Posts Made By: Phil Barker

I've been using an old (1980's) coulter 8 inch f-4.5 for some years now. It resides at the local observatory and is clearly better than a twin coulter purchased at the same time. I might test this one next. I have been cleaning the mirrors and thought what the hell I'll make a mask and foucault test her.

here's the result.

radius of curvature 1805mm
203mm

25mm plate mirror.

Looking at the mirror with a ronchi its a nice smooth looking parabola. It appears free of zones, edge isn't turned down and looks OK.

With the foucault tester it looks rather smooth in terms of figure again confirm no obvious high or low points.

Some minor roughness visible nothing to be too concerned about.

I made a mask to measure zones from .3,.6,.8 and.95 radius.

Prior to doing this I did'nt calculate the respective positions.

I took 3 sets of measurements and rotated the mirror for each set averaging out the measurements. I found little variation.

Using .3 as a starting point (therefore 0)I got 1.78mm at .6,3.81mm at .8 and 5.08mm at .95.

This is the average only the image was very stable and rather easy to read.

The mirror is therefore moderately over corrected as the correct measurements would be for a perfect parabola

0,1.56mm3.13mm and 4.66mm.

This could simply mean .3 is a low point but the scope does startest as slightly overcorrected.

Its a reasonable mirror for the price. coatings are still very good.

anyone got a program to calculate the wavefront? i suspect its under 1/4 wave but would be interested in knowing. I had a program on my last computer and forgot to transfer it.

A member of the local Society asked me to bring a mirror he's had lying round back to book.

Its a 7 inch f-7 plate mirror 25mm thick. The polish is good except for some gray at the edge.

The problem is its a severe hyperbola like I've never seen. I'm faced with how to attack it to get it spherical again. In the minimal ammount of time as well as needless to say I'll be charging him crumbs to do it.

I can think of 2 approaches

1. accentuated pressure from the edge to .5 tool on top.
This will take a long time given the seriously deep centre.

2. Full tool with graduated facets so more glass goes from the edge and less from the centre. This way it should catch up. I'll then have to recast the tool and go from there.

Any other ideas to fix this up.

Phil b

I'm about to replace the 13 f-5.6 inch mirror in my homemade dobbie with a 14.5 inch f-5 using a 1 inch plate blank.

I've read (don't agree necessarily)that plate is harder to work and needs a sealed tube.

The BVC mirror is 45mm thick. Works well but I get minor astigmatism when it cools that settles. I have an open bottom mirror box in a truss tube telescope.

Do I need to make a sealed mirror box? or is it better to have the open bottom design which surely means faster cooling with the plate mirror?

Also is plate more difficult to figure than Pyrex in this size mirror??


I refuse to believe its harder than BVC which was a nightmare to figure. I've done Pyrex in a 13 inch mirror it turned out very well and was far easier than BVC to figure.

Tried this last night with a konig 32mm 2 inch i brought off astromart.

I didn't buy the eyepiece for the newt rather for an f-15 Mak.

Its no panoptic near the edge but still a valu for money buy.

Excellant in the centre 50 degree superb contrast etc.

I compared the older Konig I have to a brand new one a friend has purchased. The older version matches the newer one except the field stop isd less distinct possible due to the loss at the edge.

The focus is different they have been completly redesigned but edge sharpness and field of view are identical.

Not a bad eyepiece but not a panoptic is my verdict.

Phil B

enjoyed a very interesting night comparing my 715 delux to a questar 7 a friend owns in good conditions.

One of the reasons I got the 715 was i was well aware of the quality images a 7 inch mak can deliver after using the Q7 on several occasions.

We concentrated on Jupiter and both scopes showed superb resolution and contrast.

The GRS was coming round and both scopes showed impressive detail in several bands including white spots ovals festoons and the division near the GRS.

The questar was beaten but only by a whisker. The image was slightly higher in contrast and all in all a harder image in the 715.

Tyhe obstruction is 33% for the Q7 and 24% for the 715 without secondary cap in place.

In terms of brightness the 715 was clearly giving brighter images over the mid 70's Questar with standard coatings.

More colour in the grs was visible in the 715 but the Q7 is a superb performer and everything visible in the 715 could be seen with patience in the Q7.

The Q7 took longer to reach thermal stability than the 715 which had a fan that appeared to prove its worth.

In terms of darkness and light scatter the Q7 showed some stray light entering the field around Jupiter not visible in the 715 hence a blacker background.

I'm not putting the boot into the Q7 but the 715 Delux has a clear advantage even if slight.

Description: 16" f/4.5

Mirror Thickness 1.625"
Substrate: Pyrex
Focal Length 72.494"
Exact F ratio 4.567

Test Results
RMS Surface Error 1/127.67 wv.
P-V Surface Error 1/34.38 wv.
Strehl Ratio 0.990
Encircled Energy Ratio 0.989
Test Wavelength 635 nm
Analysis Wavelength 550 nm
Design Conic -1.00
Only 1500US advertised on astromart.

I'm extremely skeptical of this being represented as the actual quality of the optics.

I'd be impressed if someone **randomly**(not the ones used) chose say 6 zones tested with a couder screen and it came in at better than 1/6th wave.

I now produce mirrors on a limited scale say 6 a year for local astronomers 6-16 inches in aperture in NZ and only ever claim they are a smooth consistent 1/8th wave on the wavefront.


Interested in what others think. Inwonder if this outfit can really walk on water.

Phil Barker

taken in a very dark sky with a vintage pentax screw thread
200mm shot at F4.2.

400asa 30 minutes total exposure

I saw the comet yesterday morning and to my eyes it is better than Linear T7. Coma is now about 3.8 and tail can be traced at 5-7 degrees with binoculars.

Here's hoping it continues to brighten.
Phil Barker
Christchurch
New Zealand

this time 135mm f-4 20 minutes also 400asa kodak slide film.

135mm pentax f-4.5 400asa 15 mins

I made the coma to be abou 2.5 noticeably brighter than the night before a stunning sight from a rural location.

Phil B

I've owned a pair of these and have enjoying them.

For the price they represent excellant value.

Easily better than celestron 20 x 80mm basically sharper near the edge and a lot brighter.

Colour correction and sharpness are very good amazing given the price.

made a bit flimsy I suppose to cut costs I was amazed with the views of Nebula and galaxies with these binos.

They include nebula filters I'm less sure of there effectiveness seem to dim the image somewhat possibly narrow band don't reduce nebula but cut back the light generally.

Phil Barker
Christhchurch New Zealand