DanyalAG

On 28/02/2020 at 03:59, RobertI said:

Just fitted a home-made cooling fan to the back of the scope using instructions from a fellow SGL member. I didn't do a fantastic job (missing some vital tools), but it does blow a nice breeze up the tube, has a variable speed and is vibration free. My hope is that it speeds up cool down time, but more importantly improves high powered views of lunar, planets and possibly doubles, by removing the boundary layer of warm air that sits over the mirror. It's clear outside but I have a streaming cold so will wait for another time to test it.

 

Just one question; are cooling fans required for a 6 inch Newtonian?

I've always read that 6" scopes cool down in around 20-30 minutes, while bigger aperture (8" and up) take longer to cool down, and so a cooling fan is necessary for them.

Or am I missing something here?

There are a lot of factors that can contribute to image degradation:

1. The seeing ( the usual culprit. )

2. Extinction i.e. the fact that the object you were observing(in this case, the Sun)was very low in the sky and thus the atmosphere degraded the image.

3.Inferior Eyepieces... Mostly the Eyepieces that are provided with a telescope range from being poor(kellner, ramsden, achromat lens etc) to fair(ortho, plossl)...so if you're observing with one of the cheaper ones... they could be the cause of the degraded images.

4. Even though your scope is an achromat, it has a focal ratio of F/10; at this ratio it should be very well color-corrected... however the sun is a bright object... so 'some' color fringing will occur.

Most probably, the color-fringing and washed-out effect is due to poor eyepieces. If its in the scope, you can check it at night by focusing on a bright star(vega, deneb) at high magnification, if the image is a perfect (tiny) disk, then you have nothing to worry about. If it does not come to a sharp focus, then your scope has spherical aberration... which is a fancy way of saying that the scope doesn't focus all the light to a point.

To answer your question, no bigger scopes do not usually suffer from bad images- if they are treated properly. They will usually provide you with a better image.

However, I'd recommend you get yourself a Newtonian if you want to get a bigger telescope (no color-fringing). Also you can get a Apochromat, but they're usually really expensive.

6 minutes ago, jetstream said:

Ok, I was puzzled by your answer that Mizar and Alcor are harder to split than the Double Double Epsilon Lyrae.

I just went outside not dark adapted in avg skies and find this extremely easy, but I had to check. It is nowhere near as hard as splitting Epsilon Lyrae into "2" stars.

Did you mean the Double Cluster in Perseus by chance?

In dark skies, I think Mizar and Alcor would be easier. However from my light-polluted skies, Mizar's glare makes it nearly impossible to see Alcor. On the other hand, the components of Epsilon Lyrae are almost equal magnitude. I find them much easier to focus on both of them.( I live in NELM 4 -4.5 skies )

When I was first getting into astronomy, i often heard that mass-produced Synta and GSO Dobs(Orion, Celestron, Meade) were often only average in optical quality; and often mediocre(below 1/4th p-v wavefront). That is why I decided to get an OTA from Orion Optics (VX8L) which is an 8" F/6  Newtonian OTA. I built the dobsonian mount myself, and frankly have been very pleased with it. The 1/8th p-v optics are excellent on both the planets and DSOs and I've never had any issues with it.

Recently however, I wanted a smaller scope for portability and mostly to mod, and so I thought; why not go with one of these cheap scopes from Synta and GSO. I settled upon the Starblast 6, because of its decent aperture, good portability, and good value. Now, on to the review.

When the scope first arrived, I was not at all happy with it. Star-Test showed astigmatism, and images looked distinctly soft when compared to the VX8L. For some time, I thought to replace it or better yet return it... and it spent a fair amount of time gathering dust, instead of underneath the sky. I decided to give it another shot, since you should never judge a telescope on only one star-test. this time, I noticed that the star-test wasn't showing astigmatism at all, but rather pinched optics. I was a newbie to these issues as my previous scope never had any. The next day, the scope was quickly disassembled, and I decided to do away with the mirror-retaining clips permanently. I used sillicone sealant to glue the mirror to its cell, taking care not to deform it... and left it out to dry as I waited for night to settle in.

I must say that after this small fix,  this little scope performs like a dream. The star-test is almost perfect-showing no more than 1/6th p-v undercorrection( which is I think is easily acceptable for a scope this price) also I cannot detect any roughness or zones in the optics even at 320X. Epsilon Lyrae(the double double) is resolved completely at 75X-and this is a feat that I have not been able to better in the VX8L. I have tested the scope to its very limits; which include splitting 52 and 14 Orionis (1.1" and 0.92" respectively) and seeing Pluto (in Bortle 3 skies). Optically I think, that if this is the standard for all Chinese Dobs, then it is a very good standard indeed.

On DSOs, almost everything the VX8L can show me, this can also, albeit a bit dimmer and harder to see.

I have found that planetary contrast suffers a little due to the large(33%) obstruction, but this is to be expected from an F/5 instrument.

Mechanically, I really love the tube rings, as they allow me to rotate the tube for optimum focuser position, and will also allow the mounting of the scope on other mounts.

