Bresser Messier NT150L f8 Newtonian

[900SL]

Initial impressions (first light to follow at some point..)

This is a cheapish 150mm f8 Newtonian from Bresser and sourced from JOC

I chose this for a couple of reasons, firstly because it is set up for both visual and imaging, and secondly because it appeared to be better built and specified than the usual CCC

It arrived double boxed and suspended each end in polystyrene cut outs. No obvious signs of damage in transit

 

First obvious issue: The OTA, contrary to what Bresser state on their website, is not Aluminium. It is relatively thin sheet steel, confirmed by my magnets.. It also has a spherical primary but I'll see how this goes at f8. Note: Bresser do not claim this particular 150mm f8 scope is parabolic. The 150mm f5, however, is. 

 

First impressions: Nicely put together and longer than I expected. Decent finish in white

Tube rings are OK. Vixen wedge is too short, so I've added a longer rail

Spider was centered, tight and rigid (it's not easy to deflect at all)

Tube and most of the bubbins is matt blackened, albeit with a couple of shiny fittings on the spider ends

Single speed R&P focuser appears pretty decent for the money, no noticable slop or wobble, negligible backlash, and the 1.25"/2" fittings and extension tube are well made

Mirror cell is basic but appears functional. Mirror is center spotted. 3 point perimeter support on discs. Mirror clips but no mask

Collimation of the primary after reassembly was fine, the knurled knobs work well and tightening the locking screws didn't throw it out

Some photos

 

 

 

Focuser with ~ 60mm visual extension barrel, not required for imaging

 

 

 

 

 

 

Edited August 2 by 900SL

[Astronomist]

That appears to be a nicely built scope, looks as if all the hardware bar the mirror cell is exactly the same as the 10" bresser I have, just scaled down. I look forward to hearing how the spherical mirror performs.

Edited August 1 by Astronomist

[Second Time Around]

Very useful review, thanks! 

Good that you tested the tube with a magnet and found that it's steel rather than aluminium.  I don't think many of us would have thought of that and just accepted Bresser's word.

How did you find out that the mirror was spherical?

[900SL]

39 minutes ago, Second Time Around said:

Very useful review, thanks! 

Good that you tested the tube with a magnet and found that it's steel rather than aluminium.  I don't think many of us would have thought of that and just accepted Bresser's word.

How did you find out that the mirror was spherical?

I emailed Bresser before purchase, as their advert omits to state what mirror profile is used in the 150L, unlike their other newts, which are parabolic. They confirmed spherical and low expansion glass. 

Peter Drew, of this parish, suggested this might not be a bad thing in a 150 f8 if the figure is good. I'll see.. 

[900SL]

I've also emailed Bresser to suggest that they update their advertising copy. I was initially going to return it on principle, but it seems OK in most regards so I'll see how it performs. I can always replace the mirror farther down the line

[Emperor!Takahashi!]

Thanks for posting this. This OTA has piqued my interest since 6" f/8 reflectors can be excellent visual scopes and aren't so commonly sold anymore. I look forward to your impressions of how it performs.

[Gonariu]

From the photographs the instrument looks good, Bresser could have been a little more serious by saying that the tube was made of steel and not aluminum (it will change little, but ....) and by clearly saying that the mirror was spherical and not parabolic. However, if the focal length is long enough, the differences between the performance of a spherical mirror and that of a parabolic one are minimal (there is a formula that I saw once but I don't remember it).

[CraigT82]

I have seen a calculation for the spherical aberration waver front error from spherical mirrors…. It went as below:

Error = D22/F^3

With D in inches.

So for the Bresser we are looking at (6x22)/512 = 0.258 waves of error. 

So if you take ‘diffraction limited’ to mean a quarter wavelength error then it is just slightly worse. 

Caveats to this are I have no idea what colour light this formula assumes (green?), and also that it assumes a perfect spherical figure. 
Also this is just an internet derived thing so absolutely no idea if it is correct or useful at all! 

 

Edited August 2 by CraigT82

[900SL]

5 hours ago, CraigT82 said:

I have seen a calculation for the spherical aberration waver front error from spherical mirrors…. It went as below:

Error = D22/F^3

With D in inches.

So for the Bresser we are looking at (6x22)/512 = 0.258 waves of error. 

So if you take ‘diffraction limited’ to mean a quarter wavelength error then it is just slightly worse. 

