2" diagonal

[jambouk]

What are, if any, the advantages of using a 2" diagonal in a skywatcher 180 mak, when the visual back of the ota is only 1.25"?

Thanks

James

[alan potts]

I have a 180 pro Mak on order and I thought it was a 2 inch back. That will be a pain if it is as everything I have is 2 inch. The best think with 2 inches is you can get much wider FOV and lower magnifications.

Alan.

[jambouk]

The fitting which screws onto the ota can accept 1.25" or 2", but the fundamental hole in the ota, as far as i can tell, is just a bit shy of 1.25" (as in i can't stuff a 1.25" EP into the ota itself.

Unless the thing which i'm trying to stuff an EP into also comes off, but it seems to be permanently attached.

James

[Mark at Beaufort]

I thought that the original 'gold' 180 Mak was 1.25" but the new 'black' variety is 2" and I think adverts state that situation. As Alan has stated you get a wider FOV and lower magnifications and with a focal length of 2700mm and f15 that has a great advantage.

Alan I will interested if your view of the 180 Mak as that is one scope that I have always fancied buying.

Mark

[jambouk]

This is the fundamental back as far as i can tell for the ota.

james

[Mark at Beaufort]

James that is a useful photo and shows that you have the new 'black' variety. I know that the adverts on FLO state that a 2" diagonal is provided so I assume that the system must work okay. Whether you get vignetting with a really wide 2" EP perhaps one of the users on the forum will answer that point.

James by the way I deleted your duplicated post.

Thanks for posting the image.

Mark

[jambouk]

Thanks for deleting the duplicate posting.

Do you think that comes off and i could fit a 2" directly onto the ota?

James

[jambouk]

Also, i think he mak to sct ring has a 1.25" hole in the middle, not a 2" hole, but i'd have to check and measure that when i get home.

James

[jambouk]

Taken the other night before cloud set in!

james

[Mark at Beaufort]

James that is a fabulous piece of kit. I saw the Skywatcher AZ EQ6-GT mount at SGL8 and thought what a great flexible and useful mount. Looking at your photo because you have a SCT crayford dual speed focuser screwed into back of the OTA why don't you just get a push fit 2" diagonal or am I missing something.

Mark

[jambouk]

I got the ota second hand on monday, it came with a 2" diagonal.

I can use the 2" diagonal with he crayford style focuser or with the original bits which screw onto the ota.

My question is more academic: what are the advantages of using a 2" diagonal when the fundamental hole in the ota is only 1.25".

(I'm very new to all of this)

James

[alan potts]

Mark, When it comes, might already have done so. I will be doing a full review on it and intend to try some sort of comparison with the 190mm M/N. How I am going to pull that off I have yet to work out, maybe I could start by saying they are both black. I have the range of eyepieces to compare like magnifications within a same margin, so mainly I will focus on image quality. I am sure I will cook something up. Alan.

[alan potts]

James, Just because that is the case does not mean that you will get a vignette effect, the hole only needs to be big enough to allow the column of light through which at that point will be less than the holes diameter. Hope you understand what I have rather badly written, I know what I mean. The other advantage of the 2 inch system is it costs more, this empties your pockets making your jeans last longer. Alan

[jambouk]

Ah, i see!!!!! So in effect, the light path could go through a pin hole, then open out onto a 2" diagonal.

How could i work that out? Is there some way i could asses for vignetting?

James

[John]

I don't think the aperture at the rear of the scope will vignette the light cone coming from the scope optics - it would be very bad design if it did !

The light cone from the scope does not widen out as it passes through the diagonal though.

It's possible that some of the wide field 2" eyepieces that have field stops wider than the aperture at the rear of the scope may not show a fully illuminated field of view I guess. You would not notice a sudden cut off of the light though - I think it's a gradual reduction in the light towards the edges of the field of view, if it happens at all.

[alan potts]

James, I wouldn't go so far as to say it would go through a pinhole but I think you get my point. As the light is focused into a cone and it could well be that the cross-over occurs around the point you describe, I can't check on it as I am no where near home at the moment, I'm 1600 miles away to be exact, but I think we have a similar thing on my 12 inch SC scope, this is I believe less than 2 inches. I would say that if you can afford the upgrade it is a step well worth taking. There are some nice 2 inch diagonals on the secondhand market and they come up fairly often, also with F15 you do not need to spend a fortune on larger eyepieces, even though I have. What you will need to do is be careful of which eyepieces you get but for weight factor, not quality. I have a 41mm Panoptic and a 40mm Meade SWA and both come to around a kilo in weight. The 40mm Aero which I also had would be superb in this scope and weighs about half the afore mentioned, it is also about half the price too. I am also sure you will not have a vignette problem, be guided by the sales spec, they would not sell this with a two inch if this was the case, Alan

[jambouk]

So when people talking about the "visual back" what exactly are they referring to? The bit on the ota which cannot be removed, or the bit which is supplied with the ota which screws or slots onto this?

James

[Mark at Beaufort]

Because the Mak/Cass is f15 the quality of eyepieces is not so critical as on a fast Newtonian. I would guess that the Skywatcher Panaview range might be okay especially the 38mm and they are not that expensive. - http://www.firstligh...-eyepieces.html

Mark

[jambouk]

Also, is there an easy way to see how much actual fov i get with either 1.25" or 2" set up?

[Stu]

I use a 2" diagonal on my f20 mak, as much as anything because it just feels more secure! These scopes are never about fov, so in theory there's no advantage to using 2" eps....but, as mentioned, they are very nice to use, giving high mags and maintaining good eye relief. Max fov I can get with mine is about 0.7 degrees. 31t5 gives x129....eek. I don't notice any vignetting when using 2" eps.

Stu

[rowan46]

So when people talking about the "visual back" what exactly are they referring to? The bit on the ota which cannot be removed, or the bit which is supplied with the ota which screws or slots onto this?

James

"or the bit which is supplied with the ota which screws or slots onto this?" this

[John]

A reasonable idea of the field of view that a certain eyepiece will give in a scope is to divide the apparent field of view of the eyepiece by the magnification it gives with the scope. So a 31mm Nagler has an apparent field of view of 82 degrees and, in the 180 mak, gives a magnification of 87x. 82 divided by 87 = .94 = the true field of view in degrees, ie: just a bit less than one degree.

[jambouk]

Is there an easy way to measure the fov in day light?

James

[jambouk]

So, with the crayford like focuser i've got for the scope, i take of the original "visual back" and then what part of the crayford or the mak to sct adapter referred to as a "visual back"?

James

[alan potts]

I am sure the fitting system will be the same as my SC scope. Unscrew the fitting tube off the back and just screw the focuser on the same thread. There should be some sort of adjusting screw on the focuser that you can adjust the orientation of the focuser with and Bobs your uncle. The diagonal will slide into the focuser and then you use your various eyepieces with or without the 1.25 adapter. As far as I know Visual back it just a term used to describe the size of back fitted, be it 2 inch or the smaller one.

Alan.