Telescope for doubles

[Sunshine]

That 127 Mak would be a great little scope for doubles, lunar and, planetary observing.

Edited August 4, 2020 by Sunshine

[vlaiv]

17 minutes ago, Kirby301 said:

How long does the 127 take to cool? I'll be keeping it in a wooden shed.

I have Mak102 and only thing that I can say so far is that I have not been really paying attention to this above what I usually do - take the scope and leave it for about 20 minutes to half an hour while I'm getting ready to observe or image.

I could sort of extrapolate that and say that you'll be fine with 30 to 40 minutes of cool down time as a worst case scenario, but someone who actually has this scope needs to confirm that.

[Alien 13]

13 minutes ago, vlaiv said:

I have Mak102 and only thing that I can say so far is that I have not been really paying attention to this above what I usually do - take the scope and leave it for about 20 minutes to half an hour while I'm getting ready to observe or image.

I could sort of extrapolate that and say that you'll be fine with 30 to 40 minutes of cool down time as a worst case scenario, but someone who actually has this scope needs to confirm that.

I would think that the cool down is related to the volume of glass/air in the tube, my 90mm Mak is ready to go in 10 minutes or less.. Would be interesting to know if the cool down for a 100mm Mak is any different to a 100mm refractor especially if its a triplet.

Alan

Edited August 3, 2020 by Alien 13

[A McEwan]

7 hours ago, vlaiv said:

Just realized I did not answer the question, but since I don't do double stars much, I can give what I think would be probably best beginner double star instrument taking all things into account (budget also).

6" F/8 newtonian with thin curved spider and 20% secondary obstruction would be my choice.

Example pic (no longer owned):
 

[paulastro]

In my view, in order of preference:

1  Refractor

2  Mak-Cass

3  Mak-Newt

[Kirby301]

15 hours ago, vlaiv said:

Unfortunately, this is DIY thing as far as I know.

Actually scratch that, according to this thread:

You can apparently order those online, like from here:

https://www.fpi-protostar.com/crvmnts.htm

If you want to DIY those, you'll need a thin piece of metal that you'll bend into shape (smoothly) - 180 degrees one works good.

Things to remember when choosing spider support - you need at least 180 degrees arc - less than that and you'll have only partial diffraction - only in certain directions and not evenly spread.

Length of support is related to strength of diffraction - keep the length at minimum.

Make sure all "degrees" are covered equally - having for example 270 degrees which will be 180 + 90 degrees - these 90 degrees will bias diffraction to one side.

There is a free software that will calculate resulting diffraction from central obstruction (and support) - let me see if I can find that for you.

Unfortunately I can't seem to find it, but @sharkmelley used it back in 2013 to explain some phenomena so could possibly provide a link to that?

(I found link to software in the mean time):

https://www.cloudynights.com/topic/547413-where-to-find-maskulator/

I just thought of a very quick way to remove diffraction spike on Heritage 130p - it is very easy thing to do and in principle it is "external" to scope - no modifications needed, but you will need to make aperture mask.

Take cardboard and make mask that you will place over the stalk that holds secondary mirror. Be sure that you have nice clean cut.

So it is just a series of circles (can be smaller or bigger) that you tape together to cover the stalk. Just be sure you secure them in place and they don't fall onto your primary mirror. Black cardboard works the best of course.

This will remove diffraction spike.

 

 

I've been reading and watching YouTube regarding the aperature masks. I like the idea alot. They all seem to do a single hole though. Could you explain in simple terms for a simple mind the way you mean please?

[chiltonstar]

My pennyworth.....it's horses and courses really. My first scope, mainly for planetary and doubles, was a 102mm f13 (Vixen objective) achro refractor. It still gives excellent views of doubles, but it is only 102mm aperture which limits resolution and of course limiting magnitude - there are many doubles of interest with faint secondaries. It is also long and unwieldy.

I then added a 127 Mak, which IMO is a truly excellent scope for the price, and enabled me to get to tighter doubles and fainter secondaries. To get lower, I then bought a 180 Mak which has enough aperture to reach many of the fainter doubles and (an important point) it is reasonably compact. Big newtonians and refractors may give excellent views, but they aren't that compact or convenient to use! For me personally, I have another issue as I live next to fields with wheat, barley and huge amounts of dust on occasions (eg yesterday) That makes open tube designs like newts or classical cassegrains less practical.

