Focusser Travel & Paracorr 2

[niallk]

Hi,

After several weeks, I finally took a chance with a gap in the clouds to test focussing with my newly acquired 10E.  I knew I was not going to make a night of it looking at the rainfall radar, so didn't bother cooling or putting on a shroud - it was still bright, but the moon was looking crisp!

So in with the Paracorr 2, the 10E and bingo (phew!), could reach focus - setting H on the Paracorr; 10E in 2" mode.  Some nice views along the terminator, but the key thing I wanted to check was my margin for focusser travel:

So I have say 4-5mm in-focus travel left.  I'm going to collimate with 2 knobs only from now on to avoid accidentally walking the mirror down away from the secondary. I could move the primary towards the secondary a couple of mm if I needed I guess.

What I want to ask is:

If I get on well with the 10E (I have a 20ES100 also, which I am getting to know), and am so bitten by the views that I want to get another Ethos over time, am I correct in thinking that the focusser adjustments for other TV EPs should be really pretty minor, and the tunable top should take care of EP to Paracorr spacings; the primary to Paracorr spacing should remain constant?

The reason I'm thinking this might be right is from reading the SIPS option and the installation instructions ... I'm interpreting them as suggesting that the Paracorr is at a set distance from the primary for all TV EPs, and the focusser replaces the tunable top: and focus is achieved when the EP focal plane to Paracorr2 is right ... and I guess matches the distances per using the tunable top?

I'd really like to be able to reach focus without trimming truss poles etc if I get another EP!

[Louis D]

That sounds right.  As a check, find the eyepiece in your collection that requires the most in-travel of your focuser and try to bring it to focus in your Paracorr.  If it comes to focus using just the tunable top, you should be good.

[niallk]

Thanks LouisD will do!

[spaceboy]

I will start off by saying I don't really know what I am talking about when it comes to Paracorr and coma correctors and correct spacing in general.

That said I do know that different night sky objects require varying amounts of travel to achieve focus so you may or may not find that 4mm spare eaten up ? I also know with a big dob like yours your going to be a fan of the faint stuff that really benefit from filters. These filters also vary the amount of focus travel required over the original eyepiece focal point. It will also be worth pointing out that the ES20 100° suffers from astigmatism so don't get too flustered if you seem to struggle finding the correct setting on the Paracorr. Just be aware of what is coma and what is an eyepiece aberration. I am aware that the 20ES 100° and OIII or UHC filter combo can eat up inward focus so when you next get chance this might be worth trying in your scope to see how much play you get.

As already pointed out my comments may be totally irrelevant but I hope it offers some food for thought next chance you get to see if you have the clearance so your not caught short next dark sky visit.

[niallk]

2 hours ago, spaceboy said:

I will start off by saying I don't really know what I am talking about when it comes to Paracorr and coma correctors and correct spacing in general.

That said I do know that different night sky objects require varying amounts of travel to achieve focus so you may or may not find that 4mm spare eaten up ? I also know with a big dob like yours your going to be a fan of the faint stuff that really benefit from filters. These filters also vary the amount of focus travel required over the original eyepiece focal point. It will also be worth pointing out that the ES20 100° suffers from astigmatism so don't get too flustered if you seem to struggle finding the correct setting on the Paracorr. Just be aware of what is coma and what is an eyepiece aberration. I am aware that the 20ES 100° and OIII or UHC filter combo can eat up inward focus so when you next get chance this might be worth trying in your scope to see how much play you get.

As already pointed out my comments may be totally irrelevant but I hope it offers some food for thought next chance you get to see if you have the clearance so your not caught short next dark sky visit.

That's great feedback Spaceboy - thanks a million!

I haven't had much of a chance to use the 20ES100, let alone the 10E (for various reasons) but I'm keen to face into focus and balance issues now ... ahead of the evenings getting darker again

LouisD and yourself have given me great pointers to consider and experiment on to get prepared/make the necessary adjustments.

Very much appreciated,

-Niall 

[Bart]

Niall

i just got the ES Coma corrector and found I didn't have enough infocus. I had to move the primary up about 10mm towards the focuser. I haven't yet tried to check focus as I only did it last weekend but I'm fairly confident it'll work. It was the ES 20mm 100* that was just short. It's not a biggie (or wouldn't be a second time ??) to sort out if it was necessary. Anyway, that's what the Astro summer is for, messing around with the scope? 

