What size secondary for 10" f/4.8 scope

[Chris]

yep that works cheers James:)

I've plugged in some figures which at the moment I'm guessing some of the figures such as the secondary to focus distance, I've estimated this to be about 218mm because the cell that comes with the mirror is for an 11" tube and I'm thinking about 75-80mm for the focuser. At the moment it is giving me:

minimum flat dia = 45mm

primary mirror to flat = 982mm

Its great to know the primary to secondary distance once I've got the focuser and tube dimensions, thats a great help Nigel cheers:)

I think the minimum secondary size is on the small side at 45mm because my estimate of the secondary to focus distance might be on the low side, I think this because I've read that if you have a low profile focuser you can get away with a smaller secondary, although I'm not 100% on this?

[Astrobits]

The minimum size is really only a theoretical figure. Only the central point in the field of view would be fully illuminated, it wouldn't even cover the disc of jupiter.

Nigel

[Chris]

The minimum size is really only a theoretical figure. Only the central point in the field of view would be fully illuminated, it wouldn't even cover the disc of jupiter.

Nigel

I see, that makes sense then I guess minimum has got to start somewhere:D

[Chris]

I've found the section for the secondary calculator on Stellafane as well, the graph that goes with it is very nice for visualizing the effects of different parameters:)

http://www.bbastrode...om/diagonal.htm

After inputting my likely parameters it really does back up the 63mm diagonal in that with a 1.25" eyepiece the drop off would be only mag 0.2 starting about 10mm out from the centre, this would be about mag 0.45 with a 2" eyepiece but the human eye cannot detect a 0.3 drop off according to the info under the graph so I take from this that the vignetting would be hardly noticeable with a 2" eyepiece even:)

my parameters were:

250mm

1200mm

230mm

32mm

I'll be buying the TS 63mm I think,

A BIG Thank You to everyone for your help:)

Chris

[Moonshane]

that's a great link Chris - thanks! it also confirms my 82mm secondary choice was sound. how good is SGL?

[JamesF]

that's a great link Chris - thanks! it also confirms my 82mm secondary choice was sound. how good is SGL?

Oooh, it is, too. I'd not seen that one before. Certainly it's not like the one I recall from Stellafane. Very handy.

James

[Chris]

that's a great link Chris - thanks! it also confirms my 82mm secondary choice was sound. how good is SGL?

Its rather cool isn't it Shane:) Its one of the links on the Stellafane site that James gave me the heads up about:) you should check out some of the stuff thats on there its a great resource, not as good as SGL though I'm fairly sure I have some kind of addiction to SGL:D

p.s. I really have to say how impressed I was with your 10" dob build:) I've certainly found your thread inspiring! you could do that for a living I reckon and there would be a long line of people waiting:D

[Chris]

Oooh, it is, too. I'd not seen that one before. Certainly it's not like the one I recall from Stellafane. Very handy.

James

Thanks for the Stellafane site James, some great research material on there:)

[Moonshane]

Its rather cool isn't it Shane:) Its one of the links on the Stellafane site that James gave me the heads up about:) you should check out some of the stuff thats on there its a great resource, not as good as SGL though I'm fairly sure I have some kind of addiction to SGL:D

p.s. I really have to say how impressed I was with your 10" dob build:) I've certainly found your thread inspiring! you could do that for a living I reckon and there would be a long line of people waiting:D

how kind of you to say

the 'problem' is that I care a lot about the end result and make them as though it's for myself every time. it takes so long to make them that if you work out the time taken I'd be more profitable working for min. wage. that said, I love making them and if it funds the hobby without robbing the family pot then it's good enough for me it will never make a living of course but it's a great thing to fill in the days and nights when it's not possible to observe.

I really encourage everyone to try and make their own and hope the threads help there. if nothing else, it makes you appreciate the time and dedication required and you appreciate the scope all the more when you finally get first light.

[Moonshane]

ps

if you need any guidance feel free to PM me if it's not clear from the thread.

[Joseki]

I think Kriege and Berry should be recommended to everyone planning to make their own dob in the same way Making Every Photon Count is to AP and Turn left at Orion for everyone with their first scope (even considering Albert Highe's new book which I've been reading at the mo). It costs less than a sheet of plywood..

[JamesF]

I think Kriege and Berry should be recommended to everyone planning to make their own dob in the same way Making Every Photon Count is to AP and Turn left at Orion for everyone with their first scope (even considering Albert Highe's new book which I've been reading at the mo). It costs less than a sheet of plywood..

Can't disagree. Shame it's so awkward to get hold of. Took me something like three months to get my copy.