The Alt-Az motions are OK, although they could be smoother... but it again, at this price, it is hard to beat. Of course, you do need a table to set it on-but personally-as the mount saves space-I think its a non-issue.

The only thing I seriously dislike about the scope is the plastic focuser. It has an unacceptable amount of slop, and shifts focus whenever I put in a heavier eyepiece. I will probably replace it. It's really a shame... because if Orion just replaced this with a 1.25 or 2" crayford, then this would be the perfect portable telescope.

Moving on to the eyepieces, the scope comes equipped with 25mm and 10mm Sirius Plossl eyepieces. I am of the opinion that a well-made, well-coated Plossl eyepiece is a very good eyepiece... difficult to beat in terms of contrast and light-transmissivity. I have th Meade 4000 series plossls, as well as the Orion Stratus, and they are both very good eyepieces.

The Sirius Plossls are decent quality... they are fully coated, and provide around 85-90% of the performance of the Meade Plossls.

Overall, I think, the scope is pretty decent quality. It is annoying that the mirror-clips were done too tight- and I'm sure that a beginner will struggle to deal with such issues. Here are some mods you can do if you buy or are thinking to buy this scope:

1. Star-test the scope, and if its not perfect, try loosening or even removing the mirror clips( and glueing the mirror!)

2.Add a tube-extension to the front of the scope( really helps with contrast).

3. Flocking the scope.

4.Replacing the focuser(Eventually).

All in all, I think its a pretty decent scope for what its worth.

Anyways, this is my two cents on the scope. 

1 hour ago, jetstream said:

A good test of the eyes is the Double Double- I easily see it as 2 stars naked eye- anyone else?

I have seen it naked eye on a couple of occasions, but it requires dark skies.However, in my opinion Mizar and Alcor is a finer test of visual acuity(at least for me).

20 minutes ago, Second Time Around said:

Seeing faint stars is different to how sharp your eyes are.  The former partly depends on your maximum pupil size, that tends to decrease with age, plus of course on light pollution and sky clarity.

Unfortunately the maximum pupil size in my better eye is only 4.5mm (7mm is more typical for young people) and so my naked eye limiting magnitude is well below average.

On the other hand I have very sharp vision with glasses: 6/4 in each eye, that corresponds I believe to 20/12 old style.

Oh, well I wasn't really aware of that fact. Thanks for the info. However, i do think that o Meara wasn't quite young (at least 35-40) when he did all those things.Anyways, what's the best you've been able to do?

This fact comes up quite a lot on the internet. Recently, I was reading a Sky and Telescope magazine article, and it stated that Stephen o Meara was able to see better than 7, and on some occasions even 8 magnitude stars with his naked eye alone. Is this possible? I find it very hard to believe because I've observed from Bortle 3-4 sites on a couple of occasions, and the best I could do was about mag 6.3 (with considerable effort). I am by no means an expert, and have only been involved in amateur astronomy for the best part of 2 years, but still, I know most of the techniques like dark-adaptation, averted vision and I still couldn't do any better. I have excellent eyesight (better than 20/18 in my right eye) but still, such a feat seems impossible to me. I mean no disrespect to o Meara, however is there anyone here who has observed from perfect (Bortle 1 or 2) sites and was able to replicate o Meara's feat? If not, why don't you give the faintest magnitude you've ever seen. Just a curious question on my part...

Any replies are welcome.

3 hours ago, Ouroboros said:

I like the thread title! 🙂 

Thanks. Although I'll admit I didn't really think about it.

1 hour ago, Barry-W-Fenner said:

Couple of pics of the 200p being flocked. I quite like the shot of the final strip being applied. It really shows the difference in light being absorbed.i applied mine in two inch strips down the tube and it came out really well.

There was a noticable difference in contrast when complete. One of the first objects I viewed was the ring nebula and I found it easier to locate and I could see more of it. I can't say if this was down to the flocking or better seeing that night but I am a believer that the flocking has improved my observing, I feel that it is a worth while fairly cheap upgrade which has a small hand in combating our light polluted sky's..

 

Baz

 

Did you use protostar or some other material ?

34 minutes ago, johninderby said:

I prefer to flock the entire tube altough just flocking oposite the focuser and about a foot up from the primary mirror does work nearly as well. 

You will find flocking really helps with planetary observing with much improved contrast but not that much with DSOs. 

Thanks mate.

6 hours ago, Fo_Cuss said:

If you can't get hold of recommended flocking material...
In the meantime, you can get experimental 😈

Acquire card, or rigid plastic sheet, that will self roll to the interior of the scope.

If initially, it doesn't perfectly self-roll ... you can train it, by rolling the material into a slightly smaller diameter.
In this way, when it springs out, it will grip to the tube.

Once you have cracked this setup phase, you can experiment by apply different materials to the transport material.

Find a light to point your tube at, and look down the other end.
See which materials are best at absorbing the light.

When materials are scarce, you need to be innovative 😉

🌝

Great advice. Thanks.

