I have been considering picking up a achromatic refractor for narrowband imaging. CA should not be an issue when shooting narrowband, so even the f5 achros would work fine. Looking at this one, which I could probably pickup from Telesckop-Express pretty cheaply: http://www.bresser.de/en/Astronomy/B...e-Hexafoc.html
There is also a four element 152mm made by Bresser that looks good.
Obviously even parafocal filters would need refocusing on an achro, and i suppose a flattener would be required, but as a lightvweight alternative to my heavy Mak-Newt it might be useful.
Any opinions, or experiences, shooting narrowband with acrho? What do you think?
Thanks for that link to Bert's thread. After reading that and some of the others I have found on other forums, I have decided to go ahead with project and decided on the Bresser AR-152S, (f5, 760mm fl) it has been ordered from Teleskop-Express. It has a four element lense set, the traditional achro doublet and a superachromatic-like correction group, the chromatic aberration is reduced (supposedly), but of course not much of a concern with narrowband imaging other than assisting filter refocus perhaps.
I need to source a field flattener that will work with this scope (f5, and 760mm fl), which is just outside the Orion Flattener specs but maybe it would be ok. Anyone with a Field Flattener suggestion for this scope please let me know.
Narrowband imaging with achromatic refractor should allow for pleasing results in astrophotography, given such telescope has a quality focuser. However, there will be noticeable halos around brighter stars, even when imaging with quality 3nm Astrodons, since no filter is capable of blocking 100% of unwanted light.
I believe post-processing can significantly reduce halo effects, based on online tutorials i have watched on Photoshop processing, and my own processing to date.
I will always have a probable preference for Mak-Newt images but there are trade offs in what ever platform is used. My goal firstly is to half the weight of the Mak-Newt and the Bresser 152mm does that without giving away too much aperture, and provides a slightly wider field of view.
My Bresser 152mm, f5, 760mm fl, Modified Achro arrived today, boy was that quick. I ordered it on Monday morning our time from Teleskop-Express (shipped via UPS) and it showed up this afternoon.
On first inspection, I can say this thing is a real beast, I did not realise how big it would be but not near as heavy as a Mak-Newt. There was some shipping damage, two of the nylon screws used in the finderscope bracket were broken, but that could have happened when the courier dropped the end of it right outside my door, they are easily replaced.
I went over it and tightened up the focuser adjustment screws, but I will need to check the focuser alignment with my laser and check the collimation as well. It is clean internally, and no imperfections in the objective or the rear lens Petzval group.
I have attached some photos. Hope it clears up this evening so I can do a visual star test to check it out.
Had the Bresser 152mm out for a visual first light session tonight. Collimation check with star test was perfect. Spent some time checking double star splits, the bright nebs in Sagitarius, and the field curvature and CA. The CA is actually not too bad, not that it will matter for narrowband imaging; however it certainly needs a field flattener for imaging. I have an Orion Flattener on order from Bintel, as it seems to best suit the back focus of my camera (55mm to the sensor with the filter wheel in place). Can not really image test until i get the flattener, but things still look positive for this setup to work.
Glen, I'm itching to get one of these big boys too. Do you reckon the focuser is good enough?
It was actually a bit loose on arrival but once i tightened up the three grub screws it is very solid. The hexagonal shape seems to give it good support. It handles my big heavy billet style 2" diagonal and 30 mm ep without any slop. I will know more when i get the camera and filter wheel on, hopefully tonight. Moonlight make a very nice replacement focuser and adaptor but it cost almost as much as the scope, so i am hoping the Hex focuser is imaging suitable. More soon.
I have conducted the first narrowband imaging test with the Bresser 152mm, f5, 760mm fl, modified Achro. I still have not received the Field Flattener, Bintel was out of stock and I had to order it elsewhere.
The test image on M8 Lagoon Nebula, is made up of 10 x 300" Ha subs stacked in DSS with Darks and Bias Masters.
Final full size image is posted (jpg) on Astrobin here:
Minimal processing as this was a test image, mainly to check scope performance and narrowband focus, and field curvature.
Thumbnail below is here as a courtesy, it is far to small to tell much, sugges you examine the full size one on the link above.
