Barlow lens with 10mm

[Orion90]

HI all,

Quite new to this - I am consistently having trouble focusing using a 10mm lens with a 2x Barlow on my skywatcher 130 often see what looks like my own eyelashes ha. When I use the Barlow with a 25mm it seems ok. When I use the 10mm on on its own then that is ok too.

Any thoughts?

Mark

[ghostdance]

Using what is effectively a 5mm ep requires good seeing and transparency. Higher magnification ep's can often look soft and, er, un-snappy compared to lower mag, especially when Barlowed but often without (I often can't use my Vixen LV 4mm unless my skies are exceptional, rare where I live). Focus point can also be harder to achieve unless conditions are really good and the 'point' itself is very fine ie fractional. To some degree it can depend on what ep it is, eye relief, contrast etc. Sounds like you may be having to keep your eye very close to the eyepiece, so short eye relief could be affecting your 'comfort' while using the ep. Then, at high magnifications, you may be experiencing the pesky 'floaters' dead cells within your eyeballs that love swimming about when we decide to get the low numbers out!

Plus, you may be pushing the scope to its max in terms of magnification (a 130 would be around 240x ish? Someone better qualified than I will be able to answer this).

Try over several nights, and also watch how the image changes over time at the eyepiece, as the pockets of clearer atmosphere come and go. A longer time at the eyepiece should show some changes in the detail you're able to discern (MYWMYS: more you watch, more you see )

Keep at it! It's fun, honest

[Dave In Vermont]

Okay, let's look at your telescope: It's an F/6.92, according to the manufacturer, with a focal-length (F.L.) of 900mm. It's primary-mirror has a diameter of 130mm. Now we can use any of several schools of thought to determine how much magnification this set-up is capable of supporting. The high-side is 2X the aperture - which gives the figue as 260X. And that is quite high. Another school-of-thought is 1X the aperture - which gives us 130X. Rather low, but could still present a challenge unless the seeing-conditions are very favorable. A discussion of 'seeing' can be found elsewhere in these forums.

I would say, on a personal note, that somewhere in the middle is the likely number with very good 'seeing.' Now how do we find the magnification a given eyepiece will give you? Very simple: Divide the focal-length, in millimetres, by the FL of your eyepiece. So a 10mm eyepiece, divided by 900, gives us - 900 / 10 = 90X.  So add the 2X Barlow, and you have 900 / 5 = 180X. And that's right about in the middle of the high and low maximum theories.

Your mileage may vary.

Clear Sfies & Seeing,

Dave

[Pig]

Alternatively you could remove your eyelashes

On a serious note, I would have thought your scope should be ok at 180 x magnification, maybe the Barlow you are using is not up to the job.

Have you tried a 5mm eyepiece on it own to see if you get the same soft view ?

[ronin]

To throw even more into the pie   The 10mm eyepiece is generally not great neither is the barlow so combining both really drops the quality.

5mm for an eyepiece seems to be where the good ones and the mediocre ones part company, making a short focal length objextive is more trouble on a scope so is a short focal length eyepiece. They do operate on the same principles.

Will say a barlow does not reduce the eyepiece, it extends the scope focal length. So we are discussing things incorrectly.

If your scope is the 130/900 then the mirror is spherical so that procuces a not so good image.

So when you throw everything into your setup it will take a fair bit of luck for everything to come out good.

I would ignore all values for maximum magnification, to an extent it means little, often it amounts to how bad an image is before you give up, and that is different for you and me. I have seen good sharp views of Jupiter at 50x and awful ones at 150x. The forst in a 70mm refractor the second in a 14" SCT, one night apart and under what I would say was identical conditions. Do not mistake a big image for a quality image.

There are some good priced 8mm plossl's around, if you can, and want to, get one, should do a good job.

You are giving Leeds as a location if you can get over to Green Witch near you:

3 High St, Birstall, Batley, WF17 9ES

Have a talk to Lee.

[Chronicler11]

 The 10mm eyepiece is generally not great neither is the barlow so combining both really drops the quality.

Ronin,

could you please elaborate somewhat on this? What do you mean the 10mm eyepiece is generally not great?

You're referring to a specific eyepiece? The SW 10mm that comes with the telescope maybe?

I can understand the dropping of the quality when combined with a barlow, (any barlow?) but on its own?

Thanks,

Alex 

[laudropb]

The 10 mm ep supplied with your SW is usable but not that good. I could not barlow mine and get a reasonable image, even when seeing was good. I replaced mine with the Hyoerion zoom lens and the difference was amazing

[Chronicler11]

Ok, so as expected we're specifically talking about the 10mm eyepiece that came with the dob.

Sorry about that, English is not my native language so maybe i misunderstood what i read.  

laudropb, thanks for the reply! I'm waiting for my replacement eyepiece (a Vixen SLV 10mm) by the end of the month.