Telescope for doubles

[Kirby301]

After learning about diffraction spikes on reflectors I'm left wondering what telescopes everbody else uses for the doubles, the diffraction spikes annoy me so I'm looking for ideas for my next telescope.

[andrew s]

How about a nice Maksutov? No diffraction spikes and the added bonus that the central obstruction enhances high frequency resolution compared to unobstructed due to narrowing the PSF at the expense of increasing the first ring.

Touch paper lit 👹

Regards Andrew 

[DaveL59]

or there's a very nice 6-inch frac going in the classifieds section, bargain price too 😉 

[Stu]

I think a lot depends on what you enjoy. Aesthetically a decent apo refractor will give lovely clean star images and should give you some good success.

In terms of ultimate performance, aperture counts as that defines resolution and resolving power relating to double separation. As Andrew says, something like a 180mm Mak would be a good option, or a C925 SCT?

What sort of budget would you be thinking of?

[Stu]

4 minutes ago, DaveL59 said:

or there's a very nice 6-inch frac going in the classifieds section, bargain price too 😉 

Ah yes, that would work quite well I should think.

[iPeace]

1 hour ago, Kirby301 said:

I'm left wondering what telescopes everbody else uses for the doubles

Depends on: which doubles?

• I enjoy observing plenty of them with small-aperture APO refractors. This keeps me happy most of the time.
• For doubles which are too tight and/or too faint for a small refractor, I need aperture, that means an 8" F/6 Newtonian. A 6" F/8 Newtonian also works very well. (I know, you don't like the spikes, just answering the question )

...for me, the point is that there are suitable doubles for every aperture (so to speak), so I just choose what to look at based on what I feel like carrying outside and setting up.

[vlaiv]

Not all reflectors have diffraction spikes - even newtonians.

Diffraction spikes are feature of straight spider vanes. Curved secondary support when done properly does not produce spikes and many people that don't like spikes for planetary or double star work modify their telescope with curved spider vanes.

 

[vlaiv]

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.

[johninderby]

How about a classical type of refractor.🤔

https://www.firstlightoptics.com/scopetech-telescopes/scopetech-stl-80a-maxi-80mm-f15-classical-refractor.html

 

Edited August 3, 2020 by johninderby

[andrew s]

47 minutes ago, iPeace said:

Depends on: which doubles?

• I enjoy observing plenty of them with small-aperture APO refractors. This keeps me happy most of the time.
• For doubles which are too tight and/or too faint for a small refractor, I need aperture, that means an 8" F/6 Newtonian. A 6" F/8 Newtonian also works very well. (I know, you don't like the spikes, just answering the question )

...for me, the point is that there are suitable doubles for every aperture (so to speak), so I just choose what to look at based on what I feel like carrying outside and setting up.

Yes a lot depends on what you want to do and of course budget. How close a doubles tempt you? Personally I would go with a scope that, under good conditions, can resolve 1" i.e. at least 110 to 120 mm then you will mostly be seeing limited.

Regards Andrew

[mikeDnight]

2 hours ago, andrew s said:

How about a nice Maksutov? No diffraction spikes and the added bonus that the central obstruction enhances high frequency resolution compared to unobstructed due to narrowing the PSF at the expense of increasing the first ring.

Touch paper lit 👹

Regards Andrew 

I agree! 

The amount of light that is redistributed to the first diffraction ring in a scope having a small central obstruction is likely to be so slight that it would be at the borderline of visual perception.  Diffraction from spider veins however can be distracting on brighter stars, so a good Maksutov  would be a great scope. IOptron produce a beautiful looking 6" Rumak that I hope someone will buy and review. It looks quite appealing!

[andrew s]

2 minutes ago, mikeDnight said:

I agree! 

The amount of light that is redistributed to the first diffraction ring in a scope having a small central obstruction is likely to be so slight that it would be at the borderline of visual perception.  Diffraction from spider veins however can be distracting on brighter stars, so a good Maksutov  would be a great scope. IOptron produce a beautiful looking 6" Rumak that I hope someone will buy and review. It looks quite appealing!

