Stargazing Documenting the setup of my new telescope

[collinsmark]

Collimation screw, side project (part II):

After a couple weeks of overcast nights, I finally was able to replace the collimation screws with the new solution and re-collimate.

As a refresher, the reason I'm doing this is because the Bob's Knobs screws stick out too far, risking certain damage to the Schmidt plate if I ever remove the optical tube assembly (OTA) from the mount without swapping the collimation screws back to the original ones (see Fig. 1).

Figure 1. Bob's Knobs collimation screws stick out too far.

From anybody interested, here's the specs for Celestron's collimation screws (specifically for EdgeHD scopes, shown here):

Table 1. Collimation screw specs for EdgeHD optical tubes:

(Source: https://www.celestron.com/blogs/kno...on-current-production-celestron-optical-tubes)

I've got the C14 Edge HD, so that means I need M3×0.5, with 18 mm length. I found the following parts from McMaster-Carr:

• Plastic-Head Thumb Screws, Knurled, M3 x 0.5 mm Thread, 20 mm Long (Head color: Black)
• Delrin® Acetal Plastic Washer, Water- and Steam-Resistant, M3 Screw Size, 3.2 mm ID, 8 mm OD (Thickness 0.7-0.9 mm)

Figure 2. My McMaster-Carr order.

The washers are meant to reduce the shaft length from 20 mm to something closer to 18 mm. I figured two or three washers per screw should do the trick (I went with 2 in the end). I bought plenty of washers to be on the safe side.

While it's not clear from the image and description, the screw threads are chemical-resistant 18-8 stainless steel. (More on that later; boy, I hope that means they're also corrosion resistant.)

As a gentle reminder, when replacing your collimation screws, always replace them one-at-a-time and not all at once (else your secondary mirror might fall off). See Figure 3.

Figure 3. First of three screws replaced. Top two screws are the Bob's Knobs, and the bottom screw is my solution.

Now that I had one of the Bob's Knobs screws out for comparison, I photographed it on a random book alongside one of the original Phillips-head screws, and one of the screws from my new solution. See Figure 4.

Figure 4. Collimation screw comparison. Screws were placed on some random book along with calipers for scale. Top screw is my new solution. Middle screw is one of the original Phillips-head screws. Bottom screw is one of Bob's Knobs.

Below (Figs. 5-7) are images after their full installation.

Figures 5-7. New screw solution installed.

Re-collimation went well. The new solution is almost as easy as it was with Bob's Knobs. But now there's no increased risk of Schmidt plate destruction/obliteration if I remove the OTA from the mount.

The only thing I worry about is corrosion. The original Phillips-head screws and the Bob's Knobs had threads of black oxide (or maybe black anodizing?). Where the screws in my solution are chemically resistant stainless steel.

Which is more resistant to corrosion? This isn't my area of expertise. Is switching to stainless an upgrade or a downgrade in this regard?

I've tried to look online for answers, and from what I can gather, I might be OK. But I haven't been able to find what I consider a definitive answer, and corrosion resistance isn't my field of expertise.

Besides the corrosion questions though, things look quite promising.

 

[Vanadium 50]

Which is more resistant to corrosion?

You'll find out.

Stainless is not one thing. It's a whole family of alloys. Some are better in this regard than others.

 

[collinsmark]

You'll find out.

Stainless is not one thing. It's a whole family of alloys. Some are better in this regard than others.

Right. Specifically, the new screws are 18-8 stainless steel. Most of what I find online is 18-8 corrosion resistance is "excellent." So I think I should be OK. I hope.

 

[collinsmark]

Oh, and I did check the original Philips-head screws against a magnet, and they were attracted to the magnet (indicating the original screws were steel, and not aluminum). So with that, I'm hoping I won't have any dissimilar metal problems with the new screws, beyond what would be with the original. I hope.

 

[russ_watters]

Figure 5. Demostration that they stick out past the lip. Sorry about the bad focus here, I was using my cell phone camera.

I can tell when I put the lens cap on the telescope, that it teeter-totters on the Bobs Knobs. That's not a big deal if I'm just leaving the scope outside for the night, covering it up with the TeleGizmos cover. But it is a huge deal if I ever need to tear down the setup.

Conclusion:
I will not be keeping this Bob's Knobs configuration (sorry, Bob). While Bob's Knobs might work fine for some people on some scopes, it's not for me on my telescope.

At this point I can't remember if my C11 came with knobs or if I retrofitted it, but either way I agree they are a huge benefit for collimation. For me it's about fumbling around in the dark to find the knob (instead of fumbling around with a screwdriver). Potential solution: add a stand-off insert to the lens cap by epoxying --anything to it. Pieces of wood, nuts, whatever. You only need a few mm.