Caveats to this are I have no idea what colour light this formula assumes (green?), and also that it assumes a perfect spherical figure. 
Also this is just an internet derived thing so absolutely no idea if it is correct or useful at all! 

 

Thanks Craig. I can see this becoming a project scope, I'm already looking at flocking, and mirror upgrades if this doesn't pan out.

[900SL]

7 hours ago, Gonariu said:

From the photographs the instrument looks good, Bresser could have been a little more serious by saying that the tube was made of steel and not aluminum (it will change little, but ....) and by clearly saying that the mirror was spherical and not parabolic. However, if the focal length is long enough, the differences between the performance of a spherical mirror and that of a parabolic one are minimal (there is a formula that I saw once but I don't remember it).

I have to say that Bresser have been excellent in customer support. I found a threaded screw in the focuser body (looked like some monkey had cross threaded it on assembly and screwed it up). Bressers response was to send me another hexafocal focuser.

[900SL]

18 hours ago, CraigT82 said:

I have seen a calculation for the spherical aberration waver front error from spherical mirrors…. It went as below:

Error = D22/F^3

With D in inches.

So for the Bresser we are looking at (6x22)/512 = 0.258 waves of error. 

So if you take ‘diffraction limited’ to mean a quarter wavelength error then it is just slightly worse. 

Caveats to this are I have no idea what colour light this formula assumes (green?), and also that it assumes a perfect spherical figure. 
Also this is just an internet derived thing so absolutely no idea if it is correct or useful at all! 

 

@Peter Drew (I think) mentioned that a 6"f8 spherical may actually result in a better mirror than a f8 parabola, as the spherical figure is easier to achieve in practice? This would tally with what Bresser said when I initially asked about the profile. I think their shorter focal ratio 6" mirrors are all parabolic..

[CraigT82]

8 hours ago, 900SL said:

@Peter Drew (I think) mentioned that a 6"f8 spherical may actually result in a better mirror than a f8 parabola, as the spherical figure is easier to achieve in practice? This would tally with what Bresser said when I initially asked about the profile. I think their shorter focal ratio 6" mirrors are all parabolic..

Yeah to be honest I’d trust Peter’s opinion on the matter over some random formula found on the internet! Bresser are a decent brand and so I’d expect it to be a good performer. 

[Mandy D]

@900SL you are probably better off with the steel tube as it has a lower cofficient of thermal expansion than aluminium and will only slightly impact on overall weight of a small OTA.

Edited August 3 by Mandy D

[Astronomist]

It's also nice to be able to use magnetic accessories, saves a lot of drilling.

[900SL]

Not being one to leave well enough alone, Ive dismantled the OTA and flocked the tube. The OTA was painted matt black internally, but there was a lot of reflected light at shallow angles. It was quite challenging to flock this, being only 6" diameter and around 1100mm long. I tried a few 'dry runs' before deciding to cut the flock and do the tube in two halves, with a central joint

Here's some photos of the bits:

 

 

[900SL]

Few things I noticed, some of the focuser screws project into the focuser, namely the R&P plate retainers and a safety stop screw, I might cut these down and flock the inside. The focuser itself is solid, no discernible wobble or slack. Backlash is minimal, and can be adjusted.

The secondary is 45mm on the f8, the spider and center seem firm, no flex or slack

The 'tube' is not perfect, the OD varies by 2mm across the ends when measured at right angles, but it doesn't seem to affect collimation etc, the primary has a shim on one side.  

 

I will now refit the focuser, and I have a Baader laser (and Cheshire) courtesy of @FLO which arrived accurately collimated, so I will be using this to set the focuser alignment.

Edited August 12 by 900SL

[900SL]

First light

I kept this simple, no guiding, and my old Nikon D5600 APS-C.

1200mm focal length, oversampled, 15 - 30 s exposures, slight breeze

Here's a few test images, single subs. Vega, M13, Ring Nebula. Looks like coma to perimeter, the fantastic flares on Vega, mushy M13, mis-shapen stars (not sure if this is mirror, collimation, wind, mount PE or combination of everything)

[900SL]

and an out of focus star, Arcturus

One notable.. I only achieved focus right on the limit of the focuser (fully wound in, so the focus tube is projecting into the OTA), This is with a T2 -Nikon adapter and 55mm backspace. I need to look at this as I was only just getting the crosshairs right in the bahtinov mask

 

 

[CraigT82]

The Vega image shows the ‘bites’ out of the flare due to the focuser drawtube (big bite on the left) and primary mirror clips (three smaller bites). The clips can be covered by a 3D printed ring mask but the focus drawtube is a bit harder, usually needs chopping down. You might find that your Astro cams focus a bit further out than the DSLR which may help. 

