Celestron Illuminated RACI 9x50 finderscope - trouble centering objects

[rjkh]

I recently acquired a Celestron Illuminated RACI 9x50 finderscope after nearly breaking my neck trying to use the supplied red dot finder that came with my Celestron Nexstar 8SE scope.

The package was pretty beat up when it arrived, but the initial daytime alignment went well, the unilluminated cross-hairs very clear and easy to center, and everything seemed to work fine.

Since the cross-hairs can't be seen in the dark, that night I turned on the illumination, and that's when the trouble started. I had a hard time getting the illuminated cross-hairs in clear focus by turning the knob, but good enough I guess, except that the center of the cross-hairs seemed to jump around with just the slightest movement of my head, and it was really hard to judge where exactly the center was.

So, is it just my old geezer eyes, or is there just some trick to it that I'm missing, or could it have been damaged in transit somehow, and not working correctly?

Has anyone else had this kind of difficulty, or is it just me?

Thanks for any help...

[Paz]

I have the same difficulty, the cross hairs don't adjust to eliminate parallax issues like say a rifle scope can be adjusted. I just try to  keep my eye position centred and consistent by using the field stop as a reference and making sure I can see the whole field stop equally.

[rjkh]

Thanks, Paz.

Good to know it's not just me.

Just have to learn to deal with it, I guess, using the field stop to help keep centered, as you said.

[AlexK]

The reason for that parallax effect is bad focusing.

When the starry background and crosshairs are in the focal plane it's impossible.

However, there is also the spherical aberration in the equation. When you are focusing stars, you should focus them as close to the crosshairs' center as possible. To rule out any wrong accommodation effects, it's better to defocus crosshairs as much as possible first, then focus stars by approaching the focus from an infinity side (shortest). Finally, adjust the crosshairs focus (also from an infinity), so stars and the cross are in focus simultaneously near the center.

[rjkh]

So, AlexK, are you saying that this problem can be entirely eliminated by refocusing the finderscope itself by loosening the front locking ring and rotating the front lens with the crosshairs defocused, and then refocusing the crosshairs?

I thought I'd already done all that when I set it up, but maybe not quite accurately enough? Is that what you're saying?

Do you use this same finderscope, and do you have clear and distinct ILLUMINATED crosshairs that do not jump around all over the field of view?

[rjkh]

So, tried last night, but no joy with refocusing everything.

Turning the knob that turns on the illumination makes the crosshairs brighter or dimmer but doesn't really seem to do much to focus the crosshairs.

It's almost easier to just leave the illumination off, and estimate the center of the field.

 

[Radec]

Sorry if this is a daft question, what are you using to focus the crosshairs?

[rjkh]

The only knob that seems to be available for focusing the crosshairs is the knob that turns on the illumination and controls the brightness, but it seems to have minimal effect on actually focusing the crosshairs.

In daylight, with illumination turned off, the crosshairs are of course crystal clear.

I had assumed that focusing the finderscope itself using the front lens would automatically focus the crosshairs as well, and in fact appeared to do just that in the daylight. Turning on the illumination seems to just screw everything up.

[Merlin66]

If the X wires are in good focus ( distant object) during the day they should still be in focus at night - independent of the illuminator.

 

[noah4x4]

On mine the eyepiece screws in and out to reach best focus and cross hair clarity. I have to unscrew it a long way. If screwed in too tight the view is poor. But is great when adjusted. I found battery power is also crucial. If low, the cross hairs do tend to be fuzzy. I found that I needed to replace the batteries after every (say) four hour session if I forgot to switch it off after use. Tip; you can get cards of suitable (assorted) batteries for £1 at Poundland.

[Radec]

I'm with Noah here - the crosshair focus is usually the EP end - overall focus the objective end. I tend to focus the crosshairs after the star or objective focus gives a clear view. Turn the red illumination down quite low too as I find in some (not all) systems, the presence of the light itself makes the crosshairs appear a bit fuzzy. Having said all this, the actual pin sharp requirement is limited IMO - the idea is to know the telescope is pointing at the correct spot. Good luck

[rjkh]

Partly unscrewing the eyepiece was one of the first things I tried, but maybe I just didn't unscrew it far enough.

Can't wait to try it tonight!

 

[AlexK]

On 07/03/2019 at 12:42, rjkh said:

So, AlexK, are you saying that this problem can be entirely eliminated by refocusing the finderscope itself by loosening the front locking ring and rotating the front lens with the crosshairs defocused, and then refocusing the crosshairs?

I thought I'd already done all that when I set it up, but maybe not quite accurately enough? Is that what you're saying?

Do you use this same finderscope, and do you have clear and distinct ILLUMINATED crosshairs that do not jump around all over the field of view?

Yep, your question has been answered already above: stars by the front thread, cross by the EP thread.

Yes, I dealt with many illuminated eyepieces in finders, telescope and microscope EPs (but nowadays I prefer Telrad for pointing my Dob and rarely using the optical finder; by the way, my current finder has no illumination, as it's not needed in grey/dark LPZs I'm exclusively observing from, as the sky background is perceived bright enough with pitch black crosshairs). That's a known problem, especially in high power eyepieces. 

The reason why illumination makes the crosshairs fuzzy is the mechanical construction of the illuminated crosshairs. Usually, it's a piece of glass or plastic with the reticle lines carved-in from one side (usually eye-side) to some depth. The light is shining from the side and reflecting from the walls of these deep "canyons" into your eye. Walls are of a non-zero height so some of their parts are significantly away from the focal plane creating "ghosts" susceptible to the parallax. It's hard to fight that unless you just accept that flaw and try to amend it by visually finding the brightness at which you can see a straight edge of the "canyon" to use for focusing and aligning, and later for the pointing. Some "canyons" have the center ridge, some are only fully focusable on their rims.

By the way, not lighted reticles are just painted on the glass, so almost not susceptible to the problem. Also, they are a good indicator of your darkness adaptation: if you can't see them against the sky, then you are not well dark adapted yet, so better to focus on that problem first.  In a worst case, when you can't dark adapt well enough due to the light pollution use the finger trick: put your bare finger in front of the finder's objective end, the AFOV background will be highlighted with it enough due to the light pollution (just make sure the finger is exposed to the surroundings, not inside the finder ) and reveal the crosshairs shadow.

Also, you can try to find the best reticle focusing spot by minimizing parallax instead of trying to focus on the crosshair features. Just remember about the SA, do that with focused stars in the view near the center of the crosshairs.

Finally, at any rate, I wouldn't care too much about the finder's accuracy as soon as you can get the target in the main EP reliably with it. 

[rjkh]

Wow, what a difference!

I had to unscrew the eyepiece about half way out to get those crosshairs focused, but that changed everything. I am now totally in love with my new finderscope.

Thanks to all for your fine advice and help.

Thank God for forums like this, especially when there's no instruction manual.

Clear skies!