Chris

 

 

 

Edited August 4, 2020 by chiltonstar

[John]

2 hours ago, Kirby301 said:

I've been reading and watching YouTube regarding the aperature masks. I like the idea alot. They all seem to do a single hole though. Could you explain in simple terms for a simple mind the way you mean please?

I have one for my 12 inch dob. It is a simple disk of cardboard with a hole cut in it that is the maximum size that will miss the secondary support vanes and the secondary and it's support boss. The pattern is like this with the "E" measurement in my case being around 110mm in diameter. The mask is fixed over the open top end aperture of the tube.

It works and the views I get are quite close to those that I get with my 102mm ED doublet refractor.

 

[Kirby301]

I'm pretty much set on a mak 127.

I'm a curious type of person so I'm going to try the aperture mask, the way John has described on Almach and see what difference it makes.

[F15Rules]

You might find this old thread I started a few years ago helpful?

It was a real comparison of two great scopes, either of which would be great on double stars. At F11 and F10 they aren't as narrow a field as some other Maks and the M603 is apochromatic.

If you want any further info drop me a pm🌝.

Dave

[vlaiv]

4 hours ago, Kirby301 said:

I've been reading and watching YouTube regarding the aperature masks. I like the idea alot. They all seem to do a single hole though. Could you explain in simple terms for a simple mind the way you mean please?

This is really not easy thing to explain as it involves quite a lot of math an physics, but let me try the simplest possible explanation.

It is about perpendicularity. When you have straight line - at each single point light gets diffracted perpendicular to that line. Long straight line just means a lot of light gets diffracted in just one direction - perpendicular to that line. All of that light gets "summed" (because telescope focuses light) so it all ends up in one straight line of light.

This is what you see - your stalk produces single line of diffraction.

Curved shapes and circles in particular are interesting because at each point along the circle light is also diffracted but perpendicular direction at each point along the circle is at slightly different angle - it is no longer all in the same direction. This means when you sum the light it no longer creates a single line but is spread around in a "halo".

If you think about it - telescope aperture is an edge too and it also diffracts light - it spreads it in a "halo" and when that "halo" gets focused it produces Airy pattern because aperture is circular and not straight. Secondary obstruction does the same it also produces halo and this halo gets focused and when combined with aperture edge - produces slightly different Airy pattern - moves a bit of light from central part into rings.

We want to remove diffraction of light from a stalk and spread it around so we take a bunch of circles and cover stalk with those. Now these little circles are doing the same as central obstruction and since stalk is no longer in light path - it's not doing it's own diffracting of the light and prominent spike is no longer there.

That would be explanation. Confirmation that it works can be done in two ways - one is mathematical analysis and other is experiment - rather actually doing it on your scope and looking at a bright star.

I can do math stuff for you (don't worry it won't be actual numbers - but rather images) but you'll have to do hands on approach if you are interested to try it out.

I'm now going to show you 4 different telescope apertures and diffraction effects that they produce. This will be done via simulation (Fourier optics approach).

First of course is clear aperture:

Left is image of aperture and right is image of star - Airy pattern is observed of course and no diffraction spikes ...

Here we see what newtonian scope with central obstruction and 4 spider vanes looks like. Airy disk is now a bit different as light distribution is different and we now have 4 diffraction spikes.

This is configuration with single stalk - there is only horizontal line and diffraction / spikes are actually smaller than in 4 vane configuration because length of edge that is is doing diffraction is shorter (single stalk vs 4 vanes of same length).

And here we see what happens when we add a mask of bunch of small circles to cover that stalk. We again have no prominent spike (I really stretched that image to show what is there). We can sort of see that we get like bunch of small spikes in every direction and that is the point - there is no single spike but rather "infinite" number of spikes - each one at a slightly different angle and all of them so faint that you won't see them.

