Insight into focusing

[Athena]

Hello everyone

I don't have any problems focusing using my 25 and 20mm eyepieces but find it real hard to hit critical focus using a 7mm eyepiece.

Tonight I was observing Jupiter and with my 25mm eyepiece focusing was very easy but anything above 15mm was a real problem...

Can anybody offer any words of wisdom?

Many thanks

Nikki

[moondog]

Nikki

If you were using the 8 ins SCT with the 7mm you would have 285x magnification.

Jupiter at present has a very low altitude. You would be looking through the densest part of the Earth's atmosphere. The magnification you were using would be too great for the seeing conditions. Probably about 150x was the max. you could have managed.

To use over 200x magnification Jupiter would have to be at a far greater altitude.

MD

[philsail1]

Hello "Nikki."

Well, there is no magic formula where achieving perfect focus is concerned, as there are so many "variables" involved - one's eyesight, the seeing conditions on the night, the type, quality and mechanics of the telescope's focuser, the size of scope for the power of eyepiece being used, the actual quality of the eyepiece. Also, the higher power (or shorter focal length) eyepiece one uses, the more critical the focusing becomes - with any telescope (although some are vastly better than others!).

The very design of some telescopes seem to have some small inherrant focusing difficulties built in! (large aperture SCT scopes with their rotary focusing mechanisms).

In my opinion, a "Rack and Pinion" type focuser will always be the easiest to use, and easiest to achieve a sharp focus. The "in" and "out" movement of this type of focuser is more natural to use.

The simple "Rack & Pinion" focuser has been vastly improved upon with the introduction of "Crayford" focusers. These are very "smooth" and precise in use, and are the very best type of focuser to give you the sharpest of focus for any given eyepiece. Some have double speed ratios, where you first focus "roughly" (which will give you a good enough focus anyway!), then use the 10 to 1 secondary ratio to achieve a very sharp focus. As you know, Crayfords, are expensive, but if you can invest in a good one, it is money well spent.

With regard to the best way to use any focuser. I beleive the military use a method whereby the sharpest focus is achieved by moving the focus from a wildly out of focus position, gradually "inwards" until the best focus is achieved. If you pass the focus point, run the focus back "out" well beyond the out of focus position, and gradually move it back inwards again until the best focus is achieved. Try not to "play" the focus wheel in and out, as you will not be giving the scopes mechanism, or you eye, time to accomodate the rapid fine movements in both directions. Also, you might find it better to keep both eyes open when focusing, to reduce the muscular strain around the eyes when squinting into the eyepiece with one eye. Aslo, achieving a good focus is as much about getting to know how your particular scopes focuser behaves when you use it.

Lastly, the better quality eyepiece you can afford, generally make it a little easier to achieve that sharper focus.

Oh! and finally, finally! Don't "push" your scope beyond its optical limits in trying to get huge magnifications. Even the best scopes have their limits. To push beyond that limit, is simply wasting money, and makes for uncomfortable and stressfull observing.

Regards,

philsail1

[Capricorn]

As Nikki said Jupiter is low so you get atmospheric distortions.

Also SCT's do not give the contrast that refractors do, it is a trade off of getting the aperature.

As the eyepiece FL gets less then then everything has to be more and more exact. There simply isn't the required accuracies in the system to get a good image. Short focal length eyepieces have small tolerances and the scope FL will have errors that are too much.

[kniclander]

if the focusser is too tight, that can also make it difficult. On an Newt you can usually adjust it with little grubscrews - don't know about a Cat but mut be adjustable...

[brianb]

Tonight I was observing Jupiter and with my 25mm eyepiece focusing was very easy but anything above 15mm was a real problem...

Perfectly normal, Jupiter stands high power worse than most objects; when it's low down that just makes things worse; and hot days & short nights don't help at all especially in an urban/suburban setting, when heat from the sun is retained in brick, concrete & tarmac structures & released into the night air causing extra turbulence.

7mm is almost always far too much on a f/10 SCT, whatever the subject, however well it is placed and however steady the air is. If you're getting a satisfactory image of Jupiter with 15mm you're actually doing pretty well.

[brianb]

the better quality eyepiece you can afford, generally make it a little easier to achieve that sharper focus.

Sorry but this is more or less a red herring. More expensive eyepieces may have a wider, flatter field but practically anything is OK in a f/10 scope when at the centre of the field ... in fact the simpler types usually work just as well if not better on planets than complex EPs with 10x the price tag as there is a tendency to less scatter & internal reflection.

[Psychobilly]

A larger focusing knob makes making smaller adjustments easier... or simply using a clothes peg on the existing knob as a quick fix...

Hands off focusing using a motorised unit also helps a lot as if your trying to focus an high magnifications then the tiny movement caused by touching the scope also add to the difficulties.. its really a case of making small tweaks and letting the scope settle again to see if you have made an improvement.. SCTS can also have quite a bit of backlash in the primary mirror focusing and the direction of approach can also make a difference...

Anything low down is going to be a nightmare though...

Peter...

[Whippy]

Another thing you may want to check is the scopes' collimation. If that's not right, then it'd show in the quality of the view.

Tony..

[moondog]

Nikki

An 8 ins SCT is a fine instrument for looking at the planets. The standard focusser works well. Check that the rubber focus knob is not catching on the back of the SCT and pull the rubber sleeve out slightly if it is.

It might not have quite the image contrast of a good APO refractor but it's not as bad as people make out! After all the majority of images of planets posted on astro sites tend to be taken with SCT's.

Do you know if your SCT is properly collimated? At high levels of magnification this makes a huge difference to the level of detail you can see.

Must have posted my reply at the same time as Tony's!

MD

[Whippy]

[SteveP]

Hi Athena

Do you mean you always have trouble focusing the 7mm on both of your scopes or is the issue particular to either one scope or one type of object, eg Jupiter at low altitude?

I suspect that if it is with your SCT it's because with its focal length of about 2000mm, a 7mm EP is just too powerful (>280x) for most observing in the UK and especially at low altitude. I'd be very surprised that your SCT itself is the cause. If it's just when used with your newt, I'd be looking at whether the scope needs to have its collimation checked.

Steve