3.1” vs 3.5” secondary

[Dbroz]

Hi everyone

I have built a 18” truss dob. The primary mirror is 457mm diameter, focal length is 2308mm, from primary to secondary is 1998mm and secondary to eyepiece is 310mm. My question is I was told a 80mm secondary will suit this scope best, but as I have accidentally damaged the new 80mm secondary I had to buy a new one but I couldn’t find any in stock in this size. So I ended up buying a 88mm mirror instead. It should arrive to me in about a weeks time, my concern is that this mirror might now be too big. I’ve been trying to research this but I keep coming up with mixed answers. I’m just seeing if anyone could tell me if I made a mistake buying the 88mm instead of waiting for the 80mm to be back in stock. I couldn’t wait too long as I need my scope for the snake valley star party and I didn’t know how long it would be until a 80mm will be back in the market. I would love to hear your thoughts.

Thank you
Daniel

[billdan]

Hi Daniel,

I think you will be OK with your decision, the 80mm secondary provides a 17.5% obstruction, the 88mm a 19.2% obstruction. So any loss of contrast will not be visually noticed, plus the slight extra weight should not upset the balance of the scope either.
Just make sure you collimate it as best you can and you will be OK.

Cheers
Bill

[OzEclipse]

I agree with Bill, no problem with the slight size increase especially given the focal ratio of ƒ5.05.

To ensure a fully illuminated non-vignetted field make sure you have a focuser with at least 2.5 inch inside diameter base.

There is a web-app on the stellafane web site which lets you input the construction parameters, do ray trace check for vignetting & optimise baffle locations.

https://stellafane.org/tm/newt-web/newt-web.html

Joe

[Kunama]

I also think your 3.5" diagonal will be fine, Joe's 18 F5.5 had a 3.5 to start with and I switched to 3.1 but only as I had it already. The difference is hardly noticeable.

[Dbroz]

Thanks a lot guys, it makes me feel a lot better. It should arrive later this week so I can test it out. But from your replies it looks like it will be fine.

Matt, your carbon fibre scope you built, did you build the whole thing from scratch, I mean the rocker box frame looks like aluminium but I don’t see any joins and looks professionally machine made. Do you have a thread on how you put it all together? I would love to build something like that for my next one.

Anyway thanks heaps for all your replies and reinsurance

Thank you
Daniel

[Kunama]

Thanks Daniel, Here a thread on the scope build: http://www.iceinspace.com.au/forum/showthread.php?t=164454 The main parts are aluminium and the excellent welding was farmed out to Andrew at Revolution Welding in Queanbeyan, whilst is can weld, I wanted the welds not to be seen.....

I spent many hours grinding and hand sanding the welds down so that they would not show once powder coated. The powder coat used on the mirror box and rocker is a special thin texture coat so the sanding had to be done thoroughly. The powder coat on the alt bearings is a heavy texture and hides any manner of sins.... :D

I gave the primary a proper wash on the weekend so now it is all set....

So far the scope is performing very well. It holds collimation well at all angles with less than 1mm of movement of the laser dot. I haven't tested it at high magnifications yet, only 233X.

Saturday evening we had beautiful views of the Grus Quartet and the Veil Nebula and many planetary nebulae. The Suchting mirror is superb....

Getting back to your scope, the build is looking great. It is very satisfying to make your own scope and get a good first light. Looking forward to your updates..

[Dbroz]

Thanks for that Matt, I will check out that thread now. It is very satisfying once you finish. I had the best views of Saturn with this scope . So much better than my 12@ skywatcher. Even my wife was amazed of what I achieved. I should have taken photos throughout the process, I have to admit making the mirror cell was a challenge but I got there in the end and it’s so easy to collimate. And you did give me a lot of helpful advice along the way. Thanks heaps

[ngcles]

Hi Daniel,

In a perfect world, the smaller secondary would have been the best fit, but the 88mm will be fine. 99/100 nights there will be no difference whatsoever, 1/100 nights you'll be very hard pressed to pick the difference.

Best,

L.

[Dbroz]

Yeah, I’m so disappointed that I damaged the 3.1” that I had , I ended up getting the GSO 1/14 wave 88mm, I did see some from Antares they were 1/20 wave but about $300 more expensive, do you think it’s worth spending that money on a secondary 1/20 wave, I mean will there be a big difference between Antares and GSO secondary. If there is a noticeable difference then I may save up to upgrade later on.

[Dbroz]

And Matt

I actually have an account with Ullrich Aluminum here in Melbourne, I manufacture security doors, I never thought of using Aluminum to build the scope, that is such a great idea. Plus it looks so nice. And to add the carbon fibre is such a nice touch. Must not have been cheap to make.

[OzEclipse]

With a secondary mirror sitting at 45 degrees, the wavefront error is amplified by the square root of two. So in theory, a 1/14 wave flat will yield a 1/10 wavefront error in actual use and a 1/20 wave will yield a 1/14 wave in use.

I wouldn't use a diagonal with > 1/10 wavefront error. My feeling is you won't notice the difference between 1/14 and 1/20 wave.

Joe

[Dbroz]

Thanks Joe

I am new to all of this, at least it will save me $$$$ . I was so close to buying the 1/20.

[Zubenel]

Looks like this thread is almost done but may I add that when SDM telescopes rebuilt "Zamar" ( 20" F5) part of the brief was to be able to image with it. Peter Read did the Calculations and came up with a 3.5" secondary from the original 3.1. Technically there would be a difference but as Les has stated you will never notice . I've given this much though and you would need 2 identical scopes sided by side and flick between them . It will never happen.
On the flip side you would think that the larger secondary could reduce what is visible at the extremely faint end however I confirmed seeing with AV at this years QLD Astrofest PGC 2184533 when stalking a few bucket list Globs (G352 & G351) of Andromeda . At +17.57 I would have thought not possible so I don't think the slightly larger secondary will make any difference at all. (Observing report to follow)!!
Cheers W.

[AstroJunk]

I've just installed an achingly expensive Antares 3.5 1/30th wave into my 20" dob. I was rather reluctant to do it given the exchange rate (over 700 AUD), but after Qld Astrofest this year - it was the best possible investment I could have made. In the 13 years of ownership of the scope I have not seen brighter and sharper views. The helix nebula was simply jaw dropping.

You can't beat perfection, and that doesn't come cheap.

[Billyboy78]

And several hundred ATM'ers let out a collective 'drool' ;)

[Dbroz]

My new 3.5” secondary arrived yesterday and I got to test it out. I do need to say that even though my 3.1” that is all deeply scratched still performs better than this new one. I don’t know if it’s because the night sky last night was pretty bright, I did notice that everything I viewed seemed a bit dimmer and not as crisp with the new 3.5”. After the last comment from AstroJunk I think I will still purchase the Antares 3.1” 1/20 wave secondary, the 1/30 is out of my budget at this time. I do appreciate everyone’s advice. I thank you all. I do still need to test this new secondary at a dark site which may make a big difference

Thanks again everyone

Daniel

[Kunama]

I’m curious about your original secondary, who was it made by and what was its claimed optical spec? What holder do you use?

:question:

[AstroJunk]

Couldn't tell you about it's origin - possibly Galaxy. It wasn't soo shabby a performer but had Dielectric coatings that had deteriorated and can't be recoated. It's in an AstroSystems holder and spider.

[Dbroz]

The original was still a GSO mirror they say is 1/14. It’s funny as I paid $200 for the 3.1” , I got this from Agena. The 3.5 was only $100 which I got from Andrews Communications in NSW. They are both GSO and both say 1/14