Testing of GSO RC 8"

[Paul Haese]

Ok, this is my first chance to have made any real tests of this scope. One thing that bugged me was that when I opened the box and pulled out the OTA I discovered a small scratch about 1 inch long on the carbon fibre. Probably nothing to complain.

Anyway, I have done my first check of the optics. I placed an EP on a diagonal and focused on the tree tops not far from my home at Blackwood. The image when focused is sharp and good detail can be seen in the leaves (veins). The view is very near refractor quality. Certainly promising signs from such a budget OTA. My main purpose here was to see if it would focus. I had read that some of the OTA's reviewed would not focus properly. Given that focus is sharp, this is a step in the right direction.

My next step in testing will be the star test. I will be looking for rough surfaces and turned down edges. Collimation will also be on my list of tests too.

I was going to try tonight but the cloud has moved in unexpectedly and may prevent me testing further at present.

My final test will be imaging, that is expected to take place in the next two weeks, but you never know what the weather is gong to be like.

I will add to this thread as I complete my testing regime.

[Paul Haese]

Ok Star test conducted.

Aside from miscollimation, which I will have to attend to the following was observed:

- the exhibits no astigmatism
- no roughness in the figure via star test.
- no apparent turned edges or circular zones.
- diffraction rings were even and sharp all the way from the outside to the shadow of the secondary.
- stars were as sharp as in my 4 inch Tak at focus.
- no coma was seen to the edge of a 24mm pan optic.

In short, these optics were impressive at first light. The airy disk was easily visible at focus with a nice even pattern surrounding it.

From the star test I cannot see what all the fuss was about before hand. It might not be a 24" RC or CDK, but for 2.5K it looks quite reasonable.

Now for the imaging tests. Stay tuned.

[matt]

Interesting stuff, Paul. You'd certainly be happy with what you've seen of this scope so far.

Looking forward to seeing the images it produces.

Will you have a go at planetary, even with the large-ish central obstruction?

[Paul Haese]

Matt, these scopes do look impressive so far. However, the primary strength of these scopes is for its coma free optics and sharp stars. The design of this type of telescope is not for planetary imaging in mind. The central cone would be less sharp than say in a Celestron. It is like the Meade ACF. Nice scope but not a planetary imaging scope. You just don't see images of planets taken with the Meade ACF's. Even images taken with RC's are at best pretty ordinary generally. Mind you I would like to give a 16"-24" a go just to see if it is operator error or something else.

Yeah the central obstruction would also hinder the contrast. So stick with that 9.25 Matt, they are a killer scope for planetary work. I might have a play but it is unlikely that I will use this scope for planetary imaging when I have the C14. Nor would I suggest that anyone use this type of scope for that type of imaging.

[gbeal]

Good news thus far then mate, and hopefully it will allay those rain on yer parade comments.
Gary

[matt]

Yeah...I figured you'd use it primarily for DSO...but I also suspected you wouldn't be able to help yourself turning it on a few planets....just for the sheer heck of it...;)

I know I'd be curious to see how it performs, not that I have any plans to buy one of these scopes.

Cheers...and good luck with it.

[Gama]

You can still get a airy disk and rings on a bad optical system too.
What i would do is see how well it can seperate very close stars and how much detail it can show.
I really hope its a winner, as i may even get one to use for wide field work.

Theo

[Paul Haese]

Theo,

that is the reason for my next test of imaging with the OTA. Closely separated stars will certainly show what the figure of the mirror is capable of. I did not state that a visual test was definitive, it is just one part of the testing regime I have planned.

I hope that these telescopes are winners too. It will make significant in roads into the reduction of costs for optical equipment.

[Paul Haese]

Just wanted to add one further thing. The incident of internal reflections is fairly high in this OTA. I moved Canopus from one side of the field to the other and when I was around 10mm from the apparent edge of the field there were internal reflections visible. I thought the baffling was supposed to prevent this from happening. Although I thought I read read that Jase had similar problems with his RCOS and internal reflections. I might well have been the EP design that was causing this, but it was definitely there.

[Gama]

If you look thru the focuser while the eyepiece is out, you should not be able to see anything except the secondary and main mirror reflection on the secondary. If you can see thru the front of the telescope in a straight line, then baffling is not correctly designed..

Theo.

[Kal]

Paul, I'm sure you saw this pic from the cloudynights thread, where Leonardo from Italy had some obvious reflections in his image of the horsehead from the bright star near by link (http://translate.google.com.au/translate?u=http%3A%2F%2Fstelledelc ielo.blogspot.com&sl=it&tl=en&hl=en&ie=UTF-8)

It seems strange since on first glance the scope appears to be well baffled :shrug:

[Paul Haese]

Yeah I saw that image, it is a bit disturbing to say the least. Mind you you can just tell which star is causing the internal reflections.

[avandonk]

It is internal reflections of Alnitak in the camera or filter assembly. It can also come from the immediate border of the sensor if the just out of field bright star image happens to fall on it. If the telescope caused it due to poor baffling it would be far more diffuse. This is easily solved by fitting rectangular knife edge apertures just in front of the camera or sensor.

A lot of high end astro cameras suffer this lack of tidy housekeeping. DSLR's rely on the lens exit design for adequate baffling. This is generally found on high end lenses. My 24 to 105 mm F4L has a rectangular aperture as the last thing on the lens.

I would bet one on it!

Bert

[Satchmo]

Thats correct. If for example you see the second diffraction ring from the Airy Disc almost as prominent as the first, under good seeing ,on a 4th or 5 magnitude star , with a medium sized instrument ,that's indicative of 1/2 wave wavefront or more of pure spherical aberration.

Mark

[gregbradley]

Getting reflections off of Alnitak isn't much cause for complaint as not many scopes don't.

I have seen large expensive RCOS's do the same in images.

As Bert points out there are other causes than the scope for reflections.

Unblackened edges of filters, shiny parts inside the CCD chamber and an aperture mask to the CCD can reduce these. Apogee sent one out to purchasers of their U16M camera after research found it reduced reflections.

Greg.

[Paul Haese]

An Update

Ok did another test this weekend. I attached the 40D to the OTA and tried to focus it on the trees in the nearby paddock (distance of 400m or so). With the Tak extension in and with the focusor only 20mm extended out and with the complimentary extension in place between the focusor and the OTA I acheived focus. I noted the following in my viewing through the view finder:

- There is some flop in the focusor. I applied a small amount of pressure on the underside of the camera and this managed to change the field of view. Not by a lot but enough to convince me that this focusor will have to be changed in the end. I will have to do several images to determine whether the focusor is flexing too much during an imaging run. Perhaps a better focusor will be the better option.

- I had to use a Takahashi extension to obtain focus. This means that from the rear of the OTA there is something like nearly 10"s ( guess really but looking tonight at the tape measure it seemed the correct length) to the focus point. I would expect that it would be better to have the screwed in extensions in place rather than using an extension from the focusor out.

- With the Takahashi extension I kept the flattener in place. It appears that this flattener might well be ok for imaging. The image looked nice and flat, but proof will be in imaging star fields. I also noted that there appeared to be vignetting to the corners in the view finder. I suspect the light cone is not wide enough to illuminate the whole of the chip. Again imaging will no doubt tell us the truth.

So with that I will hopefully next week obtain my first images through the OTA. I will most likely image Eta Carina. Being a region with lots of stars and nebulosity I see this as the idea test subject