James

[Joseki]

Yes it took a bit of tracking down for me too (I bought it this January). If you put a check in the post (or something) to Obsession scopes in America they'll send you a signed copy:

http://www.obsessiontelescopes.com/publications/index.php

but I ended up ordering from here:

http://www.scsastro.co.uk/catalogue/the-dobsonian-telescope.htm

thinking I'd get it quicker than ordering from America, which is slightly cheaper, however I think SCS just ordered it from America (next link), though it arrived quick enough.

http://www.willbell.com/tm/dobtel.htm

[JamesF]

SCS are just down the road -- we go past when taking the children to and from school -- so I dropped in and ordered it in person. I think they wait until they need enough stock to make a US order worthwhile though, so if you're unlucky enough to order at the wrong time you can be in for a long wait.

James

[Joseki]

Nice, that sounds like somewhere I could spend a long time browsing in (if they have display copies). I probably spend longer reading about hobbies than doing them, so it's good to know I might be better ordering direct from the US - I guess I got lucky this time.

[Chris]

Well, I placed an add for a secodary and focuser just in case really, bit of a long shot but it payed off on both counts:)

The secondary looks good and comes with the holder which is a bonus, its a little larger than I'd initially planned at 70mm but I checked on the above program and its well within range. The plus side a guess is there will be even less vignetting the downside I guess is theres going to be a little less contrast for visual. Probably slightly large than perfect but it was only 27.50 inc postage so if it doesn't work for this 10" it will be me an excuse to build a 12"

The focuser is a bog standard SW crayford for 45 quid inc postage.

[Chris]

The secondary mirror arrived today and it is slightly under 70mm at about 68.5mm for the actual reflective surface so thats sligtly closer to perfect, the coatings look good as well.

The focuser arrived also so I could more accurately calculate the diagonal to focal plane distance, I'm also adding 40mm plus 5mm extra to allow for the DSLR sensor which is set back in the camera, this is because I mainly want to build the scope for imaging. The diagonal to focal plane distance is therefore around 265mm.

When I plug these figures into the Diagonal Off-Axis Illumination Calculator and input 35mm for the eyepiece diameter field (assuming the DSLR chip is 35mm) the chip shows as being 18mm fully illuminated then drops around 0.2 mag to the edge which is negligable, this looks good to me.

The interesting thing I found was if I input the 58mm diagonal often supplied with these scopes then the graph looks rather bad, with the illumination dropping off straight the way from the centre then falling away at least 0.5 mag if not more!

I bit of further research brought up a theory that scopes could potentially be made with under sized diagonals to allow for possible poor primary mirror figuring towards the edge? If a full sized secondary was used with a mirror with a poorly figured edge then this might scatter light across the whole FOV reducing contrast? With this in mind and knowing that my secondary is 5mm over what we had worked out as being the best compromise, and it would likely use the whole light from the primary so it would be interesting to compare the contrast before and after masking the edge of the primary, this should point out any problems with the primary mirror figuring at the edge in theory? does this sound about right?

[Chris]

The twist in this thread is that that the primary mirror turned up today and even though the listing said it came with the cell plus spider they also meant secondary mirror as well! yes it came with a secondary mirror alfter all that, kind of a Doh! and Hurray! moment all rolled into one:D

The secondary it came with is the 58mmm minor axis so what I think I'll do is if I use the scope for imaging in my obsy I'll use the larger 69mm one and if the scope is too large for the obsy then I'll build a dob for visual and use the factory standard 58mm one.

Happy days! I'll start the build thread soon and see what happens:)

[TonyD]

A great read there! Thanks! Keep me posted

[Chris]

Cheers Tony will do:)

[simonmc]

Keep us posted as i already have a 10" GSO dob and would be interested in seeing what the different secondarys are like.

Simon

[Chris]

Will do Simon:) I really like the GSO stuff for the money, the only bad thing I've heard is that the mirror cells are a bit weak for the really fast f/4 scopes and theres a bit of tube flexure that shows on these models, I wouldn't hesitate to buy a slower f/ratio scope from them though:)

[Moonshane]

I doubt that visually you'd see any flex in the cell up to about 12" but probably you would if imaging.

[Chris]

Hi Shane,

Yes its only for imaging I think, the flexure occures when hanging camera and equipment off the focuser and the effects of the flexure are worse with fast f/4 scopes because the critical collimation and the trailing stars in one corner on the images due to the lack of orthoganality (if I've spelled that correctly ). For visual I have no doubt that they are fine instruments for the money:)

[simonmc]

I have to say i have not really done any imaging with mine, and not noticed any flex, but again i have never really loaded up the focuser. It is my eventual aim to try this but will probably make up a really big goto gem mount for it for imaging.. I do however have to take the mirror out for cleaning as the layer of dust on the primay is getting too much so i can have a look at the cell when im at it. Seems my job list just got longer! :grin:

Simon