Just make sure that you do your circles as round as possible. If you don't get them round enough, you'll get something like this:

Instead of nice faint halo (this is very long integration with camera on a bright star and it captured every little detail) you'll get distinct spikes. This happens if your circle is not perfectly round but rather consists out of little line segments - each little line segment will make its own small spike

Above image is because I used PVC aperture mask and I cut a hole in it with a saw so it was not very round and I did not bother too much with a file / sand paper to make it perfectly round.

Just to be clear - above image is not of this array of circle aperture mask - but a regular aperture mask I made for my achromatic refractor, it just illustrates my point about circles needing to be round.

[Kirby301]

Wow!

Thanks for explaining that, I fully understand now.

By doing the circles on the stalk I will keep most of the aperture and hide the stalk which will lose the diffraction spike, rather than one bigger hole which will hide the stalk but lose alot of aperture aswell as the diffraction spike?

[vlaiv]

8 minutes ago, Kirby301 said:

Wow!

Thanks for explaining that, I fully understand now.

By doing the circles on the stalk I will keep most of the aperture and hide the stalk which will lose the diffraction spike, rather than one bigger hole which will hide the stalk but lose alot of aperture aswell as the diffraction spike?

Exactly. Both masks will have rounded edges which is important to spread the light and prevent spikes from forming, but this one that only covers the stalk blocks much less light.

[mikeDnight]

17 hours ago, Kirby301 said:

I think the mak is going to be the best option too.

How long does the 127 take to cool? I'll be keeping it in a wooden shed.

Sometimes I think the emphasis on cool down issues can be over exaggerated. The 127 is a small bodied scope and cools quickly. A few years ago a friend brought his 127 Mak round for me to play with. As he lifted the tube assembly out of its foam lined carry case, he told me he hadn't used the scope for about 18 months. I mounted it on a Vixen Porta mount, aimed it at Jupiter which was high in the sky, and was amazed at the immediate sharpness and definition in the image. It performed straight out of the box, so by keeping the scope in a coolish room you'll pretty much guarantee a good view from the start. To be honest, I'd suggest taking the tube into the house rather than keeping it in a shed where damp and fungal growth may damage the optics.

[Pixies]

1 hour ago, vlaiv said:

Exactly. Both masks will have rounded edges which is important to spread the light and prevent spikes from forming, but this one that only covers the stalk blocks much less light.

Vlaiv,

What would happen if you tried the following type of mask to hide the 4 spider vanes?

[melsmore]

My MAK127 is kept indoors (no cool room) and It takes 30 minutes or so to cool down - almost exactly what @vlaiv extrapolated from his!  I plonk it outside first and by the time I've polar aligned my mount and faffed around it's just about good to go. Wooden sheds (in direct sunlight) can soak up a lot of heat and cool down slower than a MAK 🙂 The only (very minor for double star observing) drawback I have concerning my MAK127 is that I can't use 2" eyepieces with it .  Also, mine came with a RDF,  and as I don't have GOTO, I'd have preferred a finder scope.

@Stu recommended the C925. I've never used one, but I have had this SCT recommended to me for this by others too.  Apparently, it is good because theirs  have the same secondary mirror as in my C8 giving a much smaller % obstruction - more MAK like in fact . 

When you say "do double stars" do you mean "looking at them and seeing if you can split them", or do you mean "measuring their position angle and separation too"? If it is the latter, then there are other considerations and I would recommend you read Argyle's book before deciding exactly what you want.  I should say that I have the first not second edition, but I expect the second to be better.

Edited August 4, 2020 by melsmore
Added disclaimer on book edition.

[vlaiv]

9 minutes ago, Pixies said:

Vlaiv,

What would happen if you tried the following type of mask to hide the 4 spider vanes?

That is a good question, I know that there would be no diffraction spikes, but I don't know what total impact on the image would be.

We can run tests to see. Let me do that and I'll discuss the results.

So airy disk would be actually smaller - but rings would be quite distorted and there would be split of rings into 4 zones. This is image of airy disks without stretching - I'm now going to stretch the images to show impact in faint areas:

While central disk seems to be narrower - I think that overall blur will increase and you'll decrease resolution / resolving power of that telescope somewhat.

Probably best way to asses what is going to happen with resolution is to examine MTF of both, so here is that:

MTF is rather strange looking and it depends on how frequencies are "oriented" - in another words image of a star is not round as we have seen but has some squareness to it - diffraction rings are broken in certain places and it might visually look more like little square than a circle under very high magnification with this mask, but examining MTF reveals something interesting.

