Focusing on a Saxon 1400mm 6" Reflector

[Andrew Larmour]

Hi all, 

 

Absolute newby here - my first foray into astronomy. I bought a Saxon 1400mm 6" refractor secondhand complete with a EQ mount for a bargain price and I'm keen to get it working properly for both planetary and deep space observations.

I invested in a few extras such as a laser collimator and a 3x barlow. I think I have a handle on how it all works including the EQ mount.  I took the scope out for a test over Easter - which just happened to coincide with a ISS transit of the full moon. Rippa, I thought, that would be great to capture on my first night of observations. The problem I have is the complete inability to focus the scope to anything like sharp enough.

I have some photos I took with the scope attached: 

1. The scene with the normal camera lens for my daylight practice session
   
2. A shot of a distant house with the telescope (using a Nikon D7000 on a t-mount adaptor)
   
3. The same house with the barlow attached
   
4. A shot of the moon- no barlow - as sharp as I could get it - certainly no way to see the silhouette of the ISS with the scope this out of focus
   

When I was collumating the scope, I noticed a that the reflection on the primary mirror was not a single spot but rather a line

... which means that the laser on the collimator target is a line rather than a dot

I confirmed that this is not a problem with the shape of the laser beam coming from the collimator by showing the shape on my hand at a distance of 14m

I'm not sure if the distortion of the laser is the fault of the secondary mirror or the lens(es) in the bottom of the eyepiece mount. I'm also not sure if this distortion is what is causing the inability to focus the scope, but I suspect that both issues are symptoms of the same problem.

I'd appreciate any ideas on what to do next to resolve the focus issue. 

 

 

[Cornelius Varley]

Hello and welcome to SGL. The Saxon 1400 is a type of telescope known as s Jones Bird reflector. The telescope uses a spherical primary mirror and a corrector lens built into the focuser draw tube to correct the inherent spherical aberrations of the primary and also give the telescope its 1400mm focal length. This type of telescope tends to give a sharpish image at the centre and less sharp towards the edge. The optical design also means that laser collimators do not work with it so you will need to use the traditional methods of collimating ie collimation cap or collimating using a star.

[ollypenrice]

 

The proper way to test your collimator is to set it in V-blocks, point it at a wall a good few metres away, and rotate it. If it is true the dot on the wall will rotate on its axis, if it is is out of true the dot will describe a circle. Suitable V-blocks can be made using a block of wood and 4 large nails: just bang in the nails at an angle to cross each other to like this: X, with one x in front at the other like a miniature saw horse. For the purposes of this test they will be plenty accurate enough.

Edit: I cross posted with Peter above and didn't know that the laser wouldn't work with a Bird Jones.

Could it be that the camera simply can't reach perfect focus with this scope? Is your best focus at the limit of the focuser travel or does best focus still leave you with a bit of free movement?

 

Olly

[GazOC]

I might be wrong but I think that scope has a spherical mirror, I'm not sure they can be collimated with a laser. They also already have a built in Barlow system hence the 1400mm focal length in, what looks from the photos, a much shorter tube

 

I've no direct experience of this so I'm happy to be corrected 

 

Edit: I see I've been beaten to it ?

[Andrew Larmour]

1 minute ago, ollypenrice said:

Serveral things:

1) The proper way to test your collimator is to set it in V-blocks, point it at a wall a good few metres away, and rotate it. If it is true the dot on the wall will rotate on its axis, if it is is out of true the dot will describe a circle. Suitable 1) V-blocks can be made using a block of wood and 4 large nails: just bang in the nails at an angle to cross each other to like this: X, with one x in front at the other like a miniature saw horse. For the purposes of this test they will be plenty accurate enough.

2) It may just be that, with your camera, your focuser is not allowing sufficient travel to bring the scope's focal plane to the position of the chip on the camera. It is common to find that the chip is too far from the focal plane. This video takes you throught the issue and its options. 

If you are aiming just to use the scope visually then your camera's refusal to focus won't matter.

Olly

Thanks Olly,

I'm aware of the issue with not being able to focus some cameras because the plane of the photo-receptor cannot get close enough to the secondary mirror. That's not the problem in this case. Using the scope visually, I cannot focus the telescope. Also, when my SLR is on the telescope, I'm have plenty of movement with the focuser both in and out from the point where I got the shots.

Regards,

Andrew

[Andrew Larmour]

Thx @Cornelius Varley & @GazOC, I didn't know there was such a thing as a Jones Bird Reflector - I was wondering how Saxon managed to get 1400mm with such a short tube.

Any ideas on why I'm unable to focus? Obviously I need to properly collumate the optics (without the laser) to get the best out of it, but would that explain my inability to focus?

