Checking Telescope lenses performance?

[Dave1]

Hi guys,

I have a Towa 339 80mm, who lenses I want to send off to an expert to check there performance, if they are space correctly, if the crown and flint elements are aligned properly. Where are places you kind folks send lenses or complete telescopes to for checking the telescopes performance?

Dave

[Ricochet]

FLO send their scopes to Es Reid for optical checks. The cost might be quite prohibitive compared to the cost of your scope though. I guess it depends how much you like it.

[Fozzie]

I've spoken with Orion optics UK recently regarding their servicing service in respect to telescopes I have.. I've not made a decision yet but they were positive they could make sure all was in order in respect to the achro doublets, focuser alignment etch.. 

Might be worth a chat/email..

Ta

fozzie 

[Dave1]

Thanks for the recommendations, I will give it a go.

[Zakalwe]

What makes you think that there's a problem with the optics? Have you tried a star test?

[tuckstar]

I enquired about having my zeiss tested just to confirm the strehl rating. Was quoted around the £400 mark. Needless to say I didn't bother.

[Dave1]

I have looked at stars with it yes, all was fine on that front. Collimation is good, I checked with a Cheshire eyepiece. What makes me question the performance of it, is the fact the last time I looked at the moon with it, the moon appear more yellow than I fought it should?

If its a clear night, I'm going to take it out tonight, and take it through its paces again.

[spaceboy]

Please don't take this the wrong way but your going to be looking at a few quid for anyone worth ones salt to look at it. Personally I feel you would be better off putting your money towards a 90mm Mak https://www.firstlightoptics.com/maksutov/skywatcher-skymax-90-ota.html They come up used now and again for around £50-£65 which is more than likely going to be a drop in the ocean compared to what it is going to cost for someone to look at your Towa. The performance in the mak should far exceed the Towa. Just my 2p

[F15Rules]

Dave,

I'm sending you an email about your lens.

Dave

[Dave1]

Sorry Spaceboy I have to disagree. The Towa's are known to have good to very good optics. A 90mm Mak has what a 27mm central obstruction? When that is taken into account, I'd put money on the 90mm Mak performs worse than the Towa. It will definitely perform worse on stars.

I've actually owned a Skywatcher 180 Mak. Didn't like it at all, sent it back for a full refund.

Dave.

[spaceboy]

52 minutes ago, Dave1 said:

Sorry Spaceboy I have to disagree. The Towa's are known to have good to very good optics. A 90mm Mak has what a 27mm central obstruction? When that is taken into account, I'd put money on the 90mm Mak performs worse than the Towa. It will definitely perform worse on stars.

I've actually owned a Skywatcher 180 Mak. Didn't like it at all, sent it back for a full refund.

Dave.

No need to be sorry Dave. You know your scopes and have had a bad experience with a MAK (although I know a few who didn't favour the 180 MAK over others in the line). We all have our own preferences. My thinking was just that you could end up spending a lot of money on the scope to only find out it is what it is and the money could have gone towards something else. As you said you've already checked collimation which your happy with.

I did though forgot to mention...... have you checked there isn't a filter in the optical path. Stupid question to ask I know but I remember a thread on SGL a while back where someone had totally forgot they had put a filter in the diagonal when looking at the moon then when they next went out observing wondered why they couldn't see the same detail they had previously seen when looking at DSO's. I admit to doing it myself one time so assume it is a more common occurrence than some may think. You know.......end of a long night observing and all you want to do is quickly pack up, get inside and go to bed. By the time the skies are clear again you've long forgotten you left a filter in the diagonal or on the end of an eyepiece.

[Dave1]

When I checked the performance of the Towa last night, I checked collimation again, both intra focus and extra focus was very good. It did put up good performance on stars, last night. I didn't use any filters last night, there was no way there was a filter in use last night.

However that's not to say there wasn't a filter in use by accident last time, I observed the moon, and thought it appeared more yellow than it should. Next observing session I will test out the Towa on the moon again and see if this still holds true.

I have a French rebadge classic Vixen 80mm F15, to compare it too, on the way to me. The Vixen has 4 baffles in the OTA tube. Will be an interesting to compare the two telescopes.

[Charic]

On 04/02/2018 at 15:21, Dave1 said:

...........last time I looked..........moon appear more yellow..........

Could just have been the viewing conditions, which for your sake will cost nothing to fix!

[F15Rules]

15 hours ago, spaceboy said:

Please don't take this the wrong way but your going to be looking at a few quid for anyone worth ones salt to look at it. Personally I feel you would be better off putting your money towards a 90mm Mak https://www.firstlightoptics.com/maksutov/skywatcher-skymax-90-ota.html They come up used now and again for around £50-£65 which is more than likely going to be a drop in the ocean compared to what it is going to cost for someone to look at your Towa. The performance in the mak should far exceed the Towa. Just my 2p

I agree this is all very subjective and one person's experience will differ from anothers.

I say this as a refractor fan first and foremost, but I do love good Makustovs..I've owned Russian Intes MK66 and 67 150mm scopes and a Ylena (LOMO) Russian 150mm F14.4. All of these were absolutely superb optically.

I do know that Skywatcher's bigger Maks are stopped down a bit (I think the 180mm operates at 165-170mm), so if the 90mm is also stopped down, it might not be much more aperture than the Towa 80mm. My Moonraker (which has a Towa 80mm F15 objective) is a cracking scope for it's aperture. As will all scopes, ultimately aperture rules though ..

And a decent 90mm Mak will be amazingly portable compared to the long frac..

Dave

[Dave1]

The 180 maks are stopped down to 170mm.

Can't remember the exact science on central obstructions, but surely a 90mm makes with a 27mm instruction will have less apature than a Towa? 

I think the Braybrooke observatory wrote a good article on this. 

Anyway I'll be sticking to Fracs for now. 

Dave

 

[Ben the Ignorant]

Some achromat glasses are tinted yellow to absorb a little blue and violet light, they can't focus it so they do away with it. Maybe you observed through a white achro for a while, and then you noticed the yellow in the other by comparison?

[spaceboy]

On 2/6/2018 at 11:22, Dave1 said:

The 180 maks are stopped down to 170mm.

Can't remember the exact science on central obstructions, but surely a 90mm makes with a 27mm instruction will have less apature than a Towa? 

I think the Braybrooke observatory wrote a good article on this. 

Anyway I'll be sticking to Fracs for now. 

Dave

 

A member did send me a paper on this years ago as I always argued a central obstruction would surely reduce aperture but it isn't the case. I can't remember what the "adverse" effects were of a central obstruction other than the ones we commonly complain about, diffraction and loss of contrast, but light grasp was not one of them. The academic side of astronomy was never my strong point so I shall try to find the original thread as it will better explain it than I can.

Either way I agree completely that the views through a well corrected refractor are hard to beat and far more aesthetically pleasing than other scope designs. I don't know what it is but I also enjoy using refractors over other scopes.