Choosing a good star diagonal/prism for my telescope

[danindu001]

Hi everybody,

I was going through eBay listings and various other sites looking for a 45 degree diagonal. First I thought of getting a prism diagonal (Amici), but after seeing stuff on wikipedia, forums, and reviews it seems that prism diagonals have brightness reducing issues and aberrations. So I started looking at mirror diagonals and found various types with dielectric coatings, and various other specs which I cannot even begin to comprehend. So I need a little help on understanding these features.

I use a Celestron Powerseeker 70 refractor (fl = 700mm) and I have an Orbinar Plossl 10mm eyepiece as my default viewing eyepiece, which to the contrary of many many negative reviews here have proven to produce reasonably bright, aberration free images. I also use a FMC Barlow lens with the eyepiece, so the brightness issue has to be resolved when getting the diagonals since there will be more and more optical surfaces with each accessory, right?

And, I just need this diagonal for a comfortable viewing. It is not a problem for me if the images are upside down or laterally inverted. The thing that I really care about is the diagonal's effect on brightness of the images. So which do I choose? A prism or a mirror diagonal? (Not more than $30 and it has to be 45 degrees) Or any other suggestions are welcome!!

Thanks!! :laugh:

[ronin]

If brightness is the prime aim, and it sounds like it is, then I suspect you are looking at one of the di-electric mirrors, quartz substrate maybe, with 99% reflectivity.

However it is very doubtful that if a diagonal reflected 90% or 99% that you would see the difference. I would expect that the real difference is that the mirror is flatter and so the aberrations caused at the reflection surface are less.

To throw a spanner in the works I think BillP did a review of prism diagonals on the CN forum and found they were very good - I read some of it but not enough to get to the part of which was best, you would have to go dig out the review.

There are a few arouns, WO, Baader, Skywatcher, in the 1.25" version they seem  to be around £90-100 here in the UK, I have no idea what the cost is in Sri Lanka nor the availablity. For whatever reason WO have always made good replacement.upgrade diagonals.

[danindu001]

If brightness is the prime aim, and it sounds like it is, then I suspect you are looking at one of the di-electric mirrors, quartz substrate maybe, with 99% reflectivity.

However it is very doubtful that if a diagonal reflected 90% or 99% that you would see the difference. I would expect that the real difference is that the mirror is flatter and so the aberrations caused at the reflection surface are less.

To throw a spanner in the works I think BillP did a review of prism diagonals on the CN forum and found they were very good - I read some of it but not enough to get to the part of which was best, you would have to go dig out the review.

There are a few arouns, WO, Baader, Skywatcher, in the 1.25" version they seem  to be around £90-100 here in the UK, I have no idea what the cost is in Sri Lanka nor the availablity. For whatever reason WO have always made good replacement.upgrade diagonals.

So, as you said, if there won't be a visible difference if the reflectivity were 90% or 99%, what about an Amici erecting prism? Also, what percentage of reflectivity would be tolerable (for my telescope)?

[ronin]

No idea about the erecting prisma, I tend to go for the least number of things and a front silvered mirror is the simplest.

As said for a prism diagonal find BillP's review on CN.

The problem is that like eyepieces a cheap prism diagonal will be bad, whereas a good well made one is more cost but delivers the performance.

The simplest and easy option is a good well made, flat, front silvered mirror diagonal, such as WO, Baader etc. They are in effect a single surface, a prism will have 3, 4 or 5 faces that all have to be accurate.

[Stu]

Ronin, with a budget of $30 dollars, BillP's review is not relevant, those are all high end prisms and diagonals. It's perhaps best to fully read reviews before referencing them.

Danindu, is there a particular reason why you are looking for 45 degree? Most astronomers use 90 degree diagonals as it makes observing at the zenith and higher altitudes more comfortable.

The diagonal is nearer to focal point, so can have an effect on the image you see. I would avoid cheap amici prisms, or prisms in general and go for as good a quality 1.25" mirror diagonal as you can find. There are some over here from Celestron and Lacerta for around £20 so it looks possible.

Of course you could spend a lot more but I think something like that would be fine in your scope.

Good luck

Stu

[YKSE]

I agree with Stu, BillP's review is excellent, a very enjouyable read, but irrelevant when we're talking about $30 diagonals.

I have a WO 45 degree erecting prism diagonal for terrestrial viewing, it's noticeable dimmer than the stock 1.25" diagonals, not suitable at all for stargazing is my conclusion. Here is a good explanation by Don Pensack.

[danindu001]

Thank you all for the valuable info, guys. So a mirror diagonal it is!

And, I wanted a 45 degree one because I have a tall tripod since I use the telsecope in my backyard, and that gives me an advantage. So the 45 angle is mostly good for viewing even at lower altitudes.