possible replacement telescope

[Boros]

Hi  further to my previous post 'first telescope questions'  on relating my experience with that first telescope to a neighbour, it turned out he had bought a Bresser Messier MC 100/1400 Maksutov about a year ago and had hardly used it (seems a common story!) and he was willing to sell it. Now this seemed an ideal solution, a much smaller package that wouldn't be as unstable as the 102GT, easier to use generally, more portable and I could mount it on one of my heavy Manfrotto tripods.  So I borrowed it off him and tried it out against the 102GT.  But I have to say I was more than a little disappointed, the image appears to exhibit less contrast and definitely not as sharp.  I used the same diagonal and 15mm lens in each one. I also attached my camera and I just couldn't get a sharp image at all. The adjustment allowed it go in and out of focus, but it never got truly sharp.  Do you think these are issues with this particular Bresser or are they all likely to be the same? 

[laudropb]

Hi. I have the equivalent SW Mak and the  image is fine. Perhaps your neighbours is out of collimation. Try a star test if you get the chance.

[johninderby]

I’ve owned a Bresser mak and the images were very sharp with great contrast. Maks don’t normaly need colimating but sounds like this one is out of colimation. Can happen during shipping if knocked about while in transit or perhaps dropped?

Edited July 29, 2020 by johninderby

[Boros]

Okay thanks I'll try a star test, but the way the weather is I might have to come up with an artificial star!

[RT65CB-SWL]

Hi @Boros.

Question: Did you give the Mak. time to acclimatise to the ambient temperature before viewing through it? - I have a ETX105 and C6/SCT-xlt and give them a minimum time of thirty minutes before adding an eyepiece with the visual back pointing up to allow any heat/thermals inside the tube time to escape; (longer in the autumn/winter months, if taken outside from a warm environment, i.e. room in the house or vehicle). With a refractor 'scope this is not this is not so much of a problem.

If you can submit some de-focussed images then we be be able to advise better. You should be getting concentric circles if the primary and secondary mirrors are aligned. If they are slightly out of alignment, then it will need collimating. All you need is an 'artificial star' and can be made using a shiny ball-bearing and a bright light source shining on it and can be done during daylight hours.

[RT65CB-SWL]

BTW - I would keep the refractor as well, and think about a heavy duty mount and tripod/pier.

[Boros]

Hi White Dwarf

I never took the scope outside, the moon shot I did the other night with the 102GT was taken from inside the house with the patio door open, and the test I did with both scopes was done the same way, I probably messed around with them for a good 1 1/2 hours so I don't think temperature could have been a factor.  I think your right, I do need to keep the refractor as well.

Anyway I've just done the star test with the following results.  The first image is out of focus on the close side, the second image is just out of focus on the close side and the 3rd image is way out of focus on the far side.  Interestingly when I took the first pic it appeared to be not too bad, but as you can see when the out of focus was transferred to the far side it definitely looks to be out of alignment,  to my eyes anyway, can anyone enlighten me as to what it is and the remedy please. If it's collimation does anyone have experience of doing it on these Bresser Maks, I've searched online but can't find anything. There is 3 oval holes with 2 screws in each, but what they do is anyone's guess.  Thanks all for your help so far.

Edited July 29, 2020 by Boros

[vlaiv]

That is way too defocused for proper star test.

Last image shows out of collimation optics, but I suspect that is because focuser was racked all the way out and that could tilt the primary mirror.

You want very small level of defocus, something like this:

You want those concentric rings to still be visible. Btw - one on the right is out of collimation because rings are not concentric - that is what you want to see - either concentric rings or slightly closer on one side.

In your images above - first two images look rather concentric, but it is hard to tell because there are so many rings that they appear like flat surface. Last one is obviously not concentric.

[RT65CB-SWL]

Top one looks slightly off.
Middle one looks ok.
Last one is slightly off too, but worse than the top one.

With collimating any reflector 'scope, there are three sets of two screws... one for collimation and the other to hold it in place. The problem is... I cannot remember which one is which... doh!

To make matters worse... for my ETX105, I first have to remove the backplate.

  

[RT65CB-SWL]

I have just found this... http://www.opticstar.com/Download/Astro/Doc/Telescopes/Opticstar/Opticstar-MG152-Collimation.pdf 

[Cosmic Geoff]

Take care. Don't mess with it until it's yours and you are sure it is out of collimation.  If it is seriously out, a star test will look awful, maybe like a badminton shuttle. If you are in doubt, just hand it back to its owner as-is.

[vlaiv]

6 minutes ago, Philip R said:

Top one looks slightly off.

I'm not seeing it.

Maybe easiest way to see if it is concentric is to make mirror - either horizontal or vertical of half of the image. If it still appears circular - it is concentric.

Here is first image with right part mirrored on left side:

and here is bottom part mirrored on the top:

That looks concentric to me.

In fact, maybe this can show it even better:

Inner circle is a bit misshapen (seeing, tube currents?) but it is concentric.

[johninderby]

Here’s what the backplate of the Bresser 127 mak looks like with the rear cover removed but yours will be similar.

Edited July 29, 2020 by johninderby

[Boros]

Okay, well the middle image is the least defocused, and that doesn't look too bad, but I think I might do another test as it looks like that one might have been flawed.  After looking at your link White Dwarf it looks like I might be  opening a can of worms by attempting to collimate this.  If the second test shows the rings to be fairly concentric, is there anything else that would cause the image in the eyepiece to be soft and also prevent me from getting sharp images from the camera?  

