Tips for swapping EPs once you've tracked

[ezerasurfr]

Ok, so finally last night was a clear sky! And for the first time since I've had my scope the moon was out. So I went straight for it with the 26mm EP.

I lined up my finder scope, got the moon in the cross-hairs (I have a redot finder with 4 settings) and was spot on with the moon. The clairty was great with the 26mm, but I wanted to see 'deeper', so swapped in the 5mm.

This was my first time using the 5mm. As soon as I began swapping the EPs, the scope moved. I thought to myself, "no big deal. I'll just re-align with the finderscope." This proved WAY more difficult than expected. I targeted the moon with the finder, but the 5mm just didn't pick it up. It took so long to hone in on the moon. Swapping EPs back and forth, and trying to get the best method for swapping without disturbing the scope. When I moved onto Jupiter, this proved SO much more difficult. Tracking Jupiter's movement across the sky did not seem that challenging (at least not as tough as I expected it to be), but just getting it into view with the 5mm is proving so much of a task.

Does anyone have any tips they can share regarding swapping EPs?

[acey]

Don't make the jump too big - you went straight from 26mm to 5mm, and it's easy to lose the target that way. Would be better to have an intermediate EP at say 10mm. With a dob, high-power tracking is easiest if you use a wide-field eyepiece: with my 12" dob my highest powers are a 6mm TMB planetary (60 degree field) and a 4mm Nirvana (80 degree field). I'm fine with smaller apparent fields at lower powers, where things move slower.

[michael.h.f.wilkinson]

Alignment with a 26 mm is not that accurate. I tend to start with my 31 for coarse finder alignment, and then step up to a higher mag, and refine alignment, right up to the 10mm (203x on my scope: once you get things centred there, you are fine). I do of course use a 16x70 finder, not an RDF, so it is easier to align accurately.

[ezerasurfr]

So then, alignment is really my problem then?

My biggest concern has to do with "how to not move the scope?" when switching EPs. Should I negate this question and regard my problem more with getting better alignment?

[Capricorn]

Guess you may have to align finder with the 25mm then repeat with the 10mm.

Next owuld to consider eyepieces that are classed as parfocal - the BST are, the X-Cel's are, as well as TV plossl's. Not sure if the Vixen plossl's are (have not read about them much).

I find many of the Celeston/Skywatcher/Meade kit itemes are not.

As the field of view decreases as you push the magnification up then you have to have the object in the centre and that is not easy to determine. You then have to be both fairly quick and very smooth with the swap over. If you have to refocus (the reason for parfocal) then realistically little chance of something still being there.

It is part of owning a dobsonian, things move, you have to do the locating and the tracking, it doesn't. One of the things of a goto/tracking mount with motors, locate whatever, centre it go away, come back, it has a good chance of still being there.

[michael.h.f.wilkinson]

So then, alignment is really my problem then?

My biggest concern has to do with "how to not move the scope?" when switching EPs. Should I negate this question and regard my problem more with getting better alignment?

Hopping from 26 to 5mm means your FOV drops dramatically, and what seems to be dead centre in the 26 might be on the edge or just beyond otherwise. Moving to e.g. a 10mm reduces this risk, and you can recentre the object in that narrower FOV.

My EQ mount (motorized) does not seem to shift when swapping EPs (if the clutches are locked!), but a dob can do so. I tend to hold on to the focuser with one hand while inserting the EP with the other. I too occasionally nudge object out of the FOV while swapping.

[ezerasurfr]

Thanks for all the advice guys. I think I'm going to realign this afternoon to get my finder spot on.

So it seems like it's really a practice makes perfect issue I'm having then.

(both my EPs have 70 deg FOV, so I'm only stepping down focal length.)

Theoretically, I would run into the same issue going from 10mm, to 5mm (or even from 26mm to 10mm).

[acey]

Alignment is only part of the issue - I think the main one is stepping up the magnification too suddenly. The other issues are object drift, moving the scope, and possibly the position of the eyepiece in the focuser (i.e. not being exactly parallel to the optical axis, but instead being fractionally tilted). All of these become an issue at high magnification but are barely noticeable at low magnification. What you really need is a medium power eyepiece.

You're increasing the magnification by a factor of 5.2. The difference between my lowest eyepiece and my usual highest is 32/6 = 5.3. I never make that jump in one step - I would expect to lose the object. I'd maybe do it on a bright target like Jupiter that I can sweep back on to, but for a DSO it would be hopeless. I use a zoom for the intermediate rangei; if I were using fixed focal-length eyepieces I would have at least one and more likely two to fill in the gap.

[ezerasurfr]

Ok, I get the picture guys. Thanks. I just hate having the answer being "spend more".

[brantuk]

The other angle of course is that you might not have enough "stiction" on your axes if the dob moves so easily when just changing eyepieces. This could be down to the materials used on the Alt and Az - or the tightness of the tension bolts - and any wear on the mountings (depending on age of the scope).

[acey]

Try aligning the finder first and if that works well enough for you on the sort of targets you like looking at then you're done. But a 5mm EP is very much at the top end of what you'd expect to use with your 10", and in the long run you're going to want to have something at medium power, so that should be the next thing to think about whenever you get round to expanding your collection.

An 11mm eyepiece would mean you'd do the jump in roughly equal amounts: a factor of 2.4 magnification increase going from 26mm to 11mm, then a factor of 2.2 going from 11mm to 5mm. Very wide field wouldn't be necessary for easy tracking. The TeleVue 11mm plossl is a lovely eyepiece and would do the job perfectly.

[ezerasurfr]

Thanks again for all the advice. I aligned the finder yesterday afternoon, and tightened the tension bolt at the base. Viewing was a pleasure last night, and tracking was very easy. Thank you all so much! I will look into a medium mag EP.

[Moonshane]

Kim beat me to it. a dob should not move between eyepiece changes if balanced and set correctly. assuming your finder is aligned then I'd expect you to be able to find the moon with your highest power eyepiece in the focuser from the off. sounds like you have it sussed though

[CSM]

If you have a star in focus with a low power eyepiece and think that you have it centered then almost certainly you haven't. It is actually quite difficult to do. The best way to do it is to defocus it so as to make it a very large circle and centre that. If you then change to a medium power and do the same you will probably find that you can centre the high power lens. 26mm to 5mm was certainly too big a step.

[MorningMajor]

This is where zoom EP's have a great advantage as you can adjust the position of your object with each increase of magnification so you never lose it

[acey]

This is where zoom EP's have a great advantage as you can adjust the position of your object with each increase of magnification so you never lose it

I agree - that's one reason why I use a zoom. Bright targets can easily be re-acquired if they go out of field - but small, faint fuzzies are easily lost when swapping eyepieces, especially at high power.