I don’t know what telescope I have

[TiffsAndAstro]

1 hour ago, vlaiv said:

Two things come to mind - first is to check for off axis coma and second is to get Ronchi eyepiece:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

Ty for this. I meant more how would I know from looking at it? Big aperture shorter tube length would suggest not spherical/catadioptric? Or the opposite? Or neither

me trying to collimate it would be something I'd only try on a non spherical newt? Only for  the possibility of imaging with it.

Edited April 24 by TiffsAndAstro

[vlaiv]

10 minutes ago, TiffsAndAstro said:

Big aperture shorter tube length would suggest not spherical/catadioptric? Or the opposite? Or neither

Catadioptric has refractive glass element in optical train next to mirrors - usually as front aperture corrector - making tube closed. Sometimes this element sits inside focuser or in front of secondary - but then you'd have very short tube (Bird-Jones design) or Ruten Maksutov type (but you have newtonian, so it's not that).

If it's a longer tube - greater chance that it's spherical mirror. Fast telescopes and spherical mirrors don't get along. If it's F/8 or slower - then it might be spherical.

 

[TiffsAndAstro]

3 minutes ago, vlaiv said:

Catadioptric has refractive glass element in optical train next to mirrors - usually as front aperture corrector - making tube closed. Sometimes this element sits inside focuser or in front of secondary - but then you'd have very short tube (Bird-Jones design) or Ruten Maksutov type (but you have newtonian, so it's not that).

If it's a longer tube - greater chance that it's spherical mirror. Fast telescopes and spherical mirrors don't get along. If it's F/8 or slower - then it might be spherical.

 

Apologies, I was thinking catadioptric was another term for a spherical mirror

If I find a very cheap one locally I know what to look out for now ty

 

[Louis D]

Spherical mirrors at f/8 and slower tend to be very close in figure to a true parabola, so the images can still be quite good if the price is right.

[Xilman]

On 24/04/2024 at 17:50, vlaiv said:

If it's a longer tube - greater chance that it's spherical mirror. Fast telescopes and spherical mirrors don't get along. If it's F/8 or slower - then it might be spherical.

 

Not without corrective elements, they don't.  My  0.4m Dilworth contains nothing but spherical surfaces. A train of spherical lenses corrects for spherical aberration. Not sure of the focal ratio of the primary but guessing from the length of the OTA it is probably around f/2.5.  Take a look at http://www.astropalma.com/equipment.html to see what I mean.

For moderate focal ratios a simple meniscus lens will give most of the correction needed.

For photometry it may not matter anyway, as stars are generally defocused to spread the light over a larger number of pixels, thereby reducing the effect of pixel-to-pixel variations in sensitivity. My Beacon Hill 18" Dobsonian worked just fine as a light bucket for my (visual) variable star work, despite having very noticeable spherical aberration.

Horses for courses.

[vlaiv]

2 minutes ago, Xilman said:

Not without corrective elements, they don't.  My  0.4m Dilworth contains nothing but spherical surfaces. A train of spherical lenses corrects for spherical aberration. Not sure of the focal ratio of the primary but guessing from the length of the OTA it is probably around f/2.5.  Take a look at http://www.astropalma.com/equipment.html to see what I mean.

For newtonian with single curved mirror - it is true that spherical aberration increases rapidly with faster optics.

https://www.telescope-optics.net/reflecting.htm

For paraboloid - it's equal to 0 but for spherical with K=0, we can see that it is inverse of third power of F/ratio, so telescope has to be really slow, or have small diameter.

[Xilman]

4 minutes ago, vlaiv said:

For newtonian with single curved mirror - it is true that spherical aberration increases rapidly with faster optics.

https://www.telescope-optics.net/reflecting.htm

For paraboloid - it's equal to 0 but for spherical with K=0, we can see that it is inverse of third power of F/ratio, so telescope has to be really slow, or have small diameter.

I don't doubt it. That's why I mentioned corrective elements. A Newtonian can be, but rarely is, corrected for SA with lenses in the same way as its coma (absent in a spherical mirror) can be and usually is in fast systems.  On the same site as my Dilworth is a 0.5m f/3.5 Newtonian astrograph which absolutely requires a coma correcting refractive element.

[vlaiv]

3 minutes ago, Xilman said:

A Newtonian can be, but rarely is, corrected for SA with lenses in the same way as its coma (absent in a spherical mirror) can be and usually is in fast systems.

Well, people do have good opinions on telescopes like Maksutov Newtonian and Schmidt Newtonian. Granted, those are full aperture correctors and not sub aperture correctors, but can be quite fast systems as well- often F/4-F/5.

[Xilman]

Just now, vlaiv said:

Well, people do have good opinions on telescopes like Maksutov Newtonian and Schmidt Newtonian. Granted, those are full aperture correctors and not sub aperture correctors, but can be quite fast systems as well- often F/4-F/5.

Good point.  I was thinking of post-primary correctors rather than pre-primaries such as Schmidt and Maksutov correctors.

Note what I said about a meniscus lens. It describes  the important characteristic of a Maksutov corrector. Wikipedia has a good article on this topic, including the existence of sub-aperture correctors.

 

[davidc135]

On 24/04/2024 at 17:55, TiffsAndAstro said:

Apologies, I was thinking catadioptric was another term for a spherical mirror

If I find a very cheap one locally I know what to look out for now ty

 

The vast majority of Newts unless they are tiny or very slow will be or should be parabolised. But whether they are parabolised or have been left spherical, all will need to be collimated to perform well.

As I understand it, spherical mirrors have the same coma as equivalent paraboloids if the aperture stop coincides with the mirror. 

David

Edited April 27 by davidc135

[TiffsAndAstro]

1 hour ago, davidc135 said:

The vast majority of Newts unless they are tiny or very slow will be or should be parabolised. But whether they are parabolised or have been left spherical, all will need to be collimated to perform well.

As I understand it, spherical mirrors have the same coma as equivalent paraboloids if the aperture stop coincides with the mirror. 

David

I'm just thinking no harm in me practicing collimation if I come across something very cheap before buying something proper.