View will be dull and washed out, but you won't loose much of high frequency detail - you'll still be able to split double stars - possibly only problem will be with orienting telescope - resolution will be better in certain orientations than in others - if you have two squares they can either be turned towards each other with their side - then you can place them closer or with their corner - then corners will overlap if you put them that close.

Again I think this sort of mask is worth a try since we can't really tell how strong this funny looking diffraction rings will be - from first set of images, they don't really look that strong and might not detract too much from the view. From MTF diagram, I can see that medium and lower frequencies will be attenuate quite a bit - which means very low contrast on lunar planetary, but higher frequencies behave the same (both graphs behave almost the same from 500 on wards - to the right - those are highest frequencies of the image).

 

[melsmore]

Very sorry, I just checked the current models of C8 and C925 on FLO's website and it is certainly no longer the case concerning the secondary mirror sizes.

C925:

Secondary Mirror Obstruction: 3.35 in (85.09 mm)
Secondary Mirror Obstruction by Area: 13.1 %
Secondary Mirror Obstruction by Diameter: 36.2 %
Optical Tube Length: 22 in (558.8 mm)
Optical Tube Weight: 20 lb (9.07 kg)

C8:

Secondary Mirror Obstruction  : 2.5 in (63.5 mm)
Secondary Mirror Obstruction by Area  : 9.8 %
Secondary Mirror Obstruction by Diameter  : 31.3 %
Optical Tube Length  : 17 in (431.8 mm)
Optical Tube Weight  : 12.5 lb (5.67 kg)

Sorry, next time I'll check before I post.

[Pixies]

31 minutes ago, vlaiv said:

Again I think this sort of mask is worth a try since we can't really tell how strong this funny looking diffraction rings will be - from first set of images, they don't really look that strong and might not detract too much from the view. From MTF diagram, I can see that medium and lower frequencies will be attenuate quite a bit - which means very low contrast on lunar planetary, but higher frequencies behave the same (both graphs behave almost the same from 500 on wards - to the right - those are highest frequencies of the image).

Thanks Vlaiv,

I was thinking about making a mask to remove diffraction spikes (just a simple single circular hole), so I might go ahead and make a 4-hole one, too!

[Kirby301]

I won't be keeping my telescopes in the shed from now on.

I will only be do the visual aspect of splitting doubles not the technical type!

[RobertI]

4 hours ago, vlaiv said:

We can run tests to see. Let me do that and I'll discuss the results

Fascinating results. I have also often wondered what effect a ‘four way aperture mask’ would work - now I know!

[Kirby301]

I've hopefully found a good way to do the aperature mask. I've cut down a plastic straw to the size of the stalk, then down the length of it so it will wrap around the stalk.

I've cut out 3 penny size circles out of a cereal box and stuck them onto the stalk.

Now I've just got to test it out!

[vlaiv]

1 minute ago, Kirby301 said:

Now I've just got to test it out!

Looking forward to hearing about first hand experience on that.

[John]

This is my 12 inch F/5.3 dobsonian secondary support arrangement. Obstruction is 21% of primary diameter:

 

 

[Deflavio]

Sorry not sure if I should open a separate post for this but while talking of different spiders... just out of curiosity how the diffraction spikes from a  "double - 4 spider vanes" would look like ?

On one side, I see from teleskop-express the "double spider" can help with focusing somehow like a bahtinov mask but without the need to remove it. That would be useful but, is it true?

https://www.teleskop-express.de/shop/product_info.php/info/p6013_TS-Optics-Carbon-Double-Spider-for-225-240mm-inside-tube-diameter.html

 

On the other, digging past posts using this spider, I also see that the diffraction spikes are also "dashed". Someone mentioned the effect of NB filters but and I can see the same pattern on a colour camera. See these two posts:

 

 

So are these double vanes the cause for the interference pattern? Maybe due to not perfect focus 🤔 ??? 

Sorry to go a bit OT but I got intrigued by the possibility of using spiders for improving focus.

 

F

Edited August 20, 2020 by Deflavio