New to Newts

[JonHigh]

Hi All.  I hope you can advise on this. 

I borrowed my brother's Newtonian (SW 150PDS) to have a look if it was something worth getting as I have only ever used refractor's and Mak's. I took this 2 min sub and saved an auto stretch version in ASIFITS viewer to ask someone more knowledgable what this is? Im guessing it's a collimation issue (primary or secondary?) or is there something wrong with it optically?  I first thought it was out of focus (Used a mask) although it was saying it was focused!? I assumed if it was out of focus I would get donuts rather than odd shapes?

Please help.

Edited March 23 by JonHigh

[Elp]

Were you using a coma corrector?

It looks like a number of issues, collimation, tilt, coma and star trailing.

[ONIKKINEN]

Collimation is way, waay out for sure. Your coma has a backwards pattern in the top right, suggesting your collimation is like 3cm out of field here. The coma pattern of your top right should be in the bottom left.

The birdlike stars in the bottom look astigmatic, but you should make no conclusions on optical quality before the scope is collimated.

[JonHigh]

Thanks for the fast and sound replies! So I was going in the right direction of it being well out of collimation. Is this a Primary / secondary or both mirrors out?  Yes, indeed a coma corrector was in place but I didn't think it could present an issue as long as a corrector was used?  Maybe a better one is needed? This is a totally new scope to me so I'm a bit like a fish out of water! I'll have to sort this out and get it back up and running when clear again. Also need to sort out the tilt!! Birdlike Stars... If it turns out to be astigmatic is that the end of the scope? 

 

 

Edited March 23 by JonHigh

[ONIKKINEN]

Hmm, i had assumed you did not have a corrector because it looks so bad. But since you did the backwards coma could also come from incorrect backfocal distance between the corrector and camera sensor, most correctors need 55mm.

Collimation looks wrong for both the secondary and primary. Again, dont worry about possible optical defects before you have it collimated and get a useful look at its performance.

[JonHigh]

Oh I see. I'm so used to my triplet, Oh the fun of newts 😂 ! I will have to check with my brother as he set it up initially so I am assuming he has set it up correctly. I will check if the distance is correct maybe an extension spacer was left off? So birdlike stars is a collimation issue? What is a backwards coma? Starting to wonder if I should just stick to my frac!  Sorry for going on - Just want to understand. 👍

Edited March 23 by JonHigh

[ONIKKINEN]

Just now, JonHigh said:

Oh I see. I'm so used to my triplet, Oh the fun of newts 😂 ! I will have to check with my brother as he set it up initially so I am assuming he has set it up correctly. I will check if the distance is correct maybe a spacer was left off? But would that explain the birdlike stars? Sorry for going on - Just want to understand. 👍

Its difficult to tell what is the cause. Most likely there are a multitude of issues. The backfocal distance is easy to check, you just need to measure 55mm between the coma corrector and camera sensor. It is important to keep in mind here that this does not mean 55mm between the coma corrector and the camera body, this means 55mm between the actual sensor in the camera and the corrector. The adapter train you need will depend on the camera, if you have a camera with 17.5mm between the connecting threads and the sensor, then you need an extra 37.5mm of adapters in between. Some cameras have 12.5mm, in which case you need 42.5mm extra.

I dont think backfocal distance explains the erratic star shapes, that issue is in collimation almost certainly. This doesn't look like a slight adjustment issue, but some fairly obvious misalignment so i think you may be able to "eyeball" the collimation to be a bit better by just looking down the focuser without a specific collimation tool. But a tool of some kind is necessary for good collimation, but it need not be that expensive. A simple collimation cap will get you started, a cheshire is better, and a good collimated laser can be the best option. Although for the laser you do need to make sure it is collimated itself, and you need to make sure it sits in the focuser correctly and in the same way a camera would - very sensitive to tilt and de-centering issues.

[Elp]

As it's a PDS it's primary is centre dotted. Using a simple collimation cap they're easy to collimate.