[ollypenrice]

Hmmm, then you could, in the absence of real stars, try the ballbearing test. You seem to have space around you so you take a clean, smooth ballbearing and glue it onto the bottom of an open cardboard box which you paint mat black or flock on the inside. You then set this up at least 30 metres away with the bottom of the box vertical so that you can observe the distant BB through the scope. (I'm not making this up! ) Contrive a torch or similar to illuminate the BB without blocking your view. What will happen is that only light from the bit of the BB's surface closest to the scope will be reflected towards it, so giving you an approximation of a point source. It is vital to get the BB's image to the centre of the field of view then perform a classic star test, going just inside and outside focus, to verify your collimation. The 'star test' will Google but, basically, you want the scope to give you a set of concentric rings just inside and outside focus. Adjust collimation till you have this.

Olly

[alacant]

Hi. If you want to use the laser to collimate, you need to remove the lens at the base of the focuser tube. Loosen the 4 screws holding the focus knobs and pull out the tube. Remove the lens and replace the focuser tube. Now the laser will focus.

Be sure to replace the lens with the correct orientation.

HTH

[ollypenrice]

It's also worth warning you that the design of the scope is flawed. It attempts to make a cheap spherical mirror give decent images by the addition of a cheap lens. A bit optimistic...

Olly

[Astrobits]

The fact that the round(ish) laser spot becomes a line on the primary mirror worries me. Looks like severe astigmatism in the lens or secondary mirror. Try inserting a piece of white paper before the laser hits the secondary. If it is a line instead of a spot then the lens is a bad one or is not square on to the optical axis. I would suspect that a previous owner fiddled with it or it has moved during transport. If it is still a spot then the secondary is questionable.  Whichever is at fault you will never get good focus without fixing it.

I suggest that you go to your local astronomy club ( there is one in Melbourne ) and seek their help.

Nigel

[Astrobits]

1 hour ago, ollypenrice said:

It's also worth warning you that the design of the scope is flawed. It attempts to make a cheap spherical mirror give decent images by the addition of a cheap lens. A bit optimistic...

Olly

Actually Olly, that is the basis of the null testing set-up for parabolas. A simple positive lens is used to put in the same spherical aberration, but opposite sign, when testing a parabola at it's centre of curvature. Done properly it is very accurate. In the Jones-Bird design it is a negative lens that also has the effect of lengthening the effective focal length of the instrument.

Nigel

[Peter Drew]

The hard truth is that this type of telescope design, as marketed, is usually disappointing, particularly for high magnification. Hence the bargain secondhand prices. I have battled with the collimation with these on behalf of owners and they are a pain to collimate. Removing the "correcting" lens in order to use traditional collimation tools and procedures does not guarantee that this will hold once the lens is replaced. I've had the best level of improvement using a real or artificial star image to collimate to. A 3x Barlow is going to overstretch this instrument regardless. Terrestrial photography at 4,200mm focal length (3x Barlow) will always be challenging.   ?

[ollypenrice]

1 hour ago, Astrobits said:

Actually Olly, that is the basis of the null testing set-up for parabolas. A simple positive lens is used to put in the same spherical aberration, but opposite sign, when testing a parabola at it's centre of curvature. Done properly it is very accurate. In the Jones-Bird design it is a negative lens that also has the effect of lengthening the effective focal length of the instrument.

Nigel

OK, but as Peter says above the Bird Jones scopes as marketed are attempting and failing to 'beat the budget' don't you think?

Olly

[Andrew Larmour]

Thanks everyone for your comments and advice. I've at least got a starting point to try and resolve my issues. 

I understand the compromise that this type of reflector represents and perhaps that is why I got it for $40. The impression I got when I bought it was that the scope hadn't been used for a long time (the layer of dust on the box would support this!) and it was just taking up space - he didn't know much about it. Regardless, it is what it is and I'll make the best of it...

[mdstuart]

Do you have any local astro club as getting another pair of experienced eyes on the scope is a good way forward.

I am sure you will get a good image once you identify and resolve the issue.

Mark

[GazOC]

3 hours ago, Andrew Larmour said:

Thanks everyone for your comments and advice. I've at least got a starting point to try and resolve my issues. 

I understand the compromise that this type of reflector represents and perhaps that is why I got it for $40. The impression I got when I bought it was that the scope hadn't been used for a long time (the layer of dust on the box would support this!) and it was just taking up space - he didn't know much about it. Regardless, it is what it is and I'll make the best of it...

For $40 you didn't get a bad deal of you'd have paid that for the mount only so whatever you get out of the scope is a bit of a bonus if you find you enjoy the hobby and use this setup as a starting point ?

[Astrobits]

On 22/04/2019 at 12:53, ollypenrice said:

OK, but as Peter says above the Bird Jones scopes as marketed are attempting and failing to 'beat the budget' don't you think?

Olly

Quite. Which is why I qualified my remark with "Done properly it is very accurate". The spacing between mirror and lens is critical . Putting the lens on the end of the focussing tube is not going to give the required result.

Nigel