Skywatcher 200/1200 Dob - Undersized Secondary Mirror?

[Usman]

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.

[johninderby]

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

[Louis D]

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.

[Piero]

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).

Edited August 15, 2021 by Piero

[Usman]

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?

 

[Piero]

7 minutes ago, Usman said:

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?

 

The bottom bolt is for locking the mirror in position. The top bolt is for collimating the primary mirror. If the spring is decent, the lock bolt is not required.

Anyway, if you loosen the lock bolt, you should be able to screw the collimation bolt clockwise. This will move the primary mirror towards the back of the tube and compress the spring. As a consequence the distance P between the two mirrors will widen, making the reflection of the primary in the secondary smaller - or the secondary mirror to be larger. In addition this will slightly move the focal plane inward towards the secondary mirror (L distance). Decreasing the L distance will make the secondary mirror to appear larger. 

By decreasing the L distance you also increase the FIF (fully illuminated field) a bit.

This adjustment can be applied until the focuser drawtube doesn't enter in the telescope tube using your eyepiece with the most inward focus travel. That's why low profile focusers exist. I would not worry about this with your Skywatcher telescope.

I'd suspect that if you move the primary mirror cell few millimeters towards the end of the tube, you should be able to see the clips and the edge of your primary from the secondary.

[Usman]

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.

[Piero]

1 hour ago, Usman said:

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.

The adjustment I described previously will move the focus plane slightly inward, meaning that the focuser drawtube shifts towards the tube, not the other way around.

I don't think the drawtube will protrude into the tube though. This adjustment is minor - I doubt you will be able to move it more than 6mm or 1/4” inward - but the gained shift could be sufficient to see the whole primary.

I would just try it 🙂

Edited August 15, 2021 by Piero

[Usman]

OK. Thanks so much for your advice, Piero 🙂