Four Weeks In, My 16" Dob 'reflections' good and bad............

[barnstorm]

Hello Everyone,

I recently sold off my worn-out ETX-90 that I purchased the first quarter that Meade produced them and bought a (very) used Meade 16" Dob Starfinder.

I had not used the ETX-90 in a decade because I had worn the gears down with overuse.

I am glad to be watching the skys again!

I thought I would HATE not having a 'go-to' scope and would not be able to tolerate a scope without tracking but to my surprise I have been very happy with the Dob, especially since I installed a Telrad and bought the finder charts of Telescopic Objects, and use my iPad for star charts.

On the BAD side:

Planets don't look as good through the 16" dob as I expected.

I have a great deal more light pollution where I live then expected.

Apparently the scope attracts clouds and the moon as it seems one, the other or both are always present when I try to view.

The plastic focuser is a pain and it is unclear what focusers available on the internet will fit a 16" tube.

On the Good side:

I have seen Comet Garradd twice.

The wide field with my 30mm 2" eyepiece is fantastic.

Still Working on:

I am a bit disappointed with some of the few deep-sky objects I have found.

Since I sold my ETX-90 I don't have a smaller scope at this location to compare with and since my wife has had rotator cuff surgery last month I don't have anyone to help me load the scope up to take to a star party.

I was expecting more detail and brightness when looking at deep sky objects than I am getting.

I have the Hotech laser collimator so I know my mirrors are aligned.

I have cleaned the dust off of both mirrors carefully using directions found on this forum and others.

I have bought some reasonably high quality 2" eyepieces, and still have most of my 1.25 eps.

I am assuming my problems are:

1. My viewing has been done about 1.5 hours after sunset and 1-1.5 hours before moon rise. Perhaps the sky has not been dark enough?

2. Light Pollution (I am in a somewhat rural area but not far from some auto dealerships that leave lights on all night)

What I have viewed in the past two weeks:

Comet Garradd, faint gray gas ball with dull point in very center. Round in shape.

M81-M82, Neat to see them in the same view! With my highest power EP they both still look like very indistinct smudges, one round one long.

M51, two light roundish smoke-like smudges barely discernible from the backround. no apparent connection between them.

M97, I spent soo much time trying to find this darn thing with no luck, then by accident I happened to get it just right in the center of my view, with my 30mm 2". It was so nearly indistinguishable from the back ground that it took me some time to convince myself I was really seeing it.

It appeared as if the faintest of a wisp. It looked like a dried watermark on a glass table but almost undetectable. I went for my 2-1.25 adapter so I could go to a 12mm wide field with my 'nebula filter' and lost it.

I hope things will improve as the moon wanes because I have little hope of getting much or any dark-sky time at other locations this year.

.

[spaceboy]

You will find that most galaxies are indistinct smudges in all but the very largest of scopes under LP skies. I'm sure if you had a 6" dob to compare the views you would be able to make out some improvements in the 16" but you are always going to be at the mercy of viewing conditions. It is a shame you are unable to visit dark skies as I am sure the views would knock your socks off. On another note I'm guessing you allowed plenty of time for the mirrors to cool before observing ?? not that this makes the hugest of differences at lower magnification.

[barnstorm]

I have not checked the mirror temps yet, but I plan to do so.

I have a large reflective cover on the scope and it sits under a car-port during the day. I roll it out to the back yard an hour or two before sun-set or sometimes late evening.

The scope has a fan installed but I assumed I would not need to use it since I was not storing the scope indoors before viewing.

Because of some discussion on this forum in another thread I will try using the fan also.

At this point I am under the assumption that my mirror temps have been reasonably close to OAT.

.

[Daniel-K]

if you get m81 & 82 again try using averted vision on 82 to see the dust lane very clearly visible. have you done a star test on collimation? i know lasers are good but a good old cheshire is hard to beat. i have a buddy who swears by lasers and i put it mine to check the accuracy of a cheshire and it was nearly perfect out by about 2mm.

[Northern Soul man]

Hi Barnstorm, nice 1 getting a 16" dob, I've used a few scopes from 8 - 12" and found that using scopes visually rather than imaging seems to be disappointing for so many people, I've been observing for years and have found that where I mainly observe from (very light polluted) your only going to "detect" faint objects - even when you find them, there's no colour at, all only greyish tints.

If you observe under light polluted skies - it really doesn't matter what size scope you use - your not going to see these fainter DSO's, I found that a Right Angled Correct Image (RACI) finder (I have the 10 x 50 one from Orion) and a good set or star maps (I have sky atlas 2000) really help - your seeing in the finder exactly how the sky appears on the charts - so this really helped me locate these objects, but the main factor is Light Pollution - my light polluted skies are really poor - using low power ep's really show how bright the sky is and "washing out" anything fainter than about mag 9 - so not a lot to choose from - I stick to objects brighter than about 9.5 but you have to try each object - some are large and diffuse - but some cover smaller bits of sky and tend to be brighter - you have to make a list for each observing session - stick to it - star hop to the field the object is supposed to be in - if nothing there - then increase the mag - place a blanket over your head and cover around the ep - this helps a lot - and sweep the area a few times - but I can't stress enough that light pollution is your main factor here - if your observing against a bright sky - nearly all the objects are washed out - but increasing mag a little and sweeping with your head covered around the eyepiece and may be a UHC filter will help - its not the answer to all objects - but it certainly helps me - I've been using a 5" Mak (waiting for my 11" Celestron) and really enjoying the challenge - as I say, Ive been observing for years and keep at it night after night from a very light polluted sky.