Edited July 29, 2020 by Boros

[RT65CB-SWL]

Have you tried it 'straight through' i.e. without the diagonal in place... or with another eyepiece?

[vlaiv]

15 minutes ago, Boros said:

Okay, well the middle image is the least defocused, and that doesn't look too bad, but I think I might do another test as it looks like that one might have been flawed.  After looking at your link White Dwarf it looks like I might be  opening a can of worms by attempting to collimate this.  If the second test shows the rings to be fairly concentric, is there anything else that would cause the image in the eyepiece to be soft and also prevent me from getting sharp images from the camera?  

Did image look soft while observing at eyepiece of just with camera?

What were observing conditions like for you - what did you observe that looked blurry / soft?

[Boros]

Hi  Yes I tried it with 2 different diagonals, the one that came with it, a mirror one(cheap rubbish plastic) and the one from the 102GT which is a prism, not much to separate them to be honest. And both diagonals with 2 different eyepieces. The camera was prime focus using a barlow (without the lens)  Image was soft viewing through eyepiece and the camera lcd and the camera images are also soft and just look slightly out of focus.  I went back and forth with the same setup between the 102GT and the Bressier and each time the image from the 102GT was noticeably superior, more contrast and distinctly sharper, the Bressier  just looks soft no matter what I did.  Conditions were fine, daytime and viewing brick chimneys about 100yds away.

[Gfamily]

A couple of thoughts...

As you used the same 15mm eyepiece in testing both, you'll have been observing at 66x through the refractor and at 93x through the Mak.

That might be why the Mak was looking softer. Do you have a 20mm eyepiece so you can compare the two scopes' images at the same magnification?

Secondly, you said you were looking at chimneys as your test; I find that the daytime image is generally lower contrast than you'd expect if I'm using my SW 127mm Mak, but it's ok at night.

I don't have a refractor to compare, but I sometimes wonder if a Mak is more affected by background light.

Edited July 29, 2020 by Gfamily

[Boros]

Hello Gfamily, thanks for your thoughts, I totally understand what you mean and I will try what you have suggested, but just taking the results from the mak on their own, they are not good. Some Maks like the Celestron C90 are used as spotting scopes presumably during the day, this is unusable for that purpose. I've read that Maks are supposed to have very sharp optics, this is far from sharp. But as I say I'll give it another chance tomorrow, redo the star test and use different lenses as you suggest, but I've a funny feeling I'll be handing it back to my neighbour. Shame really as I'd hoped it could have been a more user friendly scope than the 102GT.  Thanks for all the input from everyone.

[Gfamily]

Yes, without seeing for ourselves we're in no position to judge, and it's quite likely that it's not one you'd want to take on.

Like so many things, it would be easier if local clubs were meeting and you could take it along to get a second opinion.

Best of luck though.

[Boros]

Hi  I talked to my neighbour about his Bresser, he's not in a hurry to have it back so says I'm okay to do a few more tests on it.  I had really clear skies this morning about 2am so had a good opportunity to try it on a proper subject. Did the star test and to be honest it all looked fairly concentric, but one thing I did notice was that the out of focus disc had a thin black line running vertically through the centre, this moved to either side if the disc was moved away from the centre of the frame, is this normal? Also I'm not sure about the focuser (is that a word?)  First of all I never felt the images of Saturn and Jupiter ever got truly sharp and when trying to achieve focus if I went too far and had to bring it back a tad, it went way out of focus so had to wind it back really far then try again, so I basically have to wind it to achieve focus first time, can't just get it close and jiggle it a bit.  it's impossible to fine tune the focus, it can be almost there, I'll move it just a tiny bit, but it jumps out of focus to almost a doughnut, so have to wind it way back and go in again.  Anyone had any experience of these issues?

[vlaiv]

Two things come to mind:

Vertical line is usually associated with Amici prism either 45 or 90 degrees. What sort of prism do you have in that scope?

Can you view image in daylight - like spotting scope and is that image properly oriented? If so - prism could be par of a problem here as such prisms are usually low in quality (astronomical grade Amici prisms cost a lot of money).

Second - it seems that focuser mechanism is not good / mirror is slipping. In no way there should be sudden change in focus - it should be gradual with normal working focuser.

This could also mean that mirror is not fixed properly and that it also changes position - which would cause changes in collimation depending on where you point the scope.

[johninderby]

One thing you can do is turn the the focus knob as far as it will go in one direction then to as far as it will go the other way. Repeat this a few times to make sure the grease in the focusing mechanism is evenly spread. This may cure the focusing problem.

As above the black line is most likely caused  by the diagonal. The Bresser 100 mak comes with a prism diagonal.

Edited July 31, 2020 by johninderby

[Boros]

Hi  The diagonal it came with is a rubbish plastic 90 degree mirror, not a prism,  I alternated it with the one from the 102GT which is a 45 degree prism, and on checking it is indeed an amici prism, so that probably explains the vertical black line, thank you Vlaiv.  I'll try rotating focus knob to it's extremes a few times, although it's had a good workout already, but I suspect that this is where the focus/ soft image issues are stemming from. If so and the focus racking doesn't help, then it looks like I won't be buying it!  Thanks again Vlaiv and White Giant.

[johninderby]

Have seen the scope described on one site as having a zenith prism and on another a mirror diagonal. Not that I ever use the supplied diagonals. There simply there so the scope can be used right away. I’ve seen expensive SCTs with the same cheap plastic diagonals. 

With a used scope you never inow what the original owner may have done with it or how it was stored or has it been dropped. Bresser maks are very good quality scopes so hopefully you will get it sorted.

Edited July 31, 2020 by johninderby