Dob shrouds - I wonder ...

[AlexB67]

After a nice session this evening of another GRS transit, probably will be coming to the end of Jupiter season now  I had the joy of trying my new shroud on my 10 inch flextube. 'till now this consisted of a half a plastic bucket cutout so some of it was still open/exposed at the bottom, but since I did a better job that works really nicely as described here.

http://stargazerslounge.com/topic/207066-homemade-light-shroud-for-300p-flextube/

I noticed that often it has taken quite a bit of time, and it has even proven problematic to settle my scope, attributing this to mainly the time it takes for the mirror to cool. As it turned out it was rather mild, so the temperature difference between inside and outside was not as pronounced as it has been, but the scope settled surprisingly quickly to provide some fine views this evening at 200/240x this evening.

I intend to fit a fan pretty soon anyway, but seeing the tube currents in the out of focus disk showed pretty much to be non existent, as good as I have seen them,  very steady in a shorter time than usual, I wonder if shrouds actually help with this. Would sitting near the scope (body heat )actually causes enough of a temperature gradient to interfere and that the rubber mat shroud blocks this out ??

I'll need more sessions of course seeing how it will pan out. As I said, it has been milder recently, so I may just be dreaming it up, but wondered about it all the same.

Any thoughts ?

[michaeldurban]

Hello Alex,

I have been thinking about the same issue...

As many people here will tell you the lightshroud is basically there

to block out unwanted light and reflections.

The main problem with a flextube (I have the 250D) is actually the dew forming on the

secondary mirror...

There are a lot of threads concerning this problem (from heating strips to DIY solutions)

but I have found that with a thick foam dewshield over the top OTA the problem (for me that is..)

is virtually eliminated..

The foam is a great insulator..

I am going to make a light shroud out of the leftover foam, but just because
I have leftovers...

I have flocked my whole scope, and with the dewshield (which doubles as a

light barrier) I do not notice difference in contrast or 'blackness'.

Another nice little mod is the placement of aperture rings in the top- and bottom OTA.

With those in place I have noticed a difference especially in coma reduction.

It is worth taking a look at...I used the same high density foam as I'd used for the

above mentioned dewshield...foam is 25mm thick..

good luck !

[Moonshane]

body heat can indeed be an issue with open truss systems in the same way, but less extreme, as heating vents. personally, I'd always use a shroud if I had a truss system, partly to prevent the double agony of a dropped accessory not only being dropped but hitting the primary on the way down. 

[John]

body heat can indeed be an issue with open truss systems in the same way, but less extreme, as heating vents. personally, I'd always use a shroud if I had a truss system, partly to prevent the double agony of a dropped accessory not only being dropped but hitting the primary on the way down. 

Me too. Plus stray light is an issue where I normally observe so keeping it out of the optical path, away from the secondary etc is important too.

Not sure about those aperture rings though, unless you actually want to remove the affects of a turned down mirror edge.

[Moonshane]

I tend to agree John

with 1" either side you might end up with an 8" f6 and you may as well buy one of those which will have less inherent coma?

[Phil Creed]

I tend to agree John

with 1" either side you might end up with an 8" f6 and you may as well buy one of those which will have less inherent coma?

+1.

You paid for the extra aperture vs. an 8" f/6; might as well use it.  That's what coma correctors are for, if the coma's that much of a problem.  You don't need the Type 2, Steroid-Paracorr, either.  A used Type-1 or the new ES coma corrector should work fine.

Clear Skies,

Phil

[AlexB67]

Thanks for the feedback. I was thinking not to use when taking the scope by car since it will be dark enough, but now probably will put it on every time I use it.  I'll be making an extension shield too ( along with the shroud )  but struggled a bit to find neoprene sheets that large. I suppose I could always velcro two halves together. Anyone of  places that sell larger sheets, or some material that serves as an alternative ?.  

In terms of contrast my sanded half bucket shell solution which only closed about two thirds  worked reasonably well for the shroud, I can't say the contrast was noticeably enhanced with the new shroud since I have a spot in the garden that is quite well shielded anyway, but this new rubber foam one is a lot neater and if it help with thermodynamics of the thing to settle, all good. Glad I made it now    I suspect the extension shield will probably be perhaps more useful  to prevent extra light coming in near and around the focuser.  After that I'll probably get the flocking bug, at least the top half extension of the flextube anyway.   

[AlexB67]

In reflection as well. My temporary cut open bucket solution had a gap near the bottom running parallel to the tube ( it was a bit too small in circumference, but hey, it was a temporary rough and ready hack, use what lies around the shed  

I wonder if that caused some of my issues as well, with air rising through it ending up  inside the tube, the tube currents were terrible sometimes, but it would eventually settle, but perhaps it caused as many problems as it solved even though it shielded the scope, the rubber one seals it nicely all the way around.   

[AlexB67]

Anyone of  places that sell larger sheets, or some material that serves as an alternative ?.  

Meant to say I feel the existing yoga mat material will be too floppy for an extension shield. I have some of the yoga mat left so could try it suppose, but ideally I need to something a bit more rigid, and as light as possible for the extension shield, any suggestions ?

[michaeldurban]

well..my whole point regarding these aperture circles is as follows.

the actual circle overlaps around 0.8 cm onto the mirror surface

(mirror edge starts at 1/2 inch from the OTA), thereby

(presumably) omitting the last 4mm or so which (according to some) are

not of the highest optical quality.

