws to align it to the primary mirror. Each time I tried that, though, I realised that the secondary mirror would move with respect to the focuser. I tried to hold the secondary holder with my fingers to avoid any rotation but realised that the holder itself was swiveling on its stalk. I tried tightening up the central screw as well as the alignment screws but there's still a significant amount of play in the holder (as shown in the attached photographs) and it doesn't take more than a slight touch to change its position. Can you please let me know if I am making a mistake during the collimation or whether there's a problem with the telescope holder itself? I might be wrong but shouldn't the back of the holder, where the central screw's located, always be perpendicular to the plane of the open end of the tube?

Thank you.

1 hour ago, Steve Clay said:

Had the same with my new 150PDS. I just taped a lense brush to a long stick and it moved easily.

That's a good idea! Thanks!

1 hour ago, Peter Drew said:

Difficult to avoid considering the travel distance from China. They will be dry spots and would probably blow or brush off. The good news is that they will have no effect on the performance, compare their size to that of the secondary holder and you can't even see that in use.    🙂

Thanks, that's reassuring

1 hour ago, johninderby said:

Skywatcher should copy Bresser. My new Bresser dob came with a rubberised coating inside the tube. Properly black and no chance of paint flakes.

Yes, and I don't think it would significantly increase the manufacturing costs, tbh.

Hi,
 
I received a brand new Skywatcher Heritage 130p on Wednesday and, after taking off the dust cap, saw several small black spots on the primary mirror (please see the attached photo). I think it's most likely the black paint on the inside of the tube that's flaking because when I ran my finger over the internal surface of the tube, I saw a small piece of black paint on my fingertip. I had ordered the same model about 2 years ago from another online retailer and it had an even bigger black spot on the main mirror and I returned that one right away. Has anyone had similar experiences with the Skywatcher Heritage? It seems to be a flaw in the manufacturing process, as in, the way the inner surfaces are painted or the quality of the paint.
 
Thanks.