I'm not trying to dampen your enthusiasm, but if your observing from light polluted skies, its not that your looking in the wrong place - but the sky is so bright that objects fainter than the background sky are going to be invisible - but with a few tips and a lot of patience - it becomes a little easier.

Take care and keep at it. Regards Paul.

[John]

My experience of viewing deep sky objects is that the seeing conditions and especially the amount of light pollution in the sky (whether man-made or moonlight) is the make or break factor for the quality of views you get. Collimation, unless massively out, has much more effect on planetary / lunar contrast and resolution than it does on deep sky objects. Full cooling, likewise, in my view.

Because I have to view surrounded by neighbours houses and with light pollution from Bristol and Newport on the SE and NE horizons, I've learned to pick my DSO's with care and, in the main, to wait until they are positioned well above the offending LP "murk", the closer to the zenith the better.

The views you have had of DSO's sound like those I've seen with much smaller apertures and LP is probably your main culprit, in my view.

Saying that, there are techniques such as averted vision which an observer can use to make the most of the views that are available. Spending time on an object to gradually try and tease out the detail will improve things I think.

[cs1cjc]

View Venus. If the image is smeared then it is likely that the mirror is not cool. A large mirror takes a *long* time to cool.

[swamp thing]

Interesting report.

The problems your having relate to your observing location.

Big scopes need dark skies to really show there stuff period.

You can use them under LP skies but they're never going to show the sort of detail that they can from an ink black sky.

From near London my 16" was....."well it shows a bit more than my 10". But from a dark sky it was........ "£&@)?!g hell, look at that"

You need to try it from a dark location.

Regards Steve

[barnstorm]

Thanks everyone, I am getting some great information, tips and feeling much better now.

I am going to use the fan tonight and I will see if I can find a black towel or shroud to put over my head.

if you get m81 & 82 again try using averted vision on 82 to see the dust lane very clearly visible. have you done a star test on collimation? i know lasers are good but a good old cheshire is hard to beat. i have a buddy who swears by lasers and i put it mine to check the accuracy of a cheshire and it was nearly perfect out by about 2mm.

I did try averted vision and I was not getting much but I will give it another shot.

I have not done a star test, only the laser system.

I adjust to get the dot aligned but I think there is a limit to what I can do because the *$##$ plastic focuser will put the dot just out of center with the slightest touch of the EP. I spent the extra $$ for the Hotech because it does not use a set-screw to hold the laser but I don't think I will get to see the use of this until I get a 'real' focuser.

When I look down the focuser I can see the black dot of the primary exactly on my eye.

.

[xyz]

just bear in mind that air temp usually drops continuously as the night draws on and that the big chunk of glass will also cool but at a much slower rate. The fan should help it cool down roughly in sync with the air. Success !!

[cs1cjc]

I have not done a star test, only the laser system.

I adjust to get the dot aligned but I think there is a limit to what I can do because the *$##$ plastic focuser will put the dot just out of center with the slightest touch of the EP. I spent the extra $$ for the Hotech because it does not use a set-screw to hold the laser but I don't think I will get to see the use of this until I get a 'real' focuser.

When I look down the focuser I can see the black dot of the primary exactly on my eye.

.

A star test will show whether all is properly aligned, but do worry too much about absolute precision. Small errors only move the fully sharp area (where coma is smaller than the airy disc) off centre. Another possibility is that the mirror itself may not be very good. Again a star test will show imperfections, though the results are not easy to interpret.

TMB Website

Star Testing Astronomical Telescopes, A Manual for Optical Evaluation and Adjustment, Second Edition by Harold Richard Suiter

[barnstorm]

I had some clear skies last night, at least until the moonrise.

I ran the fan and while I did not see dramatic improvement, there was no vibration and high power views did seem more stable.

thanks!

[spaceboy]

Did you do a star test ?

[barnstorm]

Did you do a star test ?

I have not yet purchased & read the book in the link so I can't say I am qualified to do a star test.

I can say that the out-of-focus stars are:

1. Round

2. Show the shadow of the secondary in the middle.

.

[John]

As I said in my earlier post, unless the collimation is miles out, which it does not sound that it is if the secondary shadow is broadly central, the factors affecting your views of deep sky objects will be primarily seeing conditions and light pollution.

[spaceboy]

Well there is only one answer to it Barnstorm. I'll have your 16" dob and you can have my 8" newt. When do you want to do the swap ? I'm free this weekend

[cs1cjc]

Hi,

There are some online resources for star testing including the following, though there may be better ones (the second is just one part of a truly wonderful resource on telescope optics which is under continual improvement; the last is an extended blurb for the book with some examples):

TMB Website

Diffraction pattern and aberrations

Star Testing Astronomical Telescopes, A Manual for Optical Evaluation and Adjustment, Second Edition by Harold Richard Suiter

[dark star]

At least you can see M51, it seems to be invisible form near London in my 10 inch reflector!

David