Unimpressive views on 2nd hand dobsonian - expectations or optics?

[Giles_B]

I finally found my 'cheap enough' second hand 10" Dobsonian with a computerised star map (a Orion10XTi Intelliscope) and, after some predicable initial teething problems on the first clear night, took it out for its next spin, in my (Bortle 6) garden last night.

I found the Intelliscope hard to use, and not much help finding anything. I think this is probably a case of just going through the manual carefully, and having a better sense of what to look for in the huge database. Luckily I've attached my trusty Starsense Explorer phone mount onto a piece of 8mm acetal, screwed this to the tube rings and it works a treat at finding my way around. The bigger worry is that the optics just didn't seem to be performing as I'd expected.

As I say, the moonlit night was light and I didn't have great expectations. I found my BST explorer eyepieces had hideous coma (the scope is f4.7) - even the 18mm which I'd heard tell was fairly good in a fast scope. I fell back on a couple Baader orthoscopic eyepieces. These are never quite as twinkly as the BSTs, but I had decent views of Pleiades and the Beehive cluster. However, even given the unfavourable conditions I was very underwhelmed with the view in comparison to my 5" Celestron Starsense 130 newtonian. The Orion Nebula had a little more detail - but only a little. I couldn't even make out Bode - not unusual on a bright night for my 5", but I'd had high expectations given the extra 5" of aperture.

My fears that there may be something fundamentally shot with the optics are already heightened by the fact I spotted a small crack at the edge of the primary mirror while collimating. The jury seems out on this looking through the forums (that is, there are many voices of doom and a few saying wait and see if it is the crack is stable), and I'd decided to wait until I had better eyepieces before deciding the coma was due to this. However, I'm now wondering if the unimpressive views may be due to some degradation of the mirror.

Now, I'm a novice, and all I have to go on is the views I'd come to expect from the Celestron 5", so my question is: I am just in the process of having my high expectations of 10" performance reduced, and essentially doing the right thing patiently waiting for some high end eyepieces to be back in stock in the UK, or are there so many red flags that I should I be thinking about a new primary mirror sooner rather than later, to improve the performance with my current equipment?

 

 

 

Edited February 15, 2022 by Giles_B
correction

[Peter Drew]

First of all, I think it is highly unlikely that the primary mirror is cracked, the material is far too thick, mirrors can easily be chipped or completely smashed but not cracked.  Expectations of a larger telescope are often too high initially, it takes time to fully appreciate the improvements.  You say that this is the first time you have had the chance to use it, I'm sure with practice you will find the push-to system easier and more accurate to set and there will be better nights to enjoy.  The eyepiece selection is going to be an issue though for best performance at F4.7        🙂 

[Pixies]

I found the 18mm BST to be pretty poor at the outside edge with an F6, let alone a F4.7. My understanding was that the 18mm was the poorest of the set. The 8 and 12mm ones are much better.

A bright near-full moon and poor transparency will limit what you can see. I'd suggest waiting for a better dark night before worrying too much. Your big light bucket will be great at picking up the bright background, especially at low powers. What FL are your orthos?

[Giles_B]

The Orthoscopics are 30mm and 6mm

Certainly I didn't go through the whole set - but went down from the 25mm to the 18mm - I probably didn't reach the lower focal lengths, so I will give them a try.

@Peter Drew I actually took a photo of the crack. It's certainly not full thickness (that is, I can't make out distortion on the mirror surface) - but I got a bit wound up by first seeing some quite hysterical posts about the aberration caused by cracking on other forums

[mikeDnight]

That looks merely like a scratch on the rear surface of the mirror which will not affect performance in any way. Have you checked the collimation is ok? If the mirrors are out of alignment it may give you an impression of hideous coma. 

[bomberbaz]

I had a 10" XTi until last year, I had owned it for around 5 years before my upgrade.

I am not sure why you are having issues with the pushto, I found it very accurate so my initial thoughts are you may be off somewhere. 

You should switch it on and the instruction, "point vertical" should appear. If pint vertical does not appear the menu should allow you to get to this, it's a common mistake. Put your dob in the instructed "vertical" position and before you do anything, scroll through the alignment star menu and decide which two stars you are intending aligning on. This last bit helps make the process of aligning more streamlined.

I used to try to get at least one star near direct east or west, Mizar was always a favourite due to it's brightness and ease of finding. Then find another in a totally different part of the sky.

