FirstScope 76 issue with altitude axis

[kvmx (Nick)]

Hi all,

I have recently had an interest in gazing upon my magnificent bortle 8 night sky and have been using a pair of old Nikon 10x25 binos to much, much enjoyment. I also found my father's old Celestron FirstScope 76, which I've tried out but I'm having trouble using. And not just because there's no finder on it.

Azimuthal traversing is fine, no issues there. The design of the mount has a tightening nut on the right side of the OTA, and I can dial it in to a perfect resistance for altitude traversal, not too tight, not too loose. The trouble is traversing higher tightens the resistance immovably so and traversing lower loosens the resistance so much it won't support itself. I can (barely) use the scope with both hands, left hand traversing Az./Alt. and right hand adjusting resistance on the fly. It's all a bit much for me with my beginners experience trying to traverse, adjust resistance and identify what I'm seeing through the eyepiece all at the same time. Intensely frustrating.

Anyone have any ideas for some modifications easily available at the local Bunnings? Is it even possible to fix? Do other dob scopes suffer the same issue? Maybe I'm just being a baby. I like the idea of using an actual telescope but unfortunately the only equipment I have is a pair of binos and this irksome FirstScope 76. I'm in two minds whether I should spring for some nice 10x50's now or save for a better scope. However I'm worried I will encounter this same issue regardless of the dob scope that I get.


Any input is greatly appreciated, thanks!

[iborg (Philip)]

Hi Nick

I was hoping someone with the same scope would be able to help.

I have a similar scope, but, have not had your issues.

An idea for something to check, look to see if there is washer or two, along with the setting circle and knob. See the attached photo.

Also, check that everything is clean.

Good luck

Philip

[Pierre_C]

Welcome, Nick.

I do not have experience with this telescope, but I wonder if there may be some age-related binding between the scope and the lock nut, which tightens and loosens with changes in altitude. I wonder if cleaning, lubrication, washers or Teflon pads may help to reduce the friction contributing to the issue.

From the design, it appears to be a top-heavy scope which may be prone to falling forward. An option may be adding weight to the bottom end of the tube for better balance so that you are fighting less with gravity. You could try adding weights to re-balance it at a 45-degree angle in such a way that the scope does not fall forward or backward when the lock nut is fully loosened.

Apart from that, the manual (p.5) seems to suggest that coarse movements are done after loosening the lock nut and then tightening it afterwards; and then finer movements are done with the lock nut mostly tightened: https://s3.amazonaws.com/celestron-s...scopemanua.pdf

I would not let this dissuade you from purchasing a larger Dobsonian telescope. The larger versions with axis attachments on both sides of the tube tend to do a lot better in this regard with better balance and tensioning systems.

Keep persevering!

[Pierre_C]

Oh, I realised after I posted that the weight of the primary mirror may somewhat offset the top-heaviness of the focusser. The principle remains the same though - adjusting the distribution of the weight of the tube to balance it evenly on the axis may help to reduce the pressure needed on the lock nut, and hence the amount of problem you are experiencing.

[Dekker (Derrick)]

Hi Nick,


I have the same scope which is also afflicted by the same problem where the tension changes when you change altitude. It may be partially self-inflicted; I think I messed up the order of the washers / spacers at some point in the past after taking the optical tube off the mount and then re-assembling.


I was able to improve the situation (but not completely resolve) by re-ordering the washers and plastic spacer. I will take some photos when I'm home and get the scope out of the cupboard.

Edit: Added pictures.
Other (better) Dobsonians do not have this problem. I replaced my FirstScope with a Skywatcher Heritage 150, and it has a roller bearing between the tension adjuster knob and mount that prevents this issue.

[kvmx (Nick)]

Thank you everyone for your replies, they have been very helpful! After making sure everything is clean and seating the washers in multiple different configurations, I have come to the conclusion that the way this scope is mounted is the real source of the issue. There's a sleeve and washer made of sticky plastic and too much pressure is being placed on these components, naturally introducing excess friction. I imagine if the scope was mounted with two arms instead of one, there wouldn't be nearly as much experienced friction.


Having said that, I have found that the scope can traverse smoothly in the altitude axis without increase or loss of desired tension. If I rotate the tightening nut simultaneously with the OTA (alt. up + loosen left, alt. down + tighten right), the OTA and tightening nut move in unison, and it traverses fluidly. I think this is about as good as I'm going to get out of this scope.


I will continue to use it here and there, but I have noticed that the eyepieces (mainly the 6mm and 4mm from the accessory pack) tend to make stars look 'flare-y' for lack of a better term. Like they've grown bright wings. I'm chalking this up to either low quality eyepieces or perhaps bad collimation. Maybe both? I honestly have no idea.

[Dekker (Derrick)]

The eyepieces that come with the Firstscope are truly woeful. It would be worth buying a reasonable quality plossl in the 5 to 10mm range, even if you have to spend more than the scope costs. At least you could then use it on your next scope as well.


Also, there is no way to adjust the primary mirror collimation... it's supposed to collimated from the factory. You can adjust the secondary mirror, but that's unlikely to be the issue.

[kvmx (Nick)]

Originally Posted by Dekker

The eyepieces that come with the Firstscope are truly woeful.

Agreed! The 20mm seems okay-ish but the 12mm/6mm/4mm eyepieces make the stars look... considerably not nice.

I'll stick with my trusty binos for now and make sure my interest in the night sky lasts more than a few months before I start buying equipment. Although I wager moving out to a site with less light than the bortle 8 backyard will afford me better views than the equipment alone will. Might drive half an hour away tonight and see how the sky looks at a supposed bortle 5 as an experiment. Fingers crossed the clouds play nice!