I haven't reduced my 10" to a 9 " although I doubt there would be a lot of difference

[John]

One reason I raised the issue of tube aperture is because I believe that might be a minimum tube opening size for a newtonian to ensure that the field of view of the primary mirror is not vignetted when using widefield, low power 2" eyepieces and that minimum is actually somewhat larger than the diameter of the primary. 

I've dredged up this ancient Cloudynights thread which discusses this:

http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/481552/Main/481111

Therefore I wonder if the aperture stop at the top end of the tube, that are right on, or even slightly smaller than the primary diameter, may actually be "stopping" a bit more than was bargained for.

[AlexB67]

I'll not be bothering with the aperture rings I think, for the aforementioned reasons and suspecting what John/Moonshane said as well, but an extension shield for sure will be made.  Just need to find some suitable material and large enough.  Ideally something light and thin that is rigid enough I prefer for the shield idea that slips over and around the outside of the OTA, tightening with some Velcro, so it is easily removed and put on, since in the car all these bits need to come off easily too, like the current shroud.

I've not often had that many dew issues to be fair, at leats I can observe for a long time before it becomes a real problem.  I think I'll probably go the heater strips route for the secondary, since when I fit a fan, I am committed down the electricity demanding route anyway with batteries and all that, so I may as well fit the secondary heater strips. 

btw, like the paint job on the mount Michael, telescope in knight rider style 

[Dreadz]

I made my dew shield from a lonsdale exercise mat from sports direct. Its thick enough to retain its shape nicely and doesnt add a huge amount of weight even though I made mine on the large side

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[AlexB67]

I made my dew shield from a lonsdale exercise mat from sports direct. Its thick enough to retain its shape nicely and doesnt add a huge amount of weight even though I made mine on the large sideuploadfromtaptalk1396291637807.jpg

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Got a a link for the product  ? they seem a bit pricy the ones I found by lonsdale. I just found this as  I was looking around and may take a punt on it, I hope it is rigid enough, but at 140g and that price worth a shot perhaps for a shield

http://www.amazon.co.uk/gp/product/B007551S9A/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=479289247&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B008XK0JPK&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=06112GG4PSC2GM0TK98X

[John]

I found a matt black excercise mat in my local 99p store that would do nicely for a dew shield for an 8" or 10" scope. It's not quite long enough for a 12" unfortunately. It's about 5mm thick so light but stiff enough for the purpose.

[Dreadz]

http://m.sportsdirect.com/mt/www.sportsdirect.com/lonsdale-fitness-mat-761112?un_jtt_v_un_PDP=yes&un_jtt_v_un_info=&un_jtt_v_un_pers=&un_jtt_redirect

I managed to get one reduced beyond that price again as the cellophane was damaged as was one of the elastic retaining straps. Bargain

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[AlexB67]

I found a matt black excercise mat in my local 99p store that would do nicely for a dew shield for an 8" or 10" scope. It's not quite long enough for a 12" unfortunately. It's about 5mm thick so light but stiff enough for the purpose.

John, that sounds ideal and very interesting if you are willing to get rid of it. PM me if interested and see If we can arrange something.

[Daniel-K]

mines a homemade astro zap shroud and without it id be dewed up in a hour. its helps with stray light and increasing contrast. do you not fancy buying one of the astrozap shields?

[russ.will]

I made a FlexTube shroud out of camping mat and velcro that doesn't intrude into the lightpath in this thread. Definitely a case of measure twice and cut once to make sure you get a good interference fit, but it looked very clean once in place.

Russell

[AlexB67]

Got a a link for the product  ? they seem a bit pricy the ones I found by lonsdale. I just found this as  I was looking around and may take a punt on it, I hope it is rigid enough, but at 140g and that price worth a shot perhaps for a shield

http://www.amazon.co.uk/gp/product/B007551S9A/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=479289247&pf_rd_s=lpo-top-stripe&pf_rd_t=201&pf_rd_i=B008XK0JPK&pf_rd_m=A3P5ROKL5A1OLE&pf_rd_r=06112GG4PSC2GM0TK98X

The link above for the extension  shield seem to work well enough, the material is a lit bit on the floppy side, but just about rigid enough to hold shape, nice thing is that it is very light indeed, the whole role weight 140 grams and by the time when cut down to size it will be quite a bit less.

One side of the role is slightly shiny but the other side roughened.  I actually did a quick test run on the moon Jupiter last night, I left it at full length, and just cut it down so it warps around, it could perhaps be shortened a little and since it is rather on the long side at the moment.    It was also a little bit breezy last night and was concerned the wind might move it about a bit, but it was surprisingly well planted. I just put Velcro strip on the shield to tighten it around and turn it into  a full cylinder so it just slips on

So now fully shrouded and an extension shield for about 20 pounds total.  Did not remove the shield to see the difference. Once I had it on I just go on with it and used it. some fine views that only lasted about 30 mins mind, clouds soon put an end to it.

Would I recommend this material, hard to say, I would probably ideally like something a little bit more rigid, but time will tell, but I do like that it is super light and just solid enough not to deform to encroach on the light path, so for now it will do.

The role has already gone up in price mind you on that link, I paid 3.99 + PP I think.