Dob Focusing Issues

[brenski]

Sky-Watcher Flextube 300P 12" SynScan GoTo Collapsible Dobsonian Telescope With Freedom Find

Having trouble with focusing. I can focus with a 15mm 2" eyepiece, but 8mm is a struggle - and only (just about) doable.

As you can see from the 2nd image - i've around ½mile of extension tubing attached to the tube just to be able to focus at 8mm!

Ideally, I'd like to add a 2x barlow so that I can see some nice detail of Jupiter and Saturn. Currently the bands on Jupiter are (just about) visible, but my understanding is that the view should be bigger/clearer. How do i achieve this? Which 2" extn tube is the one to use to be able to focus my 8mm with a 2x barlow?

[Cornelius Varley]

Is the telescope fully extended ?

[bosun21]

As has just been mentioned when extending the tube there's two positions the first one is for to allow cameras to reach focus and the second is fully extended for visual. This is why the Flex tube scopes have an oversized secondary mirror to compensate for this.

[brenski]

thanks, will check

[brenski]

tube extended another 3" or so - from its first click position. should this now focus correctly? how much of the extra extn tubing can i now remove?

[Cornelius Varley]

10 minutes ago, brenski said:

how much of the extra extn tubing can i now remove?

Probably all of it.

[brenski]

even with a barlow?

[Ratlet]

10 hours ago, brenski said:

even with a barlow?

Probably.  I've never needed to add an extension to an eyepiece to use it with a Barlow.   AFAIK the Barlow moves the euereliefe out not the focal point as much

[John]

You will find that the Hyperion 8mm (and the other Hyperions I think) work best using the 1.25 inch part of the eyepiece barrel. Using the 2 inch section directly into the focuser drawtube moves the focal plane of the eyepiece inwards significantly meaning that you have to rack the focuser even further out to reach focus. 

Using a 2x barlow with the 8mm eyepiece will give you an effective 4mm eyepiece. This means 375x magnification in your scope which will be too much for Jupiter and probably even for Saturn unless the seeing conditions are excellent and the collimation and cooling of the scope spot on. While the image scale of the planet will be larger, contrast details (eg: Jupiter's belts, red spot etc) get washed out when over-magnified.

With a 300mm scope Jupiter is generally best observed at 180x - 225x I have found. Saturn can usually benefit from a bit more magnification so 250x - 280x maybe. 

The 8mm eyepiece on it's own is probably ideal for Jupiter. Maybe a 6mm would be worthwhile for Saturn ?

A low cost way of gaining a 6mm eyepiece is to use a 14mm Baader Fine Tuning which will change your 8mm Hyperion into an effective 6mm when the FT ring is fitted. These are rather fiddly to fit but might allow you to experiment with higher magnification without undue additional cost:

 Baader Hyperion Finetuning Ring | First Light Optics

 

Edited November 1, 2023 by John