Turning stock Sky Watcher P1145 into an astrograph

[astrochumak]

Hello everyone,
Just wanted to share the process of turning this scope into actually usable astrograph

Backstory
Although I'm originally from Ukraine, right now I'm in the U.S. as an exchange student, and so I started preparing for this trip in advance. Obviously, however ridiculous it may sound, I wanted to take my astro gear here, mainly for sidewalk astronomy and some astrophotography. So the gear should've been as light as possible, as I didn't want to pay for additional luggage, and with the visual astronomy in mind, too, hence no DSLR lenses.
At the time I was thinking of whether I should buy Star Adventurer or AZ-GTi, as these options were the only ones reasonably light and available In Ukraine. I pulled the trigger and bought AZ-GTi after some half-year thinking and researching. Okay, I have the mount. Now which scope would suit it best? One guy from our local astronomy club told me about the Sky Watcher P1145 and that it has a parabolic mirror (Surprisingly! These days with Astro-Masters and Powerseekers everywhere it's easy to forget that small yet of excellent quality Newts still exist). This scope would've been perfect, but the only problem was that I had to order it from Germany, as in Ukraine everything was out of stock (with shipping costs the price would go well over 350$). With the thought that it may probably never appear in stock again I started looking at some other options. Our club has its forum with its classifieds section, and there I located the Sky-Watcher ST102 for around 100$, which surprised me. Eventually, weakened by the momentary excitement, I bought it. Long story short, I regretted it, as CA was too bad, even though some people were managed to get nice shots on Astrobin. The scope itself looked amazing though: 

Luckily, the guy accepted it back and I returned to further searching. I've been looking at WO's scopes, of course, but they were too expensive for me, just as any other ED scope (be it Astro-Tech 72ED or Sky Watcher EvoStars, which are quite rare in Ukraine). Weeks went by, and one day during the phone call I was explaining my dad how I will be ordering P1145 from Germany when I'm back from the U.S. After that, just for fun, I decided to check the marketplace... and not in vain. There it was, 80$ for the OTA on the EQ1. I quickly contacted the seller and started asking the poor guy for photos to identify the parabolic/spherical mirror. Here's what he sent:

 

Main mirror with the circle, four spider veins... I was extremely happy, and of course I bought it. I sold the mount for 40$ and OTA alone turned out to be 50$!

There are, however, some downsides to the scope. Of course, it has 1.25" plastic focuser, and, if I remember correctly, its secondary was around 35mm.
35mm secondary is bad, as the focal plane was too close to the tube, and even with the stock focuser my Canon 600D wouldn't come to focus with the mirror raised as high as the collimation screws allowed. And had they allowed the main mirror to be higher, small secondary would've been cutting the aperture, so I needed a bigger diagonal.

 

Beginning

The overall steps to turn it into astrograph were starting to be clear:
1) I needed a new 2" focuser
2) For 2" focuser the hole in the tube should be increased from ~35mm to ~55mm
3) New focuser should be installed and later squared to the tube
4) The main mirror has to be put higher:

In order to measure the rising distance for the main mirror, I took the plastic focuser off, grabbed my DSLR and started searching for the focal plane on the optical axis. When image on the screen came into focus, I measured the distance from the tube to the Canon's flange. It turned out to be exactly 40mm, and, knowing that Canon 600D's flange focal distance is 44mm, I figured that focal plane sits ~84mm above the tube (when the main mirror assembly is 30mm high). 

5) Secondary should be installed
To figure out which diameter of the secondary I needed, I used the online calculator. Basically, to get the most of the illuminated sensor and the chosen diagonal to focal plane distance calculator told me 50mm secondary would be the best. I ordered one from AliExpress (they said the mirror is 1/5 the wave, so quite good for me).

Here you can almost see me comparing two mirrors (50mm vs 35mm). In reality, it looks MUCH bigger. I was struggling with the mirror attachment as I was gluing it with silicon, and it glued incorrectly.

Thank you for reading if you came this far! I will update shortly on how I raised the mirror and what happened to the diagonal.

 

 

Edited February 5, 2022 by astrochumak

[Roy Challen]

Interesting project, astronomy doesn't have to be expensive!

[RobertI]

Great project. I think the two speed focuser will be the expensive bit but you should have a super light setup when you’re done. 🙂

[PeterStudz]

Great project. Thanks for sharing. Please let us know how you get on. 

I’ve got a 1145p on EQ1 as our first telescope. Although it’s now somewhat taken second fiddle to a Skywatcher Skyliner 200p it’s a surprisingly capable little telescope. Had a lot of fun with it even with the limitations of the mount. 

I’m now thinking of mounting it on a AZ-GTi plus better tripod. @astrochumak what tripod are you using in your pictures? I’m also thinking of turning it into a travel scope. Something that will all fit into an airline cabin bay. 

[astrochumak]

@Roy Challen that's true! As a student, I was trying to keep it as cheap as possible 😅

@RobertI I was thinking of getting the two speed focuser, but in the end went with single speed Crayford. I was unsure of how the project will turn out to be, so didn't want to invest more than I need to get things done. In the end, it works just fine! I will try to write an update today. Unfortunately, the idea of writing this post came to me later, when most of the job is done, but I will try to make it step by step, as it was when I was working on it 😃

@PeterStudz Thank you! AZ-GTi works really nicely with 1145p, and this scope is pretty much the biggest one you'd want to mount on the GTi. I'm using the "LT324C Portable 10 Layers Carbon Fiber Tripod", it turned out to be around 190$. It is quite compact and will certainly fit into an airline cabin bay. The padded bag it comes with easily fits the tripod with the Sky Watcher equatorial wedge, which was a surprise, but quite a pleasant one! I'm certain you know this, but don't forget to buy the Vixen dovetail. If you have any questions - feel free to ask 😁

[astrochumak]

Alright, follow-up.

