New bolts for a 10" dob primary mirror, how much movement is safe?

[cpoc]

I recently got back into the Dobsonian world of observing (having previously had I think a 8" but had to sell that one) with a Bintel 10" scope.
I've had some lovely views through it... but wanted to try attaching my DSLR. When attempting that I got the familiar lack of ability to focus.
I removed the eyepiece adapter and attached the T ring direct to the focus mount but still not quite there.
I bought a barlow and that didn't seem to help either.
Then I read about changing the collimation bolts for longer ones, great I thought, no permanent damage to the tube and relatively cheap.
I did a quick check and found that while my locking bolts are a fairly standard m6... the collimation bolts are m7 (of which Bunnings only had 35mm varieties).

Fortunately there's a bolt specialist near home and they set me up with some 50mm and 60mm bolts.

I've swapped all the bolts over and performed a rough collimation (my eye seems to be center of the viewfinder again).
But I'm wondering how much movement I have of the primary mirror. I'm not sure how much change I've made so far though I suspect the mirror is now at least a little bit further up the tube, I still have quite a lot of thread left on my bolts that would allow me to theoretically move it quite a bit further up... but how much movement room do I have before I'd risk damaging the mirror/telescope?


If I get clear skies tonight I will try and take it out and see if I can attain focus (though there's not much that I can spot that is likely to be easy to see on my camera, planets are too low down before dark and the moon is only a slither and also low).

[gb44 (Glenn)]

Hi Cp
If the bolts are firm there should be no issues. Just get to focus, reset collimation and go for it. If the bolts are real sloppy then the mirror could be siliconed in.

The other way is to shift the mirror cell attachment holes up the tube an inch or so.

GlennB

[cpoc]

Originally Posted by gb44

Hi Cp
If the bolts are firm there should be no issues. Just get to focus, reset collimation and go for it. If the bolts are real sloppy then the mirror could be siliconed in.

The other way is to shift the mirror cell attachment holes up the tube an inch or so.

GlennB

Thanks for that. So would I be correct in thinking that the screws/bolts around the base of the telescope hold the mirror cell in place... the bolts in the bottom of the scope move the mirror up or down within that cell?


Is there a way to know you are at the upper limit of that cell?

It was cloudy last night so I didn't get a chance to take it out to try star collimation (I've done the rough collimation of getting the reflection of my eye center in the focuser... so I think it's basically ready for star collimation).

[cpoc]

Ok so I've had a few attempts at adjusting my telescope to work with my Nikon DSLR but the biggest issue I seem to be facing is that my telescope uses M7 bolts to hold the mirror cell.
Does anyone happen to know of a reasonably cheap place to obtain some m7 bolts of 100mm or more?
Currently I have the longest M7 bolts I could get (60mm) bolted to a bracket then into the back of the telescope mirror cell and m6 bolts bolted to the nearby holes on the bracket and out the back of the telescope... this obviously isn't ideal as it means the strain is split over two bolts and the connection is off center... but I will see if I can at least get some results to find out if I'm in the ball park.

[etill (Elliot)]

I got a bunch of obscure thread types and lengths for another project in Moorabbin here: https://costlessbolts.com.au

Its not a huge place so I'd call first to check if they have them.

[cpoc]

Thanks for that, I'll have a look.

The good news is my temporary bracket mounting system worked well enough to allow me to attain prime focus. It's well colimated but it was good enough to prove it works (and it turns out my prime focus sits at close to 100mm up the tube.).

[gb44 (Glenn)]

Good job CP. Its quite a way up the tube isnt it. Can you still get good tube balance - may need weight attached to the tube.

You could draw a ray diagram and check the size of the secondary. The secondary should be oversized and not undersize.

Cheers
GlennB

[cpoc]

I think my camera is a big part of the issue, if I read correctly Nikon camera sensors tend to be some of the furthest set back of the camera brands.

I've just ordered some Volvo oil pan bolts as it turns out they can be bought in M7 at 100mm so I've ordered three (plus shipping it comes out to just under $50 but if they work they should be great).


The balance seemed fine while I was moving it about last night... holding everything in place while I carefully tried taking all the holding bolts out to put them back in again after slotting through the bracket was a bit nerve wracking... but once they were in it didn't seem all that different to normal (perhaps a slightly higher propensity to want to slowly lower at the opposite (ie viewing) end... but not overly fast and during normal operation it seemed fine.

[gb44 (Glenn)]

Wow thats a real gouge. Are they gold plated? Keep the receipt - they may not be suitable... Did you try BoltPro?

GlennB

[raymo]

I really don't mean to be rude, but your problem is lack of research before purchasing your scope. Most of Bintel's Dobs [Newts] are set up for visual,
and don't have enough back focus for using a DSLR. All SW Dobs [Newts]
can be used for either visual or AP using supplied adaptors.
raymo

[cpoc]

I won't take it as rude because I didn't buy the scope specifically for astrophotography... I was well aware that Dob's generally aren't designed for that but that you can work around that if you are motivated enough.

I am very happy with the views I get out of it (and it is also very important to note that it was a second hand scope as I can't afford a decent sized new scope... I'm very happy with the quality for what I paid for).

As regards the difference between brands and astrophotography readiness... well that just assumes I don't like the odd challenge here and there. The fact that I am now able to get prime focus with my camera means research or not, I seem to have solved the problem.

Just because one way is easier, doesn't mean it is the most rewarding.

[cpoc]

I've attempted to attach an image I managed to grab of the Orion Nebula with my Nikon and Dobsonian telescope. I still need to work on my image taking/processing as I seem to get a decent amount of noise in the final image... but I'm pretty happy with how much nebulosity is visible.

Where there's a will there's a way!

[astro744]

I was reading your post then realised it started well over a year ago. The only thing that came to my mind that another member alluded is the secondary mirror size. Moving 100mm up the tube means the secondary is intercepting a larger light cone. This would normally require a larger secondary to ensure there is sufficient illumination from centre to edge of field and that you at least get some 100% illumination in the centre. If the secondary was oversized you may be ok but if undersized then you are probably not getting the full use of the primary now both visually and photographically unless you increase the secondary size.

If it were me I would at least do the calcs to see where I stand but probably would not do anything about it as the difference may not be detectable or minimal and not be of concern. However the typical backfocus for a DSLR is approx 50mm and the fact that you had to move 100mm could indicate you had a smaller secondary because that is all was needed which is a good thing for visual as it improves contrast.

So basically you accept an illumination drop off and potentially zero 100% illumination in the centre or increase the secondary size. As to which is better would depend on just how much magnitude loss you are getting and if increasing the secondary then just how much contrast will be affected by the larger secondary. To determine contrast factor divide sec. minor axis by primary diameter, eg. 2.14” sec/10” primary =21.4% contrast factor (good is around 20% up to 25%). Better is (15-20%). Most SCTs are 30-40% albeit with less contrast (planets most noticeable visually). Don’t sweat over it because if your over 25% and are getting great views it’s probably having a good quality primary and secondary mirror (all else equal) than any affects of contrast from an oversized secondary.

If your getting illumination drop off too but have good quality primary and secondary giving you great images with great contrast then that’s fine too, it’s just not optimised fully to give full use of the primary.

You can do the calcs at https://www.bbastrodesigns.com/diagonal.htm

If you have trouble with the online calcs, sing out.