I used a crosshair eyepiece for accuracy in getting a good alignment. Or you can purposely defocus so you get a large ring which is easier to centre.

Now carry out the alignment as quickly and accurately as possible. One other thing, try and get your base fairly level, this does help.

If you cannot see bodes even on a near full moon my guess is your not pointed at it but it is just a guess. However it would be quite faint anyway.

I used top quality eyepieces in mine and it still gave bad coma, BST would be at least as bad but more likely worse.  I did start using a coma corrector which made it liveable but it wasn't a mega expensive one. I no longer use a CC, just simply live with the coma. As Mike says, check your collimation.

I did find collimation some of a pain with mine, needed a tweak each time I used it but that was because it went into a car each time it was taken out for use. 

HTH

Steve

 

[Giles_B]

Thanks @mikeDnight - I do think there is a bit more thickness to the scratch that doesn't quite show on the pictures. Put I take the point - i.e. stop fussing!

The collimation was the first thing I checked after first use - the previous owner said they'd collimated it with an included laser collimator when I picked it up.

The collimation was crazily off before I had a go with my Cheshire - less than half of the primary was visible, so I guess the laser collimator is ready for the bin....

Interesting that the coma was still bad with good quality eyepieces for you @bomberbaz. I guess a coma corrector might be further up my shopping list than I thought.

Thanks for the advice on the setup right for the push to intelliscope computer - I think from your description I'm doing it correctly. My big problems were that I was unable to get warp below about 2 - but I may have been too slow to find my reference stars, or the base may have been too off level (I hadn't consider this) - plus I was scrolling through thousands of objects that were below horizon. I think if I go out with some objects in mind it will be a bit better than random pointing!

[bomberbaz]

2 hours ago, Giles_B said:

Thanks @mikeDnight - I do think there is a bit more thickness to the scratch that doesn't quite show on the pictures. Put I take the point - i.e. stop fussing!

The collimation was the first thing I checked after first use - the previous owner said they'd collimated it with an included laser collimator when I picked it up.

The collimation was crazily off before I had a go with my Cheshire - less than half of the primary was visible, so I guess the laser collimator is ready for the bin....

Interesting that the coma was still bad with good quality eyepieces for you @bomberbaz. I guess a coma corrector might be further up my shopping list than I thought.

Thanks for the advice on the setup right for the push to intelliscope computer - I think from your description I'm doing it correctly. My big problems were that I was unable to get warp below about 2 - but I may have been too slow to find my reference stars, or the base may have been too off level (I hadn't consider this) - plus I was scrolling through thousands of objects that were below horizon. I think if I go out with some objects in mind it will be a bit better than random pointing!

exactly my point. You should find and remember how far down the list the second object is. Get to the first object then you aren't searching for the next one. See how you go. 

[cwis]

To push back a bit in the other direction with your concerns about the optics...

I got a 130mm F5 goto newt as my first scope, and after 6 months or so I "upgraded" to a 250mm F5 dob.  I say "upgraded" in quotes because upgraded would normally mean I replaced one scope with another   - of course not, I now have two scopes!  (Actually 4 but that's another story...)

Obviously it then became all about the new scope, and the 130 was neglected.

Then one night I dragged my "old" 130 out for a session -  I needed goto to find some target or other.

I was amazed how competant it was! Yes the views were dimmer, or smaller, or less sharp than the dob, (or all three!) but not by as much as I remembered - it held up very well in comparison. 

So maybe the jump in performance isn't as great as you were expecting? It may not be such a jump in badly LP skies with average UK seeing...

Try to play towards some of the inherent strengths of the larger scope notwithstanding your LP - higher mag views of the planets (not long to wait now!), or split some tight doubles - you may then see it pull away from your smaller scope in perceived performance...

[Giles_B]

Yes, those are really wise thoughts. I did laugh when you said about your 4 scopes. My other half couldn't quite believe the answer when she asked when I was getting rid of my old scope....

[Peter Drew]

I you really want to see the difference between a small and large telescope, have a look at a globular cluster.     🙂

[Louis D]

29 minutes ago, Peter Drew said:

I you really want to see the difference between a small and large telescope, have a look at a globular cluster.     🙂

Yes, GCs are very disappointing in a 90mm APO compared to an 8" Dob, let alone a 15" or larger Dob.