So this is the focuser I bought:

It is okay, but nothing special really. Just a robust single speed Crayford. It is made for 10" Newts though, and I will have troubles levelling it, which I will discuss later. I used rotary tool to enlarge the focusing hole to ~55mm. To say the least, it was painful. Steel tube took way longer to drill than I anticipated, and I lost some bits in this battle too. But in the end it turned out fine:

 

And the scope itself looks incredibly cute!

You can see that right now the collimation screws are ugly, as they are basically the original short screws connected to the M5 thread rod with the tall nut. This is temporary as I'm waiting for the 100mm bolts to arrive.

So I told you I messed up while gluing the mirror. The main problem was that silicone obviously needed some time to dry, and in order for the secondary to stay at the exact position I set it before, it needed to be fixed. I fixed it really poorly, so it glued incorrectly. I had to re-attach it while already in the U.S.

When I was peeling off the original 35mm secondary, it was fixed with the double-sided tape. I thought that 50mm is not that big of a secondary and I shouldn't bother with silicone. So I went to Walmart, bought some 3M double-sided tape and attached the mirror to the holder. For some reason, no matter the collimation, I kept on getting the egg-shaped secondary reflection pictured below (in order for it to resemble the circle at least a little but I had to take the photo off center, so this is why you can't see the main mirror clamps):

 

I decided that slightly sheering the taped secondary would be a great idea, and indeed it worked. I figured that if I attach the mirror a little bit to the left, the collimation becomes spot on, with secondary perfectly symmetrical (of course there's room for improvement in terms of collimation, but I only did this photo for you to compare the shape of the secondary with the photo above).

 

I believe this whole situation is due to the fact that focuser is not parallel to the optical axis when the secondary is glued at the center of the holder. However, it was much easier to re-tape the secondary than to struggle with leveling the focuser (I tried both).

So! I dropped the secondary. Yeah, I know. While cleaning the holder from silicon, it was lying on the table and I accidentally bumped it onto the floor. It didn't break though, and the only artifact was the internal dent. It is hard to photograph, but you can see it anyways. I thought it was okay, but it wasn't.
The problem is that now bright stars have this "silhouette" of the dent, which can be clearly seen on Sirius (I intentionally didn't crop so you can see its real size):

So I ordered another secondary, as well as the 20mm spacer for the GSO Coma Corrector. This is where the project stopped and I will update as soon as something new will happen. Feel free to ask any questions and thank you for reading!

[RobertI]

Interesting read. 🙂

[astrochumak]

Hello everyone,
Today the clouds were away and I decided to check the telescope, still with the cracked mirror. I ran into some issues and need your help 😄

It seemed like I couldn't find the focus, or at least make it as sharp as I wanted. So here are some possible reasons:
1) There was almost zero cooldown time. I thought that 4.5" Newt won't have much problems with cooldown time, but it might be the problem
2) The atmosphere wasn't calm, and I have no other telescope to check with (it looked fine though, stars weren't extremely twinkly)
3) Maybe some collimation issues? Could you tell me if it looks fine? Ignore the crack in the secondary. Here's the star test:

 

4) Can this crack make such a difference? Intuitively it seems that it shouldn't influence that much, but maybe?

 

My guess is that there was no time for it to acclimate. But I might be wrong. At low power it was more or less fine. Here are single shot with the smartphone (once again, ignore the edges, it was an Erfle ep) to the left, and the processed stack to the right (at native focal length, so quite a big crop + I don't know how to process Moon pics properly):

 

So I guess the main question is, do these look okay to you?

Edited February 14, 2022 by astrochumak

[Roy Challen]

Doesn't look too bad, I would try again on a night of good seeing allowing the scope to fully acclimate.

Is that 'bite' in your star test images due to the crack in the secondary? If so, I would change the mirror.

[astrochumak]

@Roy Challen Thanks, yeah, definitely have to try once again. This bite is indeed from the crack, so I'm waiting for the new secondary to arrive

[Shibby]

I like your project - the scope really does look cute and could end up being a very nice little imaging scope once you are finished. You will of course need a coma corrector if you want to call it an "astrograph" and these are unfortunately expensive. You can hunt for a second-hand one but it will still likely cost more than you paid for the OTA.

[astrochumak]

On 17/02/2022 at 06:23, Shibby said:

You will of course need a coma corrector

Yeah, at f/4.5 it's a must. I already have one from GSO.

Hi everyone,
Decided to make a quick update. So I finally received the secondary and successfully replaced it. Also, threading on the plastic flange of the collimating end of the tube became unusable, so I reinforced it on both sides with flange nuts, and now it works like a charm. I don't have the saw, however, to cut off the unused part of the collimating rods, so I put long nuts on them. Finally, the project is over:

 

P.S. As I've written above, I'm from Ukraine, and most of you have heard of the situation here. I decided to sell the scope somewhere. There are several reasons, but mainly a) I want to have money in case my family in Ukraine needs it, b) I don't want to bring the OTA back as Russian army bombed almost every airport in UA, so my trip back now becomes quite a fuss, so I'm not in the mood to drag the OTA around, and c) I want to donate part of the money for the needs of my country, as it's the least I can do from the U.S. Thank you all for sticking around and reading this. I enjoyed this little project

[Roy Challen]

I hope your family are safe. I've enjoyed reading about your project, it's a pity you have to sell it, would've been good to see some images taken with it.

[RobertI]

Well done on completing your project and sorry you have to sell it so soon. I hope your family and friends are safe and good luck.