[niallk]

Sounds good Barry How did you move your mirror - longer bolts or raised contact points on the flotation triangles or something?

I'm going to have a look-see at gaining some more margin via the collimation bolts ... just need to give over a night if the clouds would ever take a break, and do some methodical experimenting and checking the impact of my O-III filter etc as suggested. I really hope to get away with it if I can. If I have to do surgery, I might draw on your help/experience!

I only have 1.25" filters, so might have to look at a 2" O-III ... probably the Astronomik one - 2" stuff pricier ? I think the suggestion is to mount 2" filters screwed into the Paracorr2, rather than on an EP, as I am used to doing.

Next is balance ... yip bit of sorting out to do.

Then ... clean the mirror 

 

[Bart]

I had in the last year bought the Farpoint Collimation bolts for the LB. luckily the threads screw out so I got longer threads from wotk??. I then reintroduced them through  a wooden 'block' about 1cm thick then through the springs to the mirror cell. If you can picture it, the wooden pieces now form the back of the telescope onto which the springs now sit. A photo would help so I'll post one. It should have been easy but one of the bolts which I thought was threaded into the mirror cell, in fact, wasn't, which meant the mirror shifted and then necessitated much extra work. The air turned blue around me with the language. The mirror on the 16" LBs is one heavy mother and trying to manipulate it through the back of the tube requires the hands of a nymph with the strength of a monster. A  contortionist, I am now.   One of the many side skills one picks up through amateur astronomy.....

Still, all good now. 

[niallk]

51 minutes ago, Bart said:

I had in the last year bought the Farpoint Collimation bolts for the LB. luckily the threads screw out so I got longer threads from wotk??. I then reintroduced them through  a wooden 'block' about 1cm thick then through the springs to the mirror cell. If you can picture it, the wooden pieces now form the back of the telescope onto which the springs now sit. A photo would help so I'll post one. It should have been easy but one of the bolts which I thought was threaded into the mirror cell, in fact, wasn't, which meant the mirror shifted and then necessitated much extra work. The air turned blue around me with the language. The mirror on the 16" LBs is one heavy mother and trying to manipulate it through the back of the tube requires the hands of a nymph with the strength of a monster. A  contortionist, I am now.   One of the many side skills one picks up through amateur astronomy.....

Still, all good now. 

Your description gave me a laugh

Glad you got it sorted!

[Bart]

Off topic, but are you off to Skellig next month? If so, I could maybe organise a van from work to bring the two biggies??

[niallk]

I'm really hoping to make it, but my wife has to go into hospital the week beforehand, so I can't say for definite yet - need to see how things go the following week.  That's a kind offer though, much appreciated!!

I don't have accommodation booked, but last year I just pitched my tent on the sand dunes ... I'd do it again - its a real nice spot there

I'll be in touch - hoping to actually make it along to the Youghal Astro club get-together being organised next Saturday week - it would be great to finally meet yourself and the other folk!!  I feel bad that I haven't made it there yet: just find it hard to make any meetings - I'm only finishing up in work 30miles away up beside the airport when they are on , but want to become involved.

[Bart]

Ha! Lightweight! I finish in mallow at about 5-5:30 and have made a few! Haven't been for a while though, works been manic and been to France on hols. It'd be good if you could make the 23rd to have a chat and a pint, my wife has agreed to run me home to Killeagh, so you'd be on the way, or failing that, a taxi will definitely need to deliver me home! Hope all goes well with your wife and normality returns. As I imagine with all Astro clubs, there a fair spattering of personalities so a good night is assured.

hope to hook up soon

Barry

[Bart]

Back on-topic

pics of arrangement providing additional infocus

 

[niallk]

1 hour ago, Bart said:

Ha! Lightweight! I finish in mallow at about 5-5:30 and have made a few! Haven't been for a while though, works been manic and been to France on hols. It'd be good if you could make the 23rd to have a chat and a pint, my wife has agreed to run me home to Killeagh, so you'd be on the way, or failing that, a taxi will definitely need to deliver me home! Hope all goes well with your wife and normality returns. As I imagine with all Astro clubs, there a fair spattering of personalities so a good night is assured.

hope to hook up soon

Barry

Yeah I gotta just go in early to get home early and make some club meetings!