[900SL]

28 minutes ago, CraigT82 said:

The Vega image shows the ‘bites’ out of the flare due to the focuser drawtube (big bite on the left) and primary mirror clips (three smaller bites). The clips can be covered by a 3D printed ring mask but the focus drawtube is a bit harder, usually needs chopping down. You might find that your Astro cams focus a bit further out than the DSLR which may help. 

Cheers Craig, appreciate the feedback.

I will try using a 533MC Pro at some point binning 2x2, and add a guidescope. That should hopefully improve tracking, and will be center field so crop out the wonky edge stars. This should move the focuser out around 30mm or so. Tilt inspector also indicates that the weight of the DSLR camera is causing some tilt right to left, increasing fractionally as the scope approached zenith. This is likely due to the thin steel tube I guess. I might try a reinforcing band locally

Collimation is not as straightforward as I thought. The design of the secondary holder means that getting the rotational alignment correct takes a bit of fiddling. The secondary adjustment bolts try to sit in small dimples in the holder

The out of focus looked OK  appears to show some out of collimation in the rings. I now have a cheshire and a recollimated Bresser laser (new to newtons ). I'll try to adjust this out

The FWHM was larger than I expected. Best was 6". Pixel scale with the D5600 is around 0.7"/px. I think the spherical mirror plays a part here, but I need to set up a guided session with no wind to get decent data, with the 533MC, smaller field and focuser out of the OTA

Positive points were that the mount seems to be able to handle the OTA without issue, and the flocking and dew shield work well 

 

I think I'm starting to fall into the Newton expenditure pit. I've already spent the same as the scope, and the shopping list grows longer (new secondary holder,  milled spider, parabolic mirror anybody?   I was warned by @ONIKKINEN - I should have listened   )

 

Must resist

Edited August 27 by 900SL

[Astronomist]

2 hours ago, 900SL said:

The secondary adjustment bolts try to sit in small dimples in the holder

I fitted the secondary holder on my Bresser  with a thin steel washer to stop the screws digging into the aluminium secondary holder, very easy to precisely set the rotation now. I don't know if there happens to be a washer the right size for the smaller holder on the 6", I got lucky and found the right size on aliexpress. 

It looks like the top diffaction spike in the vega image is two spikes, is one of the vanes twisted or otherwise deformed?

BTW my 10" has the same huge flares from the primary clips (see below), I assume Bresser's clips overlap the mirror more than other manufacturers?

Edited August 27 by Astronomist

[900SL]

19 minutes ago, Astronomist said:

I fitted the secondary holder on my Bresser  with a thin steel washer to stop the screws digging into the aluminium secondary holder, very easy to precisely set the rotation now. I don't know if there happens to be a washer the right size for the smaller holder on the 6", I got lucky and found the right size on aliexpress. 

It looks like the top diffaction spike in the vega image is two spikes, is one of the vanes twisted or otherwise deformed?

BTW my 10" has the same huge flares from the primary clips (see below), I assume Bresser's clips overlap the mirror more than other manufacturers?

Thanks A.

The washer fix is a great idea, I have a box of assorted sizes, hopefully one fits.

Do the flares come from the mirror clips, or do the clips actually mask the flare (ie the clips cause the 3 shadow areas)?

[Vroobel]

23 minutes ago, Astronomist said:

BTW my 10" has the same huge flares from the primary clips (see below), I assume Bresser's clips overlap the mirror more than other manufacturers?

 

Every budget Newtonian struggles with the same: shadows caused by the primary mirror clips and (!) too-long focuser drawtube going deep inside of the OTA's tube. I solved it which is described here:

 

 

 

[900SL]

8 minutes ago, Vroobel said:

 

Every budget Newtonian struggles with the same: shadows caused by the primary mirror clips and (!) too-long focuser drawtube going deep inside of the OTA's tube. I solved it which is described here:

 

 

 

Nice work there. The stiffener plate is neat.

 

[Captain Scarlet]

You may well get an extra spike if there’s a straight edge on the secondary’s mirror silvering, where some tool was used to hold the mirror in place during the slivering.