Guiding with diagonal

[tooth_dr]

I’ve always used a diagonal to get focus with my guide camera on my ST80, I see others don’t have one. What is the way around this?

 

Cheers!

[Cornelius Varley]

Replace the diagonal with a suitable extension tube.

[tooth_dr]

Is there a specific length in these that is needed?

[vlaiv]

You can measure, or calculate the length of extension tube you would need. If you want to replace diagonal, just lookup light path for that particular diagonal and use similar length of extension tube.

Otherwise just measure focus position with piece of paper and moon and just see approximately how long extension tube you need.

You can even just guess. Take a look at this article:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

there is table of light path lengths for different diagonals, just use value for one that is the most similar to yours (really there is like average value for each of 1.25" mirror, 1.25" prism, 2" mirror and 2" prism and you won't go wrong if you use this average value because focuser travel will compensate for +/- 10mm)

[tooth_dr]

2 minutes ago, vlaiv said:

You can measure, or calculate the length of extension tube you would need. If you want to replace diagonal, just lookup light path for that particular diagonal and use similar length of extension tube.

Otherwise just measure focus position with piece of paper and moon and just see approximately how long extension tube you need.

You can even just guess. Take a look at this article:

https://www.cloudynights.com/articles/cat/articles/mirror-vs-dielectric-vs-prism-diagonal-comparison-r2877

there is table of light path lengths for different diagonals, just use value for one that is the most similar to yours (really there is like average value for each of 1.25" mirror, 1.25" prism, 2" mirror and 2" prism and you won't go wrong if you use this average value because focuser travel will compensate for +/- 10mm)

It's just the stardard diagonal that I got with the ST80.  Is it better with focuser in more and longer extension tube, or the other way around?

[vlaiv]

It really depends. In general I think for most focusers you should aim to have it racked out about 10-15mm from inward most position.

Depending on quality of focuser you want the "zone" where there is the least play / slop in it. Some focusers have a lot of play when fully racked out, some are not precise when fully in. I think that most are at "the best place" somewhere around 10-15mm. Also it depends on extension tube. It needs to be of appropriate quality not to tilt - it can tilt on connection to focuser because of the way focuser clamps the tube, or it can tilt on the other end - because the way extension tube clamps camera / eyepiece. Level of tilt depends on length of tube (well ratio of length to width of tube, I guess would be more accurate).

So if high quality focuser - choose short extension, if low quality focuser, try to rack it out just a bit and use longer extension tube. For best results don't use classical extension tube, but rather screw in extension if your focuser has some sort of thread. I believe ST80 has T2 thread, and camera probably also has that sort of thread, so something like 50mm T2 extension would be good idea - this is for imaging. For guiding you don't really care that much if star shapes are not ideal, so any sort of extension should work provided it is clamped solid to avoid differential flexure in that part of guide train.

[tooth_dr]

I'll check this out later when I get home, but a screw on one would be a good shout.  Thanks for all your advice.

[ollypenrice]

Lots of people just use an old Barlow body with the lens thrown out. With my Lodestar I need two of these which I Araldited together.

Your chip will need to be about 14 centimeters behind the rear-most part of the tube. (That's the back of the casting which carries the focuser knobs etc.)

Another thing to watch on the ST80, which is a great guidesope, is the tightness of the three screws holding that casting into the painted main tube. Give them a tweak to be sure and be firm with the drawtube lockscrew to avoid flexure.

Olly

[MarsG76]

An extension tube would solve this, but if it works for you, what wrong with using the diagonal?

[alacant]

Hi. It depends where the sensor is on your camera. A 10cm t2 extension tube works for a zwo. HTH.

 

[tooth_dr]

15 minutes ago, MarsG76 said:

An extension tube would solve this, but if it works for you, what wrong with using the diagonal?

I guess just to have it neater, and the lead comes straight out the top of the camera and a lot of dew settles around it.

[tooth_dr]

4 minutes ago, alacant said:

Hi. It depends where the sensor is on your camera. A 10cm t2 extension tube works for a zwo. HTH.

 

Sensor is close to the surface of the camera, will measure later.  DMK21

[Mered]

As a Synguider user, I use a diagonal as it puts the guider screen in a better position than using an extension tube.