Focusing a MAK

[Marky1973]

I managed to get out for an hour tonight, just to test out the Bresser Messier MC-127/1900 that I took delivery of on Thursday. It was just a quick go to see how it performed. The RDF supplied with it is a waste of space, so it took me a while to find Mars, but it looked nice and red and crisp in the 26mm supplied EP. It was boiling badly, but with the 26mm in my 2.25x Barlow, you could just make out the different coloured regions....vaguely. Jupiter looked nice an crisp in the 26mm but, again the conditions were not great, so I wasn't expecting miracles. Out of focus it looked well-collimated and the focused images were crisp.

What was problematic was the focusing. Even the lightest of touches on the focusser had the target leaping all over the place. From being central and out of focus, a little move and it was out of view on many occasions. I did have a quick play with my ZWO as well and, while I expected it to be tricky with the longer FL, it was almost impossible to focus on either planet as they just kept disappearing from view. I know about mirror flop with MAKs but I don't think this is it as it did just seem to be flying all over the view even with the slightest of touches.

I also know I am introducing vibration by touching the focusser, but I thought I was being careful.

It just seemed like more movement that was helpful whether imaging or just observing.

Is this just to be expected of a long FL Mak, or is there possibly something amiss? The scope was a clearance job which was returned for repair when the primary mirrors came loose - so I am worried this may be a recurring fault.

Maybe I am just being paranoid?

[Dave In Vermont]

The first thing that comes to my mind is the mount. What sort of mount are you using for this? Do you have a link for these? And what is under the mount would also help. I'm hoping it's something simple, such as vibrations coming up through the mount - which can be greatly alleviated by using anti-vibration pads under the legs of the tripod.

I also doubt this is mirror-flop from your description. Unfortunately I am unfamiliar with this particular Maksutov, the ones most usually encountered are from Synta and are the same telescope with Skywatcher and/or Celestron brands on them. Orion-USA too. I have a SW-branded 150mm. The image will dance if you're handling the mount, but this is normal. The question here being: How long does it take to stop dancing once you're not in physical contact with the scope or mount?

I'm sure we can help you sort the cause - and remedy.

Dave

[Marky1973]

Hi Dave

It was stuck atop my NEQ6, legs retracted, so it was a pretty solid set-up (or so I thought) and it was just on the driveway outside the house where I have set up many times before.

It generally stopped dancing fairly quickly, so vibration wasn't a major issue, I just found that the target would jump across the FOV when focusing and often not come back to centre. I've always had to keep an eye on the position of the target in my ED80, there is always going to be vibration when manually focusing, but the target usually stays fairly central as long as I don't bang the mount of scope as I am doing it and small adjustments on the handset usually do the trick.

When using the ZWOASI120MM with Sharpcap, the image would be off screen with even the lightest and slightest adjustments to the focus, so I was having to correct with the mount handset at the same time as focusing.

I'm prepared to give it a go if this sounds normal, but I don't want to waste my time trying to get something to work if there is a fault and, as I said, as the item was a return, it just makes me a little concerned. Obviously I contact the guys I got it from as well.

This is the scope.

http://www.telescopehouse.com/telescopes/telescopes-by-brand/brand-bresser-telescopes/bresser-messier-mc-127-1900-ota-optical-tube-assembly.html

I will try again, it is obviously new, and there is going to be an element to getting used to a new scope - if I get another clear night before the 14 day return period is up!

[Stu]

If the vibrations damp down quickly then it does sound like mirror flop which can be a significant problem in these scopes. It certainly sounds like it is more significant than it should be and you are only using x73. At high powers I suspect it would be unusable so I would get in touch with your supplier to see whether the repair was actually done/done properly.

[Marky1973]

Cheers Stu. I really hate returning stuff, so was hoping it was just me! :-)

Something I also meant to mention was that, when de-focussed, the collimation looked good, but there was a line that cut through the donut, although not centrally - like the picture below. again, I wasn't sure if this was a peculiarity to MAKS, the equivalent of the spider vanes on a NEWT, but it wasn't visible on the focused image, so I wasn't so bothered about it.

 

[Louis D]

10 hours ago, Marky1973 said:

Something I also meant to mention was that, when de-focussed, the collimation looked good, but there was a line that cut through the donut, although not centrally - like the picture below. again, I wasn't sure if this was a peculiarity to MAKS, the equivalent of the spider vanes on a NEWT, but it wasn't visible on the focused image, so I wasn't so bothered about it.

Have you tried looking through the scope from both ends up close and from a distance in daylight to see if you can identify some foreign object in the light path?  It's most likely closer to the eyepiece end rather than the corrector end if you can make it out at all in the eyepiece when defocussed.

[Marky1973]

Hi Louis. I had a good look, but couldn't see anything obvious - as I say, it wasn't visible when properly focused. In the end I have decided to send it back to the supplier, I just don't think I could live with the movement in the mirror. Maybe time to save up for something bigger! ;-)

[Stu]

13 hours ago, Marky1973 said:

Cheers Stu. I really hate returning stuff, so was hoping it was just me! :-)

Something I also meant to mention was that, when de-focussed, the collimation looked good, but there was a line that cut through the donut, although not centrally - like the picture below. again, I wasn't sure if this was a peculiarity to MAKS, the equivalent of the spider vanes on a NEWT, but it wasn't visible on the focused image, so I wasn't so bothered about it.

 

That doesn't look right! I suspect they got it back, did a quick check and just put it back up for sale.