Looking pretty good Glen! I'd be interested to see how it performs in RGB on this area, how much haloing you get
It'll help when your flattener arrives, the edges aren't as bad as I thought that they might be, not a huge sensor though (bigger than mine)!
Thanks Colin, i believe the rear mounted petzval style corrector group may have some affect on the field, it is better than i expected i have to admit. I do not plan in using this scope for RGB imaging it is going to be used exclusively for narrowband. I was pleasantly surprised by how well it worked on the NEQ6, the guide graph was pretty much flat lining and this does help image quality. I did not even need to use the counterweight extension bar. The MN190 on the other hand is hard to guide well on my mount even though balanced to produce a good graph in the eastern sky it struggles with the weight. I will use the MN190 for RGB. With a mix of three imaging scope of various focal lengths i think i have a stable i can work with over the next few years ( 760mm, 1000mm, & 1250mm). The 10" carbon strut newt is the long one. They are also all f5 scopes, which i prefer for its speed. I am through with RCs forever.
So when the moon is around the Bresser will be used for all wider field narrowband imaging, the MN190 for more reach, both are well baffled too. When the moon is not present and in RGB mode, the MN190 and the 10" Newt will be used. That's the plan at least.
Glen this is much better than I expected - I think you'll go ahead and order one too. Which Ha filter did you use?
And just out of curiosity, what didn't you like in the RC?
I have the Baader narrowband filter set (36mm in my QHY 5 position filter wheel), camera is the ASI1600MM-Cool. It would be wise to match your camera and scope for pixel scale. What camera would you be using? At the 760mm focal length you want a fairly small pixel.
I got one of Bresser scopes also.
Purchased and upgraded the focuser with the 10:1 wheel.
Pretty smooth now.
Mine also arrived loose, had to adjust the whole focuser.
The optics had what appeared to be flock gassing on them, had to clean them.
Some screws arrived loose and after inspection the holes are all loose on the main scope. Maybe its the cheap aluminum.
After removing the dew shield I noticed a crack on the end on the main scope. Okay I can live with that.
Overall its Chinese built Meade, the outer visual is good looking but mine had those few hidden faults.
Views were stunning and I added the 2" W.O. Durabright rotolock diagonal.
Need to start taking pictures with it.
I got one of Bresser scopes also.
Purchased and upgraded the focuser with the 10:1 wheel.
Pretty smooth now.
Mine also arrived loose, had to adjust the whole focuser.
The optics had what appeared to be flock gassing on them, had to clean them.
Some screws arrived loose and after inspection the holes are all loose on the main scope. Maybe its the cheap aluminum.
After removing the dew shield I noticed a crack on the end on the main scope. Okay I can live with that.
Overall its Chinese built Meade, the outer visual is good looking but mine had those few hidden faults.
Views were stunning and I added the 2" W.O. Durabright rotolock diagonal.
Need to start taking pictures with it.
John where did you get the 10:1 gear set wheel? I know they are available from Bresser direct but do any US retailers sell them? Are they the same as the Explore Scientific gear set?
John where did you get the 10:1 gear set wheel? I know they are available from Bresser direct but do any US retailers sell them? Are they the same as the Explore Scientific gear set?
Originally bought the scope from APM in Germany.
Ordered the 10:1 from them.
Tried USA first and Bresser or Explore Scientific didn't carry them.
Went to a star party a few weeks ago and saw a 102mm Explore Scientific and looked at the focuser real close.
Mine is the new Hexafoc but it looked very much the same as the standard focuser except now it has flat sides.
Ask the guys at APM Germany they know a ton about these scopes.
The 10:1 makes it real nice and smooth now.
Focusing is way better for my old eyes.
The upgrade gear for the refractor is slowly coming together; the Hexfoc 10:1 gear set has been shipped from Teleskop-Express, and the Hotech Field Flattener is on the way from Agena Astro. I had to go with the Hotech after running into trouble trying to buy the Orion Flattener from Opt Corp - it seems Orion would not permit them to export it (so they say) and it would void the warranty, which is pretty disappointing when Orion's local distributor does not have any stock (I tried to buy local). Anyway the Hotech unit seems to be well regarded. http://www.hotechusa.com/category-s/4.htm
I was only looking at those two as they best matched the backfocus distance (without additional extensions) of my camera and filter wheel combo. I should have these items towards the end of next week hopefully.