[JonHigh]

Thanks for the detailed info. To be honest I thought it was something I may have inadvertently done to it!  - I will check the back focusing spacers just in case which is an easy thing to do. However it's the collimation I don't know at all. I will get a cheshire to start with so at least I can bring it to where it should be. This was one reason why I didn't go for an RC scope and got a my Triplet some time ago now. Was thinking about selling it and getting a Newt as a replacement but I'm not so sure now. Maybe my brother should take it back but not before I have played! 🤣

Thanks again 

 

PS. Aha yes there is a central dot. Right I get it now! 🙄

Edited March 23 by JonHigh

[Elp]

Get a collimation cap first, or make your own. No need to spend excess money. Once you've used it you can check the collimation on a defocused star, the shadow of the secondary should be in the centre of the star as a black circle, the defocused star should form concentric rings around it. If not, then you need to adjust one secondary screw at a time to see what direction the star pattern goes until you've got it central.

Edited March 23 by Elp

[Ratlet]

Or if you are particularly lazy like me, you can just use a cheapy laser (just make your you collimate the laser first, which is even easier than collimating a newt).

[Fir Chlis]

There are many guides out there, but this is one that I like - particularly use of the coloured card.

https://www.stargeezer.co.uk/guides/130eq/mirror-alignment-and-collimation

I’d also recommend getting a collimating cap (or make one), as you can get a pretty good collimation without spending much. I’m rather obsessed with collimation, so I went further - see my posting at the end of this thread

 https://forums.sharpcap.co.uk/viewtopic.php?t=5610

I was pleased to find that the collimation cap result was good, and it was just some very fine tuning using the SharpCap method. I have a 200PDS. 
 

Geoff

[JonHigh]

Awesome. Thanks so much for your help! Really is very much appreciated! I feel like a newbie all over again!! 😂 

[wimvb]

10 hours ago, Fir Chlis said:

I’d also recommend getting a collimating cap (or make one),

The cheapest way:

SW newtonians come with a cheap 2× barlow. Unscrew the lower black ring that holds the optics. Drill a 1.5-2 mm hole in the very centre if the top cover. Voilà a budget collimation cap. Not the very best, but it will do for rough collimation.

Btw, if you have a laser collimator, use that with the barlow (with optics in place). The barlow will spread the laser beam, and you centre the shadow of the primary mirror's marker ring on the laser's bullseye. That's how I collimate my MakNewt. 

[Ratlet]

9 minutes ago, wimvb said:

The cheapest way:

SW newtonians come with a cheap 2× barlow. Unscrew the lower black ring that holds the optics. Drill a 1.5-2 mm hole in the very centre if the top cover. Voilà a budget collimation cap. Not the very best, but it will do for rough collimation.

Btw, if you have a laser collimator, use that with the barlow (with optics in place). The barlow will spread the laser beam, and you centre the shadow of the primary mirror's marker ring on the laser's bullseye. That's how I collimate my MakNewt. 

I use the laser without Barlow to make sure my laser is central to the dot and then the barlowed laser to line up the primary.  It's pretty darn precise and quick.

[bosun21]

24 minutes ago, Ratlet said:

I use the laser without Barlow to make sure my laser is central to the dot and then the barlowed laser to line up the primary.  It's pretty darn precise and quick.

You can still get the laser dot on the central spot on the primary with a secondary mirror that is not correctly positioned. I initially use a concenter to set the secondary correctly and then a cheshire and cap for the primary. Once set I then use a Hotech laser for a check and tweak before every session.

[ONIKKINEN]

Another trick is to install an aperture stop (less than 1mm) to the collimation laser, which will make the laser spot on the primary large enough to cover the entire center marker. Then you can collimate using the shadow of the center marker rather than just the reflected laser. Makes it easy to see how much adjustment is needed and to what direction, since the reflected shadow is now bigger than the central hole on the target plate the laser has.

I have not had any luck with barlows on my laser, have tried 2 barlow elements and a few different methods of laser to barlow attachment and they all disagree with each other - so not accurate.