[Dave In Vermont]

That's what I'm thinking as well. And I think you know (and dread) what the 'fix' would be - send it back.

One thing you said is causing me to have to ask this, but..... You do know that any object you view at higher magnification is going to move across the view, as if you are using 73X, the percieved rotation of the Earth is also magnified 73X faster than if you were looking at the object with just your eyes? Sorry to ask, but your missive made that a bit murky. Now onto the business of 'mirror-flop.'

It seems that Synta has re-tooled their stock focuser supplied on the Skywatcher and Celestron Maksutovs. I'm hearing from recent buyers of these that the focus has been very good and doesn't require re-adjustment to compensate. As a recent buyer of the Skywatcher 150mm Mak - this has been my experience as well. But I went ahead with changing out the back of the scope to a 10:1 Crayford dual-focuser* (a GSO works great!). If whoever makes your Bresser hasn't caught up with Synta, that would solve the mirror-flop issue. I simply set the mirror to about center in the 'sled' regards it's movement range, and then use the Crayford for it's fine-tuning of dialing-in the sharpest image.

So there is a solution to mirror-flop.

Until later -

Dave

*

 

[John]

14 hours ago, Marky1973 said:

Cheers Stu. I really hate returning stuff, so was hoping it was just me! :-)

Something I also meant to mention was that, when de-focussed, the collimation looked good, but there was a line that cut through the donut, although not centrally - like the picture below. again, I wasn't sure if this was a peculiarity to MAKS, the equivalent of the spider vanes on a NEWT, but it wasn't visible on the focused image, so I wasn't so bothered about it.

 

I've seen a line like that when I tried using a 45 degree erecting type prism for astro purposes

If the scope has had issues in the past I'd suspect that those might be coming back to haunt you again though.

[Marky1973]

12 hours ago, John said:

I've seen a line like that when I tried using a 45 degree erecting type prism for astro purposes

If the scope has had issues in the past I'd suspect that those might be coming back to haunt you again though.

Cheers John - I was swapping out the 90 degree diagonal supplied and checking with a spare 45 degree just to see if there was any difference - so maybe that was introducing the line - as I say it wasn't an issue when properly focused, but that would explain it!

[Marky1973]

12 hours ago, Dave In Vermont said:

That's what I'm thinking as well. And I think you know (and dread) what the 'fix' would be - send it back.

One thing you said is causing me to have to ask this, but..... You do know that any object you view at higher magnification is going to move across the view, as if you are using 73X, the percieved rotation of the Earth is also magnified 73X faster than if you were looking at the object with just your eyes? Sorry to ask, but your missive made that a bit murky. Now onto the business of 'mirror-flop.'

It seems that Synta has re-tooled their stock focuser supplied on the Skywatcher and Celestron Maksutovs. I'm hearing from recent buyers of these that the focus has been very good and doesn't require re-adjustment to compensate. As a recent buyer of the Skywatcher 150mm Mak - this has been my experience as well. But I went ahead with changing out the back of the scope to a 10:1 Crayford dual-focuser* (a GSO works great!). If whoever makes your Bresser hasn't caught up with Synta, that would solve the mirror-flop issue. I simply set the mirror to about center in the 'sled' regards it's movement range, and then use the Crayford for it's fine-tuning of dialing-in the sharpest image.

So there is a solution to mirror-flop.

Until later -

Dave

*

 

Thanks Dave. I did look at those upgrades when I was thinking about a 150/C8/C9.25 but for the Bresser the upgrade was almost as much as the scope! :-)

I could see some drift due to the Earth's rotation - I was tracking, but only did a quick polar alignment, so that was no surprise - but the movement when focussing was more than that and the image was disappearing from view and not coming back, making centering very difficult. I would presume, even with mirror flop, that the target would stay (fairly) central if focusing caused some movement.

Maybe it is time to save up for a better scope - I think I have a couple of years before the planets are back in better positions! ;-)

I didn't want to send it back, but I am going to as I would have struggled with that movement. Might be worth trying to have a look through a larger Mak to see how it compares some time in the future.

[Stu]

I found the mirror flop with my C8 EdgeHD too much for my liking and added a Baader SteelTrack and motor drive. Interestingly I'm using a borrowed C925 currently and the mirror flop seems far less on that, quite acceptable to me. It implies different engineering between the two, but I don't know if that's correct.

Cool down and dewing are two issues that you need to get right in order to get the best out of the larger Maks. I had an OMC200, and rarely got the best out of it because the cooling was very tricky to get right.

[Marky1973]

Maybe I am just too critical or expecting too much? But I'd rather spend a couple of hundred on a focuser to upgrade a C9.25....seems more economical. :-)

The MAK had been outside for a couple of hours cooling before I started using it, so hopefully that wasn't an issue.

I probably need to try a few more models before I buy in case this is something I'm not going to be able to live with... or buy a massive refractor..... :-)

[billyharris72]

If it helps, I've got the 102 and 150 mm Skywatcher Skymax Maks, and have not had any issues with the focus on either. Movement is slight, if any - if I was to upgrade the focuser it would be for fine tuning rather than reducing mirror flop. You might consider one of these, though I suspect you just got a duff one from Bresser; I doubt they're all like that.

[Marky1973]

I suspect you are right Billy - it was a customer return following problems with the mirror, so I thought I'd take a punt, it just didn't work out this time. I suspect it was a just a bad apple. The one review I could find was complimentary to the scope and Telescope House have been very helpful through the buying/return process.