Glass spider

[kalasinman]

Interesting?

http://www.astronomyhints.com/spider.html

[Peter Drew]

I'd say go for it as well as there is relatively little time and finance at risk, just be prepared to read a few "told you so" posts if it doesn't work out . Solar film material is available without coating which can be used to close off open tube telescopes, no chance of course to mount a spiderless secondary though!

[Alien 13]

I would go for it i have taken pictures and used a telescope before through double glazing with supprisingly little effect.

Alan

[John]

....... As there is only one contributor who has any hands on experience with this idea, and his remarks were conditionally  positive (thanks again Derek),....

I think some of the other contributors to this thread do have extensive practical experience of designing and building scopes but are being modest about it

Good luck with the project though

[long_arms]

Good to see that you're going ahead with this   

I would love to know specific details, like how you will mount the secondary holder? And the window itself? And will you be drilling the central hole yourself?

Thanks, 
Dan 

[kalasinman]

@John, I certainly didn't imply that members here lacked experience, far from it. My comment was that only one respondent had used this particular approach.

@ Dan, thanks. I am planning to drill the hole. I've been around equipment most of my working life. Subject to change, however.

Right now, I'm starting to consider secondary support designs. I've seen several designs in articles about DIY curved spiders. Not too fond of any, yet.

I'd like to make the window support fairly easy to remove, so the whole assy could be removed for cleaning and service if needed. Wish I knew someone with a 3D printer...;-)

After reading reviews of the MN190, I'm going to diligently stay on a weight saving regime.

The window itself will most likely be a pre-made pre-multi-coated item off the shelf from Edmund.

[Scott]

Thanks to all for the information so far.

@long_arm , interesting and happy to find someone interested in the same concept. I can get a 203mm blank quite cheap. I'll look around for a shop to take it to 1/4 wave or so, as it comes 4-6 wave. I'm not sure that it would need tilting if I have it multi-coated. Dew here is rarely a problem at 25c, perhaps during our winter it will show up. We have 3 weeks or so of 15c then. With inside and outside temps being within a degree of each other, cool down is not much of a consideration either, although I am taking a hard look at a cool box for the DSLR.

One point I'm curious about. The criticallity of conditions affecting the light path, seems to relate to their type and location relative to the focal plain. I don't know whether a 4-6 wave window at the front of the scope would have a large impact on image quality or not. Seems like it would, as mirrors like to be 1/4 wave or better, but I don't know.

The candidates at this point are: stock newt, newt with curved spider, windowed newt.

In the case of the MN190, I just can't justify spending that much on my pension, $2100 USD plus shipping? I can get a decent 8"newt delivered to me here for $405 USD.

In my case, I've been in the technical/mechanical trades all my working life. My time costs me nothing, so fiddling with it to get it right is actually welcome if it sorts in the end.

As I'm in no hurry to do this, just accumulating data.--Jack

Just be sure to check how your warrantee will work. If the scope is delivered faulty, will you be required to send it back to the states at your expense? something to think about.

[Gina]

Wish I knew someone with a 3D printer...;-)

What would you like printed?  If you post some details or a design I'll see what I can do

[rfdesigner]

@John, I certainly didn't imply that members here lacked experience, far from it. My comment was that only one respondent had used this particular approach.

@ Dan, thanks. I am planning to drill the hole. I've been around equipment most of my working life. Subject to change, however.

Right now, I'm starting to consider secondary support designs. I've seen several designs in articles about DIY curved spiders. Not too fond of any, yet.

I'd like to make the window support fairly easy to remove, so the whole assy could be removed for cleaning and service if needed. Wish I knew someone with a 3D printer...;-)

After reading reviews of the MN190, I'm going to diligently stay on a weight saving regime.

The window itself will most likely be a pre-made pre-multi-coated item off the shelf from Edmund.

Can I suggest starting by just trying the proposed window and leaving your spider in place, just cap the tube..no centre hole, KISS.

Cutting the hole could release a strain, and give you a false negative.  As an engineer I like to take single steps, especially on "research" projects, which arguably this is.  One unknown at a time..   Just take the piece of glass, hold it over the top and see if you still have a telescope or a light scattering tool, also means you can do a with/without test under identical conditions.

If it succeeds, cut the hole, and test it..  if that succeeds, mount the secondary in the hole, and so on..  

One step at a time..

In my case David Sindon ground mine..  I knew it would work, this is cheaper but you're taking a risk, if it doesn't go right it's worth knowing exactly what caused the failure in case you want a second go.

Good luck

[HD-AP]

Hi

I have a 10 inch f8 newtonian with an optical window designed for visual planetary /lunar observation all optics were/are by david sinden of the sinden optical co primary in zerodur better than 1/20th wave secondary flat in quartz 1/30th wave optical window 1.10th wave or better .The optical window is 11 inches dia housed in a 13 inch dia tube with a dew cap to protect the optical window from dewing .i will / have attatched a report detailing the manufacture of the optical window ,for interest . 

Optical windows of high accuracy are very expensive to produce as what you have is a large diameter optical flat but worked both sides not one ,and on a substrate that is refactive in transmission not reflective ,so has to be high quality.

[kalasinman]

@Auspom. Good idea. Shipping out of Thailand is relatively cheap. I'm intending to buy the GSO from Germany, as the price and shipping are over $100 cheaper. The vendor there also checks the optics and scope before shipping.

@Gina, when the time comes, i'll pm you. Thanks very much. BTW probably the holder for the window.

@rfdesigner, that sounds like the best way to proceed. Great suggestion.

@HD-AP, Interesting. Are you pleased with the final result?