I'm thinking of trying different materials and then using a flashlight to see how reflective it is at oblique angles. Then I'll choose the material that reflects the least light.

Right now, I have a found a sheet that is used in home-theaters.It is marketed to minimize reflections. So I'm thinking it will (probably) work.

Also, I have another question. I've been reading on the net, and most sites say that only flocking the tube opposite the focuser and 6 inches up from the primary mirror (along with a tube extension) has any real effect. Is this true? or should I flock the whole tube?

Again, Thanks to everyone for the advice.

Thanks a lot mate.

Hello Everyone. I'm going to flock a 6" f/5 Newtonian(Orion Starblast 6) but I have a problem.Purpose-built flocking materials are currently not available(not shipped) in my country. My question is were I to flock my telescope with some black foam sheets (from hobby stores), would it make any difference? Maybe I could even paint the sheets with flat-black paint?

And if this is not a suitable material or method for flocking, could anybody suggest some alternative flocking materials to the purpose-built ones like Protostar?. Thanks.

Thanks a lot, I really appreciate all the help. That low-profile focuser surely is a thing of beauty.

Hey Chriske, really love your focusers, especially the crayford and the bearing-helical. I'm looking to mod my scope (smaller secondary+low-profile focuser), and looking online, I quickly found out that low-profile focusers cost a lot. Now I am trying to find resources on any good crayford focuser design (low-profile). The profile I want is between 1.5 to 2 inches racked-in height (1.6 inches would be ideal).

Now I don't really have access to a 3D printer or a lathe, The only tools I have are hand-tools, and some basic power-tools(drill, hole-saw). 

I just wanted to ask if it is possible to build a decent low-profile crayford focuser out of some wood,PVC pipe and ball bearings...You look like you have a lot of experience regarding focuser-making, so any advice or help? Thanks.

13 hours ago, CraigT82 said:

Yup! Have a look here for some ideas (most here involve 3D printing but might serve as inspiration).

 

Thanks a lot man.

Hello Everybody, my question is very simple:

with quality ball bearings, a PVC shaft, and some wood, is it possible to build a decently-working low profile crayford focuser?

By low profile, I mean something with a racked-in height of 1.5-2 inches. 1.6 inches would be ideal.

Any help,advice or useful links would be appreciated.

48 minutes ago, Ricochet said:

Are you aware that the focal length of the scope is fixed and a function of the primary mirror? If you reduce the secondary - focal plane distance by 65mm then you will have to increase the primary - secondary distance by 65mm. This means that you will have to move both the focuser and secondary spider supports up the tube. I don't think that the tube is long enough to support such a movement unless you also plan on extending/replacing the tube as well. I think the most cost effective solution for this scope is to get a replacement mirror that is sized for the current position of the secondary. Depending on the length of time you have had the scope and the warranty provided in your country, a direct replacement from the manufacturer might be available (if you bought it new).

Yes, I am aware of that fact, but I think that 6.5 cm of space is available at the front of the tube. if it's not, then maybe I'll extend the tube(with PVC).I really want to optimize the scope, as I think that planetary views are a bit too low on contrast.Anyways, thanks a lot.

Hello, everybody. I have a starblast 6 (6" f/5), and I'm looking to modify it in several different ways-namely:

1. when I first recieved the scope, I was a complete newbie, and so didn't know anything about star-testing and optical quality. However, after a week or so of searching online, I learned how to star-test, and quickly figured out that the scope has (mild) astigmatism. A little more research, and after rotating the primary mirror several times in its cell, I was able to confirm that the primary actually has excellent optics, but the secondary mirror suffers from astigmatism.Now, I am also not quite happy with the focuser on this scope. So, I want the change the secondary mirror and the focuser for a low-profile one. The secondary mirror has a minor axis of 47 mm(1.85 inches). After some searching and measuring the scope, I came up with these measurements:

I. Primary mirror to secondary mirror distance = 570 mm (22.44 inches)

II.Secondary mirror to focal plane distance = 180 mm (7.08 inches)

I reckon that if I reduce the secondary-focal plane distance to around 115 mm(4.5 inches), I can get away with a secondary mirror size of 35 mm(1.3 inches) for a fully illuminated field of around 0.4 degrees and an edge magnitude drop-off of 0.3 magnitudes. However this will reduce the obstruction percentage  from 31% to 23% by diameter, and from 9% to around 5% by area. My question is, will I be able to detect the better contrast of this smaller secondary mirror or not? And if yes, how much of a difference will it actually make?

2. I am also going to modify the mount, as the azimuth motions have gotten a bit iffy. Any good links, or has anyone done this before?

Any help regarding these issues would be greatly appreciated.

Hey Alan, I also have a Starblast 6, but I'm really curious about the level of optical perfection in this scope. I'm an absolute newbie, but from what I've learned, F/5 is a fast scope, and so will suffer from many optical abberations. I've no idea how to star-test, so any feedback from you regarding the optical quality of the scope would be highly appreciated.I would prefer a comparison with that Takahashi Apo you have. Thanks. 

(P.S. I also hate the focuser)