Yes I was tempted by the IOptron Rumak before going for the 180 Tak Mewlon. 

Regards Andrew 

[iPeace]

1 minute ago, andrew s said:

How close a doubles tempt you? Personally I would go with a scope that, under good conditions, can resolve 1" i.e. at least 110 to 120 mm then you will mostly be seeing limited.

It goes without saying that magnitude/apparent brightness is also a factor; I may have split doubles of which I couldn't see the faint secondary...but I wouldn't know.

My refractors are 60-85mm, so my limit is usually closer to 2" - as long as the stars are bright enough for me to enjoy.

A multiple like Tegmine is attractive enough to push out a newt, but it's usually the other way around: I'll try for Tegmine if I happen to be using the newt anyway.

I'm sure your assessment makes sense, using a nice 120mm APO to get down to 1" subject to seeing. In my own case, I didn't enjoy using that large a refractor as much as I do enjoy using a larger Newtonian - and I don't mind the spikes.

[Kirby301]

So so much to consider! 

I can narrow down easily with budget.

I would say around £500.

No apos or big maks then 😆

I saw the cr6 in the for sale section and immediately liked it, only problem is with a big refractor comes a heavy mount, im used to a heritage 130p!  and its in Wales!

This brings me onto the maks and the portability of them.  I was looking at the skymax 127 but then heard about the long cool down times.

Ideally I would like something easy to grab and go, not have to wait too long for cooling down and to see a good amount of doubles with  and for £500 🤣

Unfortunately I know there's not something like that unless I do go down the newtonian route, @vlaiv any good links to curved secondary support I will have a look into it.

[mikeDnight]

If you keep the 127 Mak in a cool dry place it will give great views. Another alternative could be a second-hand 100mm ED which could also come within your budget. Both the above will be excellent double star scopes!

[iPeace]

I used a Sky-Watcher Skymax 102 for a while, it didn't need to cool for very long at all.

Very portable and easily mounted.

https://www.firstlightoptics.com/maksutov/skywatcher-skymax-102-ota.html

[iPeace]

Just thinking...

24 minutes ago, Kirby301 said:

im used to a heritage 130p

...which should be able to show a lot of worthwhile stuff...

I know this is over-sensible, but I'd also consider enjoying the spikes for a while longer (like stars on Christmas cards).

 

[JeremyS]

1 hour ago, andrew s said:

Yes I was tempted by the IOptron Rumak before going for the 180 Tak Mewlon. 

Regards Andrew 

Heart win over head, Andrew? 🙂

 

[Stu]

55 minutes ago, Kirby301 said:

im used to a heritage 130p!

They are actually very capable scopes. With decent eyepieces in, they will split quite tight doubles. I have managed Pi Aquilae with mine, which is 1.4”, not bad at all!

Does £500 have to include a mount?

[andrew s]

8 minutes ago, JeremyS said:

Heart win over head, Andrew? 🙂

 

No, I ideally wanted mirrors only and the best optical quality I could get. When one came up second hand at good price it was a no brainer (which somehow seems strangely contradictory).

Regards Andrew 

Edited August 3, 2020 by andrew s

[vlaiv]

1 hour ago, Kirby301 said:

Unfortunately I know there's not something like that unless I do go down the newtonian route, @vlaiv any good links to curved secondary support I will have a look into it.

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.

 

 

[John]

Personally I prefer using refractors for double star observing. On your budget, if you have the mount for it, I might be looking our for one of the Bresser 127L F/9.3 achromats.

Mak-cassegrains are also good and mak-newtonians too. They produce star images that are really very "refractor like" and you can buy aperture for less than good refractors per cm.

 

[Kirby301]

Unfortunately I would have to include a mount in the budget too 🙁

I will have to try the aperture mask on the 130p.

[vlaiv]

25 minutes ago, Kirby301 said:

Unfortunately I would have to include a mount in the budget too

What's the budget? £500?

Very simple solution would be:

https://www.firstlightoptics.com/sky-watcher-az-gti-wifi/sky-watcher-skymax-127-az-gti.html

 

[Kirby301]

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.