You know I might just take you up on that sound offer of a lift back - a few pints would be nice . Probably better if the pub option is gone for - let's face it: the odds are generally stacked against us for observing! Might be a good call to leave the scopes at home - I once tried to use the scope after just a glass, ok two, of wine (just killing time while the mirror cooled a bit...) - literally could not merge the images in my binoviewer... I was appalled ?

Neat job on the mirror position adjustment!  Yeah it's when I have to get out the pipe cutters that I get a bit nervous...

[Don Pensack]

On 7/13/2016 at 15:28, niallk said:

Hi,

After several weeks, I finally took a chance with a gap in the clouds to test focussing with my newly acquired 10E.  I knew I was not going to make a night of it looking at the rainfall radar, so didn't bother cooling or putting on a shroud - it was still bright, but the moon was looking crisp!

So in with the Paracorr 2, the 10E and bingo (phew!), could reach focus - setting H on the Paracorr; 10E in 2" mode.  Some nice views along the terminator, but the key thing I wanted to check was my margin for focusser travel:

So I have say 4-5mm in-focus travel left.  I'm going to collimate with 2 knobs only from now on to avoid accidentally walking the mirror down away from the secondary. I could move the primary towards the secondary a couple of mm if I needed I guess.

What I want to ask is:

If I get on well with the 10E (I have a 20ES100 also, which I am getting to know), and am so bitten by the views that I want to get another Ethos over time, am I correct in thinking that the focusser adjustments for other TV EPs should be really pretty minor, and the tunable top should take care of EP to Paracorr spacings; the primary to Paracorr spacing should remain constant?

The reason I'm thinking this might be right is from reading the SIPS option and the installation instructions ... I'm interpreting them as suggesting that the Paracorr is at a set distance from the primary for all TV EPs, and the focusser replaces the tunable top: and focus is achieved when the EP focal plane to Paracorr2 is right ... and I guess matches the distances per using the tunable top?

I'd really like to be able to reach focus without trimming truss poles etc if I get another EP!

You've got it--the tunable top essentially parfocalizes your eyepiece set, so after setting the Paracorr for that eyepiece whose setting you know, all other eyepieces will focus with the tunable top to find their correct settings.

For fine focus, you may have a +/- 1mm range after that, but your additional in-travel is sufficient.

The good thing for that position is that the weight of the Paracorr + eyepiece will cause less sag in the focuser.

[niallk]

46 minutes ago, Don Pensack said:

You've got it--the tunable top essentially parfocalizes your eyepiece set, so after setting the Paracorr for that eyepiece whose setting you know, all other eyepieces will focus with the tunable top to find their correct settings.

For fine focus, you may have a +/- 1mm range after that, but your additional in-travel is sufficient.

The good thing for that position is that the weight of the Paracorr + eyepiece will cause less sag in the focuser.

Hi Don - thanks for your response  Good to hear, and a very good point re  reduced potential for focusser sag under weight!

All the best,

-Niall 

[niallk]

Got out last night, but plagued by clouds

However, I got to check focussing with a 2" O-III in place (in barrel of the Paracorr 2) - negligible impact on focusser travel 

Had some great views of M57 through a gap ... love the 100° in the 10E, but eye placement is key I'm finding (I can see the field stop when I'm in the right spot). I think I need to grow by about 1" to not teeter a little at the eyepiece near zenith .  Hopefully with more use, eye placement will be second nature.  My 68° Pan24 with 2.5x PM is lovely to use, but the fov is like looking through a toilet roll insert in comparison...

[Louis D]

7 hours ago, niallk said:

My 68° Pan24 with 2.5x PM is lovely to use, but the fov is like looking through a toilet roll insert in comparison...

Yes, the danger in going with ever wider AFOVs.  First, 60 to 65 makes 45 to 50 seems claustrophobic, then 70 to 82 makes 60 to 65 seem narrow